[PATCH bpf-next v3 00/16] Clean up and document RCU-based object protection for XDP_REDIRECT

[PATCH bpf-next v3 00/16] Clean up and document RCU-based object protection for XDP_REDIRECT

[Toke Høiland-Jørgensen]

During the discussion[0] of Hangbin's multicast patch series, Martin pointed out
that the lifetime of the RCU-protected  map entries used by XDP_REDIRECT is by
no means obvious. I promised to look into cleaning this up, and Paul helpfully
provided some hints and a new unrcu_pointer() helper to aid in this.

It seems[1] that back in the early days of XDP, local_bh_disable() did not
provide RCU protection, which is why the rcu_read_lock() calls were added
to drivers in the first place. But according to Paul[2], in recent kernels
a local_bh_disable()/local_bh_enable() pair functions as one big RCU
read-side section, so no further protection is needed. This even applies to
-rt kernels, which has an explicit rcu_read_lock() in place as part of the
local_bh_disable()[3].

This patch series is mostly a documentation exercise, cleaning up the
description of the lifetime expectations and adding __rcu annotations so
sparse and lockdep can help verify it.

Patches 1 and 2 are preparatory: Patch 1 adds Paul's unrcu_pointer()
helper (which has already been added to his tree), which we need for some
of the operations in devmap, and patch 2 adds bh context as a valid
condition for map lookups. Patch 3 is the main bit that adds the __rcu
annotations and updates documentation comments, and the rest are patches
updating the drivers, with one patch per distinct maintainer.

Unfortunately I don't have any hardware to test any of the driver patches;
Jesper helpfully verified that it doesn't break anything on i40e, but the rest
of the driver patches are only compile-tested.

[0] https://lore.kernel.org/bpf/20210415173551.7ma4slcbqeyiba2r@kafai-mbp.dhcp.thefacebook.com/
[1] https://lore.kernel.org/bpf/c5192ab3-1c05-8679-79f2-59d98299095b@iogearbox.net/
[2] https://lore.kernel.org/bpf/20210417002301.GO4212@paulmck-ThinkPad-P17-Gen-1/
[3] https://lore.kernel.org/bpf/20210419165837.GA975577@paulmck-ThinkPad-P17-Gen-1/

Changelog:
v3:
  - Remove one other unnecessary change to hlist_for_each_entry_rcu()
  - Carry forward another ACK
v2:
  - Add a comment about RCU protection to the drivers where rcu_read_lock()
    is removed
  - Drop unnecessary patch 3 which changed dev_get_by_index_rcu()
  - Add some more text with the history to cover letter
  - Fix a few places where the wrong RCU checks were used in cpumap and
    xskmap code
  - Carry forward ACKs

Paul E. McKenney (1):
  rcu: Create an unrcu_pointer() to remove __rcu from a pointer

Toke Høiland-Jørgensen (15):
  bpf: allow RCU-protected lookups to happen from bh context
  xdp: add proper __rcu annotations to redirect map entries
  ena: remove rcu_read_lock() around XDP program invocation
  bnxt: remove rcu_read_lock() around XDP program invocation
  thunderx: remove rcu_read_lock() around XDP program invocation
  freescale: remove rcu_read_lock() around XDP program invocation
  net: intel: remove rcu_read_lock() around XDP program invocation
  marvell: remove rcu_read_lock() around XDP program invocation
  mlx4: remove rcu_read_lock() around XDP program invocation
  nfp: remove rcu_read_lock() around XDP program invocation
  qede: remove rcu_read_lock() around XDP program invocation
  sfc: remove rcu_read_lock() around XDP program invocation
  netsec: remove rcu_read_lock() around XDP program invocation
  stmmac: remove rcu_read_lock() around XDP program invocation
  net: ti: remove rcu_read_lock() around XDP program invocation

 drivers/net/ethernet/amazon/ena/ena_netdev.c  |  6 +--
 drivers/net/ethernet/broadcom/bnxt/bnxt_xdp.c |  5 +-
 .../net/ethernet/cavium/thunder/nicvf_main.c  |  5 +-
 .../net/ethernet/freescale/dpaa/dpaa_eth.c    | 11 ++---
 .../net/ethernet/freescale/dpaa2/dpaa2-eth.c  |  6 +--
 drivers/net/ethernet/intel/i40e/i40e_txrx.c   |  5 +-
 drivers/net/ethernet/intel/i40e/i40e_xsk.c    | 11 ++---
 drivers/net/ethernet/intel/ice/ice_txrx.c     |  6 +--
 drivers/net/ethernet/intel/ice/ice_xsk.c      |  6 +--
 drivers/net/ethernet/intel/igb/igb_main.c     |  5 +-
 drivers/net/ethernet/intel/igc/igc_main.c     | 10 ++--
 drivers/net/ethernet/intel/ixgbe/ixgbe_main.c |  5 +-
 drivers/net/ethernet/intel/ixgbe/ixgbe_xsk.c  |  9 ++--
 .../net/ethernet/intel/ixgbevf/ixgbevf_main.c |  5 +-
 drivers/net/ethernet/marvell/mvneta.c         |  6 ++-
 .../net/ethernet/marvell/mvpp2/mvpp2_main.c   |  8 +--
 drivers/net/ethernet/mellanox/mlx4/en_rx.c    |  8 +--
 .../ethernet/netronome/nfp/nfp_net_common.c   |  6 ++-
 drivers/net/ethernet/qlogic/qede/qede_fp.c    |  7 +--
 drivers/net/ethernet/sfc/rx.c                 | 12 ++---
 drivers/net/ethernet/socionext/netsec.c       |  7 +--
 .../net/ethernet/stmicro/stmmac/stmmac_main.c | 13 ++---
 drivers/net/ethernet/ti/cpsw_priv.c           | 13 ++---
 include/linux/rcupdate.h                      | 14 ++++++
 include/net/xdp_sock.h                        |  2 +-
 kernel/bpf/cpumap.c                           | 13 +++--
 kernel/bpf/devmap.c                           | 49 ++++++++-----------
 kernel/bpf/hashtab.c                          | 21 +++++---
 kernel/bpf/helpers.c                          |  6 +--
 kernel/bpf/lpm_trie.c                         |  6 ++-
 net/core/filter.c                             | 28 +++++++++++
 net/xdp/xsk.c                                 |  4 +-
 net/xdp/xsk.h                                 |  4 +-
 net/xdp/xskmap.c                              | 29 ++++++-----
 34 files changed, 194 insertions(+), 157 deletions(-)

-- 
2.32.0

[PATCH bpf-next v3 01/16] rcu: Create an unrcu_pointer() to remove __rcu from a pointer

[Toke Høiland-Jørgensen]

From: "Paul E. McKenney" <paulmck@kernel.org>

The xchg() and cmpxchg() functions are sometimes used to carry out RCU
updates.  Unfortunately, this can result in sparse warnings for both
the old-value and new-value arguments, as well as for the return value.
The arguments can be dealt with using RCU_INITIALIZER():

        old_p = xchg(&p, RCU_INITIALIZER(new_p));

But a sparse warning still remains due to assigning the __rcu pointer
returned from xchg to the (most likely) non-__rcu pointer old_p.

This commit therefore provides an unrcu_pointer() macro that strips
the __rcu.  This macro can be used as follows:

        old_p = unrcu_pointer(xchg(&p, RCU_INITIALIZER(new_p)));

Reported-by: Toke Høiland-Jørgensen <toke@redhat.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Toke Høiland-Jørgensen <toke@redhat.com>
---
 include/linux/rcupdate.h | 14 ++++++++++++++
 1 file changed, 14 insertions(+)

diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
index 9455476c5ba2..d7895b81264e 100644
--- a/include/linux/rcupdate.h
+++ b/include/linux/rcupdate.h
@@ -363,6 +363,20 @@ static inline void rcu_preempt_sleep_check(void) { }
 #define rcu_check_sparse(p, space)
 #endif /* #else #ifdef __CHECKER__ */
 
+/**
+ * unrcu_pointer - mark a pointer as not being RCU protected
+ * @p: pointer needing to lose its __rcu property
+ *
+ * Converts @p from an __rcu pointer to a __kernel pointer.
+ * This allows an __rcu pointer to be used with xchg() and friends.
+ */
+#define unrcu_pointer(p)						\
+({									\
+	typeof(*p) *_________p1 = (typeof(*p) *__force)(p);		\
+	rcu_check_sparse(p, __rcu); 					\
+	((typeof(*p) __force __kernel *)(_________p1)); 		\
+})
+
 #define __rcu_access_pointer(p, space) \
 ({ \
 	typeof(*p) *_________p1 = (typeof(*p) *__force)READ_ONCE(p); \
-- 
2.32.0

[PATCH bpf-next v3 02/16] bpf: allow RCU-protected lookups to happen from bh context

[Toke Høiland-Jørgensen]

XDP programs are called from a NAPI poll context, which means the RCU
reference liveness is ensured by local_bh_disable(). Add
rcu_read_lock_bh_held() as a condition to the RCU checks for map lookups so
lockdep understands that the dereferences are safe from inside *either* an
rcu_read_lock() section *or* a local_bh_disable() section. While both
bh_disabled and rcu_read_lock() provide RCU protection, they are
semantically distinct, so we need both conditions to prevent lockdep
complaints.

This change is done in preparation for removing the redundant
rcu_read_lock()s from drivers.

Acked-by: Martin KaFai Lau <kafai@fb.com>
Signed-off-by: Toke Høiland-Jørgensen <toke@redhat.com>
---
 kernel/bpf/hashtab.c  | 21 ++++++++++++++-------
 kernel/bpf/helpers.c  |  6 +++---
 kernel/bpf/lpm_trie.c |  6 ++++--
 3 files changed, 21 insertions(+), 12 deletions(-)

diff --git a/kernel/bpf/hashtab.c b/kernel/bpf/hashtab.c
index 6f6681b07364..72c58cc516a3 100644
--- a/kernel/bpf/hashtab.c
+++ b/kernel/bpf/hashtab.c
@@ -596,7 +596,8 @@ static void *__htab_map_lookup_elem(struct bpf_map *map, void *key)
 	struct htab_elem *l;
 	u32 hash, key_size;
 
-	WARN_ON_ONCE(!rcu_read_lock_held() && !rcu_read_lock_trace_held());
+	WARN_ON_ONCE(!rcu_read_lock_held() && !rcu_read_lock_trace_held() &&
+		     !rcu_read_lock_bh_held());
 
 	key_size = map->key_size;
 
@@ -989,7 +990,8 @@ static int htab_map_update_elem(struct bpf_map *map, void *key, void *value,
 		/* unknown flags */
 		return -EINVAL;
 
-	WARN_ON_ONCE(!rcu_read_lock_held() && !rcu_read_lock_trace_held());
+	WARN_ON_ONCE(!rcu_read_lock_held() && !rcu_read_lock_trace_held() &&
+		     !rcu_read_lock_bh_held());
 
 	key_size = map->key_size;
 
@@ -1082,7 +1084,8 @@ static int htab_lru_map_update_elem(struct bpf_map *map, void *key, void *value,
 		/* unknown flags */
 		return -EINVAL;
 
-	WARN_ON_ONCE(!rcu_read_lock_held() && !rcu_read_lock_trace_held());
+	WARN_ON_ONCE(!rcu_read_lock_held() && !rcu_read_lock_trace_held() &&
+		     !rcu_read_lock_bh_held());
 
 	key_size = map->key_size;
 
@@ -1148,7 +1151,8 @@ static int __htab_percpu_map_update_elem(struct bpf_map *map, void *key,
 		/* unknown flags */
 		return -EINVAL;
 
-	WARN_ON_ONCE(!rcu_read_lock_held() && !rcu_read_lock_trace_held());
+	WARN_ON_ONCE(!rcu_read_lock_held() && !rcu_read_lock_trace_held() &&
+		     !rcu_read_lock_bh_held());
 
 	key_size = map->key_size;
 
@@ -1202,7 +1206,8 @@ static int __htab_lru_percpu_map_update_elem(struct bpf_map *map, void *key,
 		/* unknown flags */
 		return -EINVAL;
 
-	WARN_ON_ONCE(!rcu_read_lock_held() && !rcu_read_lock_trace_held());
+	WARN_ON_ONCE(!rcu_read_lock_held() && !rcu_read_lock_trace_held() &&
+		     !rcu_read_lock_bh_held());
 
 	key_size = map->key_size;
 
@@ -1276,7 +1281,8 @@ static int htab_map_delete_elem(struct bpf_map *map, void *key)
 	u32 hash, key_size;
 	int ret;
 
-	WARN_ON_ONCE(!rcu_read_lock_held() && !rcu_read_lock_trace_held());
+	WARN_ON_ONCE(!rcu_read_lock_held() && !rcu_read_lock_trace_held() &&
+		     !rcu_read_lock_bh_held());
 
 	key_size = map->key_size;
 
@@ -1311,7 +1317,8 @@ static int htab_lru_map_delete_elem(struct bpf_map *map, void *key)
 	u32 hash, key_size;
 	int ret;
 
-	WARN_ON_ONCE(!rcu_read_lock_held() && !rcu_read_lock_trace_held());
+	WARN_ON_ONCE(!rcu_read_lock_held() && !rcu_read_lock_trace_held() &&
+		     !rcu_read_lock_bh_held());
 
 	key_size = map->key_size;
 
diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c
index 544773970dbc..e880f6bb6f28 100644
--- a/kernel/bpf/helpers.c
+++ b/kernel/bpf/helpers.c
@@ -28,7 +28,7 @@
  */
 BPF_CALL_2(bpf_map_lookup_elem, struct bpf_map *, map, void *, key)
 {
-	WARN_ON_ONCE(!rcu_read_lock_held());
+	WARN_ON_ONCE(!rcu_read_lock_held() && !rcu_read_lock_bh_held());
 	return (unsigned long) map->ops->map_lookup_elem(map, key);
 }
 
@@ -44,7 +44,7 @@ const struct bpf_func_proto bpf_map_lookup_elem_proto = {
 BPF_CALL_4(bpf_map_update_elem, struct bpf_map *, map, void *, key,
 	   void *, value, u64, flags)
 {
-	WARN_ON_ONCE(!rcu_read_lock_held());
+	WARN_ON_ONCE(!rcu_read_lock_held() && !rcu_read_lock_bh_held());
 	return map->ops->map_update_elem(map, key, value, flags);
 }
 
@@ -61,7 +61,7 @@ const struct bpf_func_proto bpf_map_update_elem_proto = {
 
 BPF_CALL_2(bpf_map_delete_elem, struct bpf_map *, map, void *, key)
 {
-	WARN_ON_ONCE(!rcu_read_lock_held());
+	WARN_ON_ONCE(!rcu_read_lock_held() && !rcu_read_lock_bh_held());
 	return map->ops->map_delete_elem(map, key);
 }
 
diff --git a/kernel/bpf/lpm_trie.c b/kernel/bpf/lpm_trie.c
index 1b7b8a6f34ee..423549d2c52e 100644
--- a/kernel/bpf/lpm_trie.c
+++ b/kernel/bpf/lpm_trie.c
@@ -232,7 +232,8 @@ static void *trie_lookup_elem(struct bpf_map *map, void *_key)
 
 	/* Start walking the trie from the root node ... */
 
-	for (node = rcu_dereference(trie->root); node;) {
+	for (node = rcu_dereference_check(trie->root, rcu_read_lock_bh_held());
+	     node;) {
 		unsigned int next_bit;
 		size_t matchlen;
 
@@ -264,7 +265,8 @@ static void *trie_lookup_elem(struct bpf_map *map, void *_key)
 		 * traverse down.
 		 */
 		next_bit = extract_bit(key->data, node->prefixlen);
-		node = rcu_dereference(node->child[next_bit]);
+		node = rcu_dereference_check(node->child[next_bit],
+					     rcu_read_lock_bh_held());
 	}
 
 	if (!found)
-- 
2.32.0

[PATCH bpf-next v3 03/16] xdp: add proper __rcu annotations to redirect map entries

[Toke Høiland-Jørgensen]

XDP_REDIRECT works by a three-step process: the bpf_redirect() and
bpf_redirect_map() helpers will lookup the target of the redirect and store
it (along with some other metadata) in a per-CPU struct bpf_redirect_info.
Next, when the program returns the XDP_REDIRECT return code, the driver
will call xdp_do_redirect() which will use the information thus stored to
actually enqueue the frame into a bulk queue structure (that differs
slightly by map type, but shares the same principle). Finally, before
exiting its NAPI poll loop, the driver will call xdp_do_flush(), which will
flush all the different bulk queues, thus completing the redirect.

Pointers to the map entries will be kept around for this whole sequence of
steps, protected by RCU. However, there is no top-level rcu_read_lock() in
the core code; instead drivers add their own rcu_read_lock() around the XDP
portions of the code, but somewhat inconsistently as Martin discovered[0].
However, things still work because everything happens inside a single NAPI
poll sequence, which means it's between a pair of calls to
local_bh_disable()/local_bh_enable(). So Paul suggested[1] that we could
document this intention by using rcu_dereference_check() with
rcu_read_lock_bh_held() as a second parameter, thus allowing sparse and
lockdep to verify that everything is done correctly.

This patch does just that: we add an __rcu annotation to the map entry
pointers and remove the various comments explaining the NAPI poll assurance
strewn through devmap.c in favour of a longer explanation in filter.c. The
goal is to have one coherent documentation of the entire flow, and rely on
the RCU annotations as a "standard" way of communicating the flow in the
map code (which can additionally be understood by sparse and lockdep).

The RCU annotation replacements result in a fairly straight-forward
replacement where READ_ONCE() becomes rcu_dereference_check(), WRITE_ONCE()
becomes rcu_assign_pointer() and xchg() and cmpxchg() gets wrapped in the
proper constructs to cast the pointer back and forth between __rcu and
__kernel address space (for the benefit of sparse). The one complication is
that xskmap has a few constructions where double-pointers are passed back
and forth; these simply all gain __rcu annotations, and only the final
reference/dereference to the inner-most pointer gets changed.

With this, everything can be run through sparse without eliciting
complaints, and lockdep can verify correctness even without the use of
rcu_read_lock() in the drivers. Subsequent patches will clean these up from
the drivers.

[0] https://lore.kernel.org/bpf/20210415173551.7ma4slcbqeyiba2r@kafai-mbp.dhcp.thefacebook.com/
[1] https://lore.kernel.org/bpf/20210419165837.GA975577@paulmck-ThinkPad-P17-Gen-1/

Signed-off-by: Toke Høiland-Jørgensen <toke@redhat.com>
---
 include/net/xdp_sock.h |  2 +-
 kernel/bpf/cpumap.c    | 13 +++++++----
 kernel/bpf/devmap.c    | 49 ++++++++++++++++++------------------------
 net/core/filter.c      | 28 ++++++++++++++++++++++++
 net/xdp/xsk.c          |  4 ++--
 net/xdp/xsk.h          |  4 ++--
 net/xdp/xskmap.c       | 29 ++++++++++++++-----------
 7 files changed, 80 insertions(+), 49 deletions(-)

diff --git a/include/net/xdp_sock.h b/include/net/xdp_sock.h
index 9c0722c6d7ac..fff069d2ed1b 100644
--- a/include/net/xdp_sock.h
+++ b/include/net/xdp_sock.h
@@ -37,7 +37,7 @@ struct xdp_umem {
 struct xsk_map {
 	struct bpf_map map;
 	spinlock_t lock; /* Synchronize map updates */
-	struct xdp_sock *xsk_map[];
+	struct xdp_sock __rcu *xsk_map[];
 };
 
 struct xdp_sock {
diff --git a/kernel/bpf/cpumap.c b/kernel/bpf/cpumap.c
index a1a0c4e791c6..480e936c54d0 100644
--- a/kernel/bpf/cpumap.c
+++ b/kernel/bpf/cpumap.c
@@ -74,7 +74,7 @@ struct bpf_cpu_map_entry {
 struct bpf_cpu_map {
 	struct bpf_map map;
 	/* Below members specific for map type */
-	struct bpf_cpu_map_entry **cpu_map;
+	struct bpf_cpu_map_entry __rcu **cpu_map;
 };
 
 static DEFINE_PER_CPU(struct list_head, cpu_map_flush_list);
@@ -469,7 +469,7 @@ static void __cpu_map_entry_replace(struct bpf_cpu_map *cmap,
 {
 	struct bpf_cpu_map_entry *old_rcpu;
 
-	old_rcpu = xchg(&cmap->cpu_map[key_cpu], rcpu);
+	old_rcpu = unrcu_pointer(xchg(&cmap->cpu_map[key_cpu], RCU_INITIALIZER(rcpu)));
 	if (old_rcpu) {
 		call_rcu(&old_rcpu->rcu, __cpu_map_entry_free);
 		INIT_WORK(&old_rcpu->kthread_stop_wq, cpu_map_kthread_stop);
@@ -551,7 +551,7 @@ static void cpu_map_free(struct bpf_map *map)
 	for (i = 0; i < cmap->map.max_entries; i++) {
 		struct bpf_cpu_map_entry *rcpu;
 
-		rcpu = READ_ONCE(cmap->cpu_map[i]);
+		rcpu = rcu_dereference_raw(cmap->cpu_map[i]);
 		if (!rcpu)
 			continue;
 
@@ -562,6 +562,10 @@ static void cpu_map_free(struct bpf_map *map)
 	kfree(cmap);
 }
 
+/* Elements are kept alive by RCU; either by rcu_read_lock() (from syscall) or
+ * by local_bh_disable() (from XDP calls inside NAPI). The
+ * rcu_read_lock_bh_held() below makes lockdep accept both.
+ */
 static void *__cpu_map_lookup_elem(struct bpf_map *map, u32 key)
 {
 	struct bpf_cpu_map *cmap = container_of(map, struct bpf_cpu_map, map);
@@ -570,7 +574,8 @@ static void *__cpu_map_lookup_elem(struct bpf_map *map, u32 key)
 	if (key >= map->max_entries)
 		return NULL;
 
-	rcpu = READ_ONCE(cmap->cpu_map[key]);
+	rcpu = rcu_dereference_check(cmap->cpu_map[key],
+				     rcu_read_lock_bh_held());
 	return rcpu;
 }
 
diff --git a/kernel/bpf/devmap.c b/kernel/bpf/devmap.c
index 2a75e6c2d27d..2f6bd75cd682 100644
--- a/kernel/bpf/devmap.c
+++ b/kernel/bpf/devmap.c
@@ -73,7 +73,7 @@ struct bpf_dtab_netdev {
 
 struct bpf_dtab {
 	struct bpf_map map;
-	struct bpf_dtab_netdev **netdev_map; /* DEVMAP type only */
+	struct bpf_dtab_netdev __rcu **netdev_map; /* DEVMAP type only */
 	struct list_head list;
 
 	/* these are only used for DEVMAP_HASH type maps */
@@ -226,7 +226,7 @@ static void dev_map_free(struct bpf_map *map)
 		for (i = 0; i < dtab->map.max_entries; i++) {
 			struct bpf_dtab_netdev *dev;
 
-			dev = dtab->netdev_map[i];
+			dev = rcu_dereference_raw(dtab->netdev_map[i]);
 			if (!dev)
 				continue;
 
@@ -259,6 +259,10 @@ static int dev_map_get_next_key(struct bpf_map *map, void *key, void *next_key)
 	return 0;
 }
 
+/* Elements are kept alive by RCU; either by rcu_read_lock() (from syscall) or
+ * by local_bh_disable() (from XDP calls inside NAPI). The
+ * rcu_read_lock_bh_held() below makes lockdep accept both.
+ */
 static void *__dev_map_hash_lookup_elem(struct bpf_map *map, u32 key)
 {
 	struct bpf_dtab *dtab = container_of(map, struct bpf_dtab, map);
@@ -410,15 +414,9 @@ static void bq_xmit_all(struct xdp_dev_bulk_queue *bq, u32 flags)
 	trace_xdp_devmap_xmit(bq->dev_rx, dev, sent, cnt - sent, err);
 }
 
-/* __dev_flush is called from xdp_do_flush() which _must_ be signaled
- * from the driver before returning from its napi->poll() routine. The poll()
- * routine is called either from busy_poll context or net_rx_action signaled
- * from NET_RX_SOFTIRQ. Either way the poll routine must complete before the
- * net device can be torn down. On devmap tear down we ensure the flush list
- * is empty before completing to ensure all flush operations have completed.
- * When drivers update the bpf program they may need to ensure any flush ops
- * are also complete. Using synchronize_rcu or call_rcu will suffice for this
- * because both wait for napi context to exit.
+/* __dev_flush is called from xdp_do_flush() which _must_ be signalled from the
+ * driver before returning from its napi->poll() routine. See the comment above
+ * xdp_do_flush() in filter.c.
  */
 void __dev_flush(void)
 {
@@ -433,9 +431,9 @@ void __dev_flush(void)
 	}
 }
 
-/* rcu_read_lock (from syscall and BPF contexts) ensures that if a delete and/or
- * update happens in parallel here a dev_put won't happen until after reading
- * the ifindex.
+/* Elements are kept alive by RCU; either by rcu_read_lock() (from syscall) or
+ * by local_bh_disable() (from XDP calls inside NAPI). The
+ * rcu_read_lock_bh_held() below makes lockdep accept both.
  */
 static void *__dev_map_lookup_elem(struct bpf_map *map, u32 key)
 {
@@ -445,12 +443,14 @@ static void *__dev_map_lookup_elem(struct bpf_map *map, u32 key)
 	if (key >= map->max_entries)
 		return NULL;
 
-	obj = READ_ONCE(dtab->netdev_map[key]);
+	obj = rcu_dereference_check(dtab->netdev_map[key],
+				    rcu_read_lock_bh_held());
 	return obj;
 }
 
-/* Runs under RCU-read-side, plus in softirq under NAPI protection.
- * Thus, safe percpu variable access.
+/* Runs in NAPI, i.e., softirq under local_bh_disable(). Thus, safe percpu
+ * variable access, and map elements stick around. See comment above
+ * xdp_do_flush() in filter.c.
  */
 static void bq_enqueue(struct net_device *dev, struct xdp_frame *xdpf,
 		       struct net_device *dev_rx, struct bpf_prog *xdp_prog)
@@ -735,14 +735,7 @@ static int dev_map_delete_elem(struct bpf_map *map, void *key)
 	if (k >= map->max_entries)
 		return -EINVAL;
 
-	/* Use call_rcu() here to ensure any rcu critical sections have
-	 * completed as well as any flush operations because call_rcu
-	 * will wait for preempt-disable region to complete, NAPI in this
-	 * context.  And additionally, the driver tear down ensures all
-	 * soft irqs are complete before removing the net device in the
-	 * case of dev_put equals zero.
-	 */
-	old_dev = xchg(&dtab->netdev_map[k], NULL);
+	old_dev = unrcu_pointer(xchg(&dtab->netdev_map[k], NULL));
 	if (old_dev)
 		call_rcu(&old_dev->rcu, __dev_map_entry_free);
 	return 0;
@@ -851,7 +844,7 @@ static int __dev_map_update_elem(struct net *net, struct bpf_map *map,
 	 * Remembering the driver side flush operation will happen before the
 	 * net device is removed.
 	 */
-	old_dev = xchg(&dtab->netdev_map[i], dev);
+	old_dev = unrcu_pointer(xchg(&dtab->netdev_map[i], RCU_INITIALIZER(dev)));
 	if (old_dev)
 		call_rcu(&old_dev->rcu, __dev_map_entry_free);
 
@@ -1031,10 +1024,10 @@ static int dev_map_notification(struct notifier_block *notifier,
 			for (i = 0; i < dtab->map.max_entries; i++) {
 				struct bpf_dtab_netdev *dev, *odev;
 
-				dev = READ_ONCE(dtab->netdev_map[i]);
+				dev = rcu_dereference(dtab->netdev_map[i]);
 				if (!dev || netdev != dev->dev)
 					continue;
-				odev = cmpxchg(&dtab->netdev_map[i], dev, NULL);
+				odev = unrcu_pointer(cmpxchg(&dtab->netdev_map[i], RCU_INITIALIZER(dev), NULL));
 				if (dev == odev)
 					call_rcu(&dev->rcu,
 						 __dev_map_entry_free);
diff --git a/net/core/filter.c b/net/core/filter.c
index caa88955562e..0b7db5c70385 100644
--- a/net/core/filter.c
+++ b/net/core/filter.c
@@ -3922,6 +3922,34 @@ static const struct bpf_func_proto bpf_xdp_adjust_meta_proto = {
 	.arg2_type	= ARG_ANYTHING,
 };
 
+/* XDP_REDIRECT works by a three-step process, implemented in the functions
+ * below:
+ *
+ * 1. The bpf_redirect() and bpf_redirect_map() helpers will lookup the target
+ *    of the redirect and store it (along with some other metadata) in a per-CPU
+ *    struct bpf_redirect_info.
+ *
+ * 2. When the program returns the XDP_REDIRECT return code, the driver will
+ *    call xdp_do_redirect() which will use the information in struct
+ *    bpf_redirect_info to actually enqueue the frame into a map type-specific
+ *    bulk queue structure.
+ *
+ * 3. Before exiting its NAPI poll loop, the driver will call xdp_do_flush(),
+ *    which will flush all the different bulk queues, thus completing the
+ *    redirect.
+ *
+ * Pointers to the map entries will be kept around for this whole sequence of
+ * steps, protected by RCU. However, there is no top-level rcu_read_lock() in
+ * the core code; instead, the RCU protection relies on everything happening
+ * inside a single NAPI poll sequence, which means it's between a pair of calls
+ * to local_bh_disable()/local_bh_enable().
+ *
+ * The map entries are marked as __rcu and the map code makes sure to
+ * dereference those pointers with rcu_dereference_check() in a way that works
+ * for both sections that to hold an rcu_read_lock() and sections that are
+ * called from NAPI without a separate rcu_read_lock(). The code below does not
+ * use RCU annotations, but relies on those in the map code.
+ */
 void xdp_do_flush(void)
 {
 	__dev_flush();
diff --git a/net/xdp/xsk.c b/net/xdp/xsk.c
index cd62d4ba87a9..996da915f520 100644
--- a/net/xdp/xsk.c
+++ b/net/xdp/xsk.c
@@ -749,7 +749,7 @@ static void xsk_unbind_dev(struct xdp_sock *xs)
 }
 
 static struct xsk_map *xsk_get_map_list_entry(struct xdp_sock *xs,
-					      struct xdp_sock ***map_entry)
+					      struct xdp_sock __rcu ***map_entry)
 {
 	struct xsk_map *map = NULL;
 	struct xsk_map_node *node;
@@ -785,7 +785,7 @@ static void xsk_delete_from_maps(struct xdp_sock *xs)
 	 * might be updates to the map between
 	 * xsk_get_map_list_entry() and xsk_map_try_sock_delete().
 	 */
-	struct xdp_sock **map_entry = NULL;
+	struct xdp_sock __rcu **map_entry = NULL;
 	struct xsk_map *map;
 
 	while ((map = xsk_get_map_list_entry(xs, &map_entry))) {
diff --git a/net/xdp/xsk.h b/net/xdp/xsk.h
index edcf249ad1f1..a4bc4749faac 100644
--- a/net/xdp/xsk.h
+++ b/net/xdp/xsk.h
@@ -31,7 +31,7 @@ struct xdp_mmap_offsets_v1 {
 struct xsk_map_node {
 	struct list_head node;
 	struct xsk_map *map;
-	struct xdp_sock **map_entry;
+	struct xdp_sock __rcu **map_entry;
 };
 
 static inline struct xdp_sock *xdp_sk(struct sock *sk)
@@ -40,7 +40,7 @@ static inline struct xdp_sock *xdp_sk(struct sock *sk)
 }
 
 void xsk_map_try_sock_delete(struct xsk_map *map, struct xdp_sock *xs,
-			     struct xdp_sock **map_entry);
+			     struct xdp_sock __rcu **map_entry);
 void xsk_clear_pool_at_qid(struct net_device *dev, u16 queue_id);
 int xsk_reg_pool_at_qid(struct net_device *dev, struct xsk_buff_pool *pool,
 			u16 queue_id);
diff --git a/net/xdp/xskmap.c b/net/xdp/xskmap.c
index 9df75ea4a567..2e48d0e094d9 100644
--- a/net/xdp/xskmap.c
+++ b/net/xdp/xskmap.c
@@ -12,7 +12,7 @@
 #include "xsk.h"
 
 static struct xsk_map_node *xsk_map_node_alloc(struct xsk_map *map,
-					       struct xdp_sock **map_entry)
+					       struct xdp_sock __rcu **map_entry)
 {
 	struct xsk_map_node *node;
 
@@ -42,7 +42,7 @@ static void xsk_map_sock_add(struct xdp_sock *xs, struct xsk_map_node *node)
 }
 
 static void xsk_map_sock_delete(struct xdp_sock *xs,
-				struct xdp_sock **map_entry)
+				struct xdp_sock __rcu **map_entry)
 {
 	struct xsk_map_node *n, *tmp;
 
@@ -124,6 +124,10 @@ static int xsk_map_gen_lookup(struct bpf_map *map, struct bpf_insn *insn_buf)
 	return insn - insn_buf;
 }
 
+/* Elements are kept alive by RCU; either by rcu_read_lock() (from syscall) or
+ * by local_bh_disable() (from XDP calls inside NAPI). The
+ * rcu_read_lock_bh_held() below makes lockdep accept both.
+ */
 static void *__xsk_map_lookup_elem(struct bpf_map *map, u32 key)
 {
 	struct xsk_map *m = container_of(map, struct xsk_map, map);
@@ -131,12 +135,11 @@ static void *__xsk_map_lookup_elem(struct bpf_map *map, u32 key)
 	if (key >= map->max_entries)
 		return NULL;
 
-	return READ_ONCE(m->xsk_map[key]);
+	return rcu_dereference_check(m->xsk_map[key], rcu_read_lock_bh_held());
 }
 
 static void *xsk_map_lookup_elem(struct bpf_map *map, void *key)
 {
-	WARN_ON_ONCE(!rcu_read_lock_held());
 	return __xsk_map_lookup_elem(map, *(u32 *)key);
 }
 
@@ -149,7 +152,8 @@ static int xsk_map_update_elem(struct bpf_map *map, void *key, void *value,
 			       u64 map_flags)
 {
 	struct xsk_map *m = container_of(map, struct xsk_map, map);
-	struct xdp_sock *xs, *old_xs, **map_entry;
+	struct xdp_sock __rcu **map_entry;
+	struct xdp_sock *xs, *old_xs;
 	u32 i = *(u32 *)key, fd = *(u32 *)value;
 	struct xsk_map_node *node;
 	struct socket *sock;
@@ -179,7 +183,7 @@ static int xsk_map_update_elem(struct bpf_map *map, void *key, void *value,
 	}
 
 	spin_lock_bh(&m->lock);
-	old_xs = READ_ONCE(*map_entry);
+	old_xs = rcu_dereference_protected(*map_entry, lockdep_is_held(&m->lock));
 	if (old_xs == xs) {
 		err = 0;
 		goto out;
@@ -191,7 +195,7 @@ static int xsk_map_update_elem(struct bpf_map *map, void *key, void *value,
 		goto out;
 	}
 	xsk_map_sock_add(xs, node);
-	WRITE_ONCE(*map_entry, xs);
+	rcu_assign_pointer(*map_entry, xs);
 	if (old_xs)
 		xsk_map_sock_delete(old_xs, map_entry);
 	spin_unlock_bh(&m->lock);
@@ -208,7 +212,8 @@ static int xsk_map_update_elem(struct bpf_map *map, void *key, void *value,
 static int xsk_map_delete_elem(struct bpf_map *map, void *key)
 {
 	struct xsk_map *m = container_of(map, struct xsk_map, map);
-	struct xdp_sock *old_xs, **map_entry;
+	struct xdp_sock __rcu **map_entry;
+	struct xdp_sock *old_xs;
 	int k = *(u32 *)key;
 
 	if (k >= map->max_entries)
@@ -216,7 +221,7 @@ static int xsk_map_delete_elem(struct bpf_map *map, void *key)
 
 	spin_lock_bh(&m->lock);
 	map_entry = &m->xsk_map[k];
-	old_xs = xchg(map_entry, NULL);
+	old_xs = unrcu_pointer(xchg(map_entry, NULL));
 	if (old_xs)
 		xsk_map_sock_delete(old_xs, map_entry);
 	spin_unlock_bh(&m->lock);
@@ -231,11 +236,11 @@ static int xsk_map_redirect(struct bpf_map *map, u32 ifindex, u64 flags)
 }
 
 void xsk_map_try_sock_delete(struct xsk_map *map, struct xdp_sock *xs,
-			     struct xdp_sock **map_entry)
+			     struct xdp_sock __rcu **map_entry)
 {
 	spin_lock_bh(&map->lock);
-	if (READ_ONCE(*map_entry) == xs) {
-		WRITE_ONCE(*map_entry, NULL);
+	if (rcu_access_pointer(*map_entry) == xs) {
+		rcu_assign_pointer(*map_entry, NULL);
 		xsk_map_sock_delete(xs, map_entry);
 	}
 	spin_unlock_bh(&map->lock);
-- 
2.32.0

[PATCH bpf-next v3 04/16] ena: remove rcu_read_lock() around XDP program invocation

[Toke Høiland-Jørgensen]

The ena driver has rcu_read_lock()/rcu_read_unlock() pairs around XDP
program invocations. However, the actual lifetime of the objects referred
by the XDP program invocation is longer, all the way through to the call to
xdp_do_flush(), making the scope of the rcu_read_lock() too small. This
turns out to be harmless because it all happens in a single NAPI poll
cycle (and thus under local_bh_disable()), but it makes the rcu_read_lock()
misleading.

Rather than extend the scope of the rcu_read_lock(), just get rid of it
entirely. With the addition of RCU annotations to the XDP_REDIRECT map
types that take bh execution into account, lockdep even understands this to
be safe, so there's really no reason to keep it around.

Cc: Guy Tzalik <gtzalik@amazon.com>
Cc: Saeed Bishara <saeedb@amazon.com>
Signed-off-by: Toke Høiland-Jørgensen <toke@redhat.com>
---
 drivers/net/ethernet/amazon/ena/ena_netdev.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/drivers/net/ethernet/amazon/ena/ena_netdev.c b/drivers/net/ethernet/amazon/ena/ena_netdev.c
index 881f88754bf6..cd5f03691b4c 100644
--- a/drivers/net/ethernet/amazon/ena/ena_netdev.c
+++ b/drivers/net/ethernet/amazon/ena/ena_netdev.c
@@ -385,7 +385,9 @@ static int ena_xdp_execute(struct ena_ring *rx_ring, struct xdp_buff *xdp)
 	u64 *xdp_stat;
 	int qid;
 
-	rcu_read_lock();
+	/* This code is invoked within a single NAPI poll cycle and thus under
+	 * local_bh_disable(), which provides the needed RCU protection.
+	 */
 	xdp_prog = READ_ONCE(rx_ring->xdp_bpf_prog);
 
 	if (!xdp_prog)
@@ -443,8 +445,6 @@ static int ena_xdp_execute(struct ena_ring *rx_ring, struct xdp_buff *xdp)
 
 	ena_increase_stat(xdp_stat, 1, &rx_ring->syncp);
 out:
-	rcu_read_unlock();
-
 	return verdict;
 }
 
-- 
2.32.0

[PATCH bpf-next v3 05/16] bnxt: remove rcu_read_lock() around XDP program invocation

[Toke Høiland-Jørgensen]

The bnxt driver has rcu_read_lock()/rcu_read_unlock() pairs around XDP
program invocations. However, the actual lifetime of the objects referred
by the XDP program invocation is longer, all the way through to the call to
xdp_do_flush(), making the scope of the rcu_read_lock() too small. This
turns out to be harmless because it all happens in a single NAPI poll
cycle (and thus under local_bh_disable()), but it makes the rcu_read_lock()
misleading.

Rather than extend the scope of the rcu_read_lock(), just get rid of it
entirely. With the addition of RCU annotations to the XDP_REDIRECT map
types that take bh execution into account, lockdep even understands this to
be safe, so there's really no reason to keep it around.

Cc: Michael Chan <michael.chan@broadcom.com>
Signed-off-by: Toke Høiland-Jørgensen <toke@redhat.com>
---
 drivers/net/ethernet/broadcom/bnxt/bnxt_xdp.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/drivers/net/ethernet/broadcom/bnxt/bnxt_xdp.c b/drivers/net/ethernet/broadcom/bnxt/bnxt_xdp.c
index ec9564e584e0..f38e6ce9b40e 100644
--- a/drivers/net/ethernet/broadcom/bnxt/bnxt_xdp.c
+++ b/drivers/net/ethernet/broadcom/bnxt/bnxt_xdp.c
@@ -138,9 +138,10 @@ bool bnxt_rx_xdp(struct bnxt *bp, struct bnxt_rx_ring_info *rxr, u16 cons,
 	xdp_prepare_buff(&xdp, *data_ptr - offset, offset, *len, false);
 	orig_data = xdp.data;
 
-	rcu_read_lock();
+	/* This code is invoked within a single NAPI poll cycle and thus under
+	 * local_bh_disable(), which provides the needed RCU protection.
+	 */
 	act = bpf_prog_run_xdp(xdp_prog, &xdp);
-	rcu_read_unlock();
 
 	tx_avail = bnxt_tx_avail(bp, txr);
 	/* If the tx ring is not full, we must not update the rx producer yet
-- 
2.32.0

[PATCH bpf-next v3 06/16] thunderx: remove rcu_read_lock() around XDP program invocation

[Toke Høiland-Jørgensen]

The thunderx driver has rcu_read_lock()/rcu_read_unlock() pairs around XDP
program invocations. However, the actual lifetime of the objects referred
by the XDP program invocation is longer, all the way through to the call to
xdp_do_flush(), making the scope of the rcu_read_lock() too small. This
turns out to be harmless because it all happens in a single NAPI poll
cycle (and thus under local_bh_disable()), but it makes the rcu_read_lock()
misleading.

Rather than extend the scope of the rcu_read_lock(), just get rid of it
entirely. With the addition of RCU annotations to the XDP_REDIRECT map
types that take bh execution into account, lockdep even understands this to
be safe, so there's really no reason to keep it around.

Cc: Sunil Goutham <sgoutham@marvell.com>
Cc: linux-arm-kernel@lists.infradead.org
Signed-off-by: Toke Høiland-Jørgensen <toke@redhat.com>
---
 drivers/net/ethernet/cavium/thunder/nicvf_main.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/drivers/net/ethernet/cavium/thunder/nicvf_main.c b/drivers/net/ethernet/cavium/thunder/nicvf_main.c
index c33b4e837515..1d752815c69a 100644
--- a/drivers/net/ethernet/cavium/thunder/nicvf_main.c
+++ b/drivers/net/ethernet/cavium/thunder/nicvf_main.c
@@ -555,9 +555,10 @@ static inline bool nicvf_xdp_rx(struct nicvf *nic, struct bpf_prog *prog,
 	xdp_prepare_buff(&xdp, hard_start, data - hard_start, len, false);
 	orig_data = xdp.data;
 
-	rcu_read_lock();
+	/* This code is invoked within a single NAPI poll cycle and thus under
+	 * local_bh_disable(), which provides the needed RCU protection.
+	 */
 	action = bpf_prog_run_xdp(prog, &xdp);
-	rcu_read_unlock();
 
 	len = xdp.data_end - xdp.data;
 	/* Check if XDP program has changed headers */
-- 
2.32.0

[PATCH bpf-next v3 06/16] thunderx: remove rcu_read_lock() around XDP program invocation

[Toke Høiland-Jørgensen]

The thunderx driver has rcu_read_lock()/rcu_read_unlock() pairs around XDP
program invocations. However, the actual lifetime of the objects referred
by the XDP program invocation is longer, all the way through to the call to
xdp_do_flush(), making the scope of the rcu_read_lock() too small. This
turns out to be harmless because it all happens in a single NAPI poll
cycle (and thus under local_bh_disable()), but it makes the rcu_read_lock()
misleading.

Rather than extend the scope of the rcu_read_lock(), just get rid of it
entirely. With the addition of RCU annotations to the XDP_REDIRECT map
types that take bh execution into account, lockdep even understands this to
be safe, so there's really no reason to keep it around.

Cc: Sunil Goutham <sgoutham@marvell.com>
Cc: linux-arm-kernel@lists.infradead.org
Signed-off-by: Toke Høiland-Jørgensen <toke@redhat.com>
---
 drivers/net/ethernet/cavium/thunder/nicvf_main.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/drivers/net/ethernet/cavium/thunder/nicvf_main.c b/drivers/net/ethernet/cavium/thunder/nicvf_main.c
index c33b4e837515..1d752815c69a 100644
--- a/drivers/net/ethernet/cavium/thunder/nicvf_main.c
+++ b/drivers/net/ethernet/cavium/thunder/nicvf_main.c
@@ -555,9 +555,10 @@ static inline bool nicvf_xdp_rx(struct nicvf *nic, struct bpf_prog *prog,
 	xdp_prepare_buff(&xdp, hard_start, data - hard_start, len, false);
 	orig_data = xdp.data;
 
-	rcu_read_lock();
+	/* This code is invoked within a single NAPI poll cycle and thus under
+	 * local_bh_disable(), which provides the needed RCU protection.
+	 */
 	action = bpf_prog_run_xdp(prog, &xdp);
-	rcu_read_unlock();
 
 	len = xdp.data_end - xdp.data;
 	/* Check if XDP program has changed headers */
-- 
2.32.0


_______________________________________________
linux-arm-kernel mailing list
linux-arm-kernel@lists.infradead.org
http://lists.infradead.org/mailman/listinfo/linux-arm-kernel

[PATCH bpf-next v3 07/16] freescale: remove rcu_read_lock() around XDP program invocation

[Toke Høiland-Jørgensen]

The dpaa and dpaa2 drivers have rcu_read_lock()/rcu_read_unlock() pairs
around XDP program invocations. However, the actual lifetime of the objects
referred by the XDP program invocation is longer, all the way through to
the call to xdp_do_flush(), making the scope of the rcu_read_lock() too
small. This turns out to be harmless because it all happens in a single
NAPI poll cycle (and thus under local_bh_disable()), but it makes the
rcu_read_lock() misleading.

Rather than extend the scope of the rcu_read_lock(), just get rid of it
entirely. With the addition of RCU annotations to the XDP_REDIRECT map
types that take bh execution into account, lockdep even understands this to
be safe, so there's really no reason to keep it around.

Cc: Madalin Bucur <madalin.bucur@nxp.com>
Cc: Ioana Ciornei <ioana.ciornei@nxp.com>
Cc: Ioana Radulescu <ruxandra.radulescu@nxp.com>
Reviewed-by: Camelia Groza <camelia.groza@nxp.com>
Signed-off-by: Toke Høiland-Jørgensen <toke@redhat.com>
---
 drivers/net/ethernet/freescale/dpaa/dpaa_eth.c   | 11 ++++-------
 drivers/net/ethernet/freescale/dpaa2/dpaa2-eth.c |  6 +++---
 2 files changed, 7 insertions(+), 10 deletions(-)

diff --git a/drivers/net/ethernet/freescale/dpaa/dpaa_eth.c b/drivers/net/ethernet/freescale/dpaa/dpaa_eth.c
index 177c020bf34a..98fdcbde687a 100644
--- a/drivers/net/ethernet/freescale/dpaa/dpaa_eth.c
+++ b/drivers/net/ethernet/freescale/dpaa/dpaa_eth.c
@@ -2558,13 +2558,9 @@ static u32 dpaa_run_xdp(struct dpaa_priv *priv, struct qm_fd *fd, void *vaddr,
 	u32 xdp_act;
 	int err;
 
-	rcu_read_lock();
-
 	xdp_prog = READ_ONCE(priv->xdp_prog);
-	if (!xdp_prog) {
-		rcu_read_unlock();
+	if (!xdp_prog)
 		return XDP_PASS;
-	}
 
 	xdp_init_buff(&xdp, DPAA_BP_RAW_SIZE - DPAA_TX_PRIV_DATA_SIZE,
 		      &dpaa_fq->xdp_rxq);
@@ -2585,6 +2581,9 @@ static u32 dpaa_run_xdp(struct dpaa_priv *priv, struct qm_fd *fd, void *vaddr,
 	}
 #endif
 
+	/* This code is invoked within a single NAPI poll cycle and thus under
+	 * local_bh_disable(), which provides the needed RCU protection.
+	 */
 	xdp_act = bpf_prog_run_xdp(xdp_prog, &xdp);
 
 	/* Update the length and the offset of the FD */
@@ -2638,8 +2637,6 @@ static u32 dpaa_run_xdp(struct dpaa_priv *priv, struct qm_fd *fd, void *vaddr,
 		break;
 	}
 
-	rcu_read_unlock();
-
 	return xdp_act;
 }
 
diff --git a/drivers/net/ethernet/freescale/dpaa2/dpaa2-eth.c b/drivers/net/ethernet/freescale/dpaa2/dpaa2-eth.c
index 8433aa730c42..964d85c9e37d 100644
--- a/drivers/net/ethernet/freescale/dpaa2/dpaa2-eth.c
+++ b/drivers/net/ethernet/freescale/dpaa2/dpaa2-eth.c
@@ -352,8 +352,6 @@ static u32 dpaa2_eth_run_xdp(struct dpaa2_eth_priv *priv,
 	u32 xdp_act = XDP_PASS;
 	int err, offset;
 
-	rcu_read_lock();
-
 	xdp_prog = READ_ONCE(ch->xdp.prog);
 	if (!xdp_prog)
 		goto out;
@@ -363,6 +361,9 @@ static u32 dpaa2_eth_run_xdp(struct dpaa2_eth_priv *priv,
 	xdp_prepare_buff(&xdp, vaddr + offset, XDP_PACKET_HEADROOM,
 			 dpaa2_fd_get_len(fd), false);
 
+	/* This code is invoked within a single NAPI poll cycle and thus under
+	 * local_bh_disable(), which provides the needed RCU protection.
+	 */
 	xdp_act = bpf_prog_run_xdp(xdp_prog, &xdp);
 
 	/* xdp.data pointer may have changed */
@@ -414,7 +415,6 @@ static u32 dpaa2_eth_run_xdp(struct dpaa2_eth_priv *priv,
 
 	ch->xdp.res |= xdp_act;
 out:
-	rcu_read_unlock();
 	return xdp_act;
 }
 
-- 
2.32.0

[PATCH bpf-next v3 08/16] net: intel: remove rcu_read_lock() around XDP program invocation

[Toke Høiland-Jørgensen]

The Intel drivers all have rcu_read_lock()/rcu_read_unlock() pairs around
XDP program invocations. However, the actual lifetime of the objects
referred by the XDP program invocation is longer, all the way through to
the call to xdp_do_flush(), making the scope of the rcu_read_lock() too
small. This turns out to be harmless because it all happens in a single
NAPI poll cycle (and thus under local_bh_disable()), but it makes the
rcu_read_lock() misleading.

Rather than extend the scope of the rcu_read_lock(), just get rid of it
entirely. With the addition of RCU annotations to the XDP_REDIRECT map
types that take bh execution into account, lockdep even understands this to
be safe, so there's really no reason to keep it around.

Cc: Jesse Brandeburg <jesse.brandeburg@intel.com>
Cc: Tony Nguyen <anthony.l.nguyen@intel.com>
Cc: intel-wired-lan@lists.osuosl.org
Tested-by: Jesper Dangaard Brouer <brouer@redhat.com> # i40e
Signed-off-by: Toke Høiland-Jørgensen <toke@redhat.com>
---
 drivers/net/ethernet/intel/i40e/i40e_txrx.c       |  5 +++--
 drivers/net/ethernet/intel/i40e/i40e_xsk.c        | 11 +++++------
 drivers/net/ethernet/intel/ice/ice_txrx.c         |  6 +-----
 drivers/net/ethernet/intel/ice/ice_xsk.c          |  6 +-----
 drivers/net/ethernet/intel/igb/igb_main.c         |  5 +++--
 drivers/net/ethernet/intel/igc/igc_main.c         | 10 +++++-----
 drivers/net/ethernet/intel/ixgbe/ixgbe_main.c     |  5 +++--
 drivers/net/ethernet/intel/ixgbe/ixgbe_xsk.c      |  9 ++++-----
 drivers/net/ethernet/intel/ixgbevf/ixgbevf_main.c |  5 +++--
 9 files changed, 28 insertions(+), 34 deletions(-)

diff --git a/drivers/net/ethernet/intel/i40e/i40e_txrx.c b/drivers/net/ethernet/intel/i40e/i40e_txrx.c
index de70c16ef619..7fc5bdb5cd9e 100644
--- a/drivers/net/ethernet/intel/i40e/i40e_txrx.c
+++ b/drivers/net/ethernet/intel/i40e/i40e_txrx.c
@@ -2298,7 +2298,6 @@ static int i40e_run_xdp(struct i40e_ring *rx_ring, struct xdp_buff *xdp)
 	struct bpf_prog *xdp_prog;
 	u32 act;
 
-	rcu_read_lock();
 	xdp_prog = READ_ONCE(rx_ring->xdp_prog);
 
 	if (!xdp_prog)
@@ -2306,6 +2305,9 @@ static int i40e_run_xdp(struct i40e_ring *rx_ring, struct xdp_buff *xdp)
 
 	prefetchw(xdp->data_hard_start); /* xdp_frame write */
 
+	/* This code is invoked within a single NAPI poll cycle and thus under
+	 * local_bh_disable(), which provides the needed RCU protection.
+	 */
 	act = bpf_prog_run_xdp(xdp_prog, xdp);
 	switch (act) {
 	case XDP_PASS:
@@ -2329,7 +2331,6 @@ static int i40e_run_xdp(struct i40e_ring *rx_ring, struct xdp_buff *xdp)
 		break;
 	}
 xdp_out:
-	rcu_read_unlock();
 	return result;
 }
 
diff --git a/drivers/net/ethernet/intel/i40e/i40e_xsk.c b/drivers/net/ethernet/intel/i40e/i40e_xsk.c
index 46d884417c63..a5982cd112be 100644
--- a/drivers/net/ethernet/intel/i40e/i40e_xsk.c
+++ b/drivers/net/ethernet/intel/i40e/i40e_xsk.c
@@ -153,8 +153,10 @@ static int i40e_run_xdp_zc(struct i40e_ring *rx_ring, struct xdp_buff *xdp)
 	struct bpf_prog *xdp_prog;
 	u32 act;
 
-	rcu_read_lock();
-	/* NB! xdp_prog will always be !NULL, due to the fact that
+	/* This code is invoked within a single NAPI poll cycle and thus under
+	 * local_bh_disable(), which provides the needed RCU protection.
+	 *
+	 * NB! xdp_prog will always be !NULL, due to the fact that
 	 * this path is enabled by setting an XDP program.
 	 */
 	xdp_prog = READ_ONCE(rx_ring->xdp_prog);
@@ -162,9 +164,7 @@ static int i40e_run_xdp_zc(struct i40e_ring *rx_ring, struct xdp_buff *xdp)
 
 	if (likely(act == XDP_REDIRECT)) {
 		err = xdp_do_redirect(rx_ring->netdev, xdp, xdp_prog);
-		result = !err ? I40E_XDP_REDIR : I40E_XDP_CONSUMED;
-		rcu_read_unlock();
-		return result;
+		return !err ? I40E_XDP_REDIR : I40E_XDP_CONSUMED;
 	}
 
 	switch (act) {
@@ -184,7 +184,6 @@ static int i40e_run_xdp_zc(struct i40e_ring *rx_ring, struct xdp_buff *xdp)
 		result = I40E_XDP_CONSUMED;
 		break;
 	}
-	rcu_read_unlock();
 	return result;
 }
 
diff --git a/drivers/net/ethernet/intel/ice/ice_txrx.c b/drivers/net/ethernet/intel/ice/ice_txrx.c
index e2b4b29ea207..1a311e91fb6d 100644
--- a/drivers/net/ethernet/intel/ice/ice_txrx.c
+++ b/drivers/net/ethernet/intel/ice/ice_txrx.c
@@ -1129,15 +1129,11 @@ int ice_clean_rx_irq(struct ice_ring *rx_ring, int budget)
 		xdp.frame_sz = ice_rx_frame_truesize(rx_ring, size);
 #endif
 
-		rcu_read_lock();
 		xdp_prog = READ_ONCE(rx_ring->xdp_prog);
-		if (!xdp_prog) {
-			rcu_read_unlock();
+		if (!xdp_prog)
 			goto construct_skb;
-		}
 
 		xdp_res = ice_run_xdp(rx_ring, &xdp, xdp_prog);
-		rcu_read_unlock();
 		if (!xdp_res)
 			goto construct_skb;
 		if (xdp_res & (ICE_XDP_TX | ICE_XDP_REDIR)) {
diff --git a/drivers/net/ethernet/intel/ice/ice_xsk.c b/drivers/net/ethernet/intel/ice/ice_xsk.c
index faa7b8d96adb..d6da377f5ac3 100644
--- a/drivers/net/ethernet/intel/ice/ice_xsk.c
+++ b/drivers/net/ethernet/intel/ice/ice_xsk.c
@@ -463,7 +463,6 @@ ice_run_xdp_zc(struct ice_ring *rx_ring, struct xdp_buff *xdp)
 	struct ice_ring *xdp_ring;
 	u32 act;
 
-	rcu_read_lock();
 	/* ZC patch is enabled only when XDP program is set,
 	 * so here it can not be NULL
 	 */
@@ -473,9 +472,7 @@ ice_run_xdp_zc(struct ice_ring *rx_ring, struct xdp_buff *xdp)
 
 	if (likely(act == XDP_REDIRECT)) {
 		err = xdp_do_redirect(rx_ring->netdev, xdp, xdp_prog);
-		result = !err ? ICE_XDP_REDIR : ICE_XDP_CONSUMED;
-		rcu_read_unlock();
-		return result;
+		return !err ? ICE_XDP_REDIR : ICE_XDP_CONSUMED;
 	}
 
 	switch (act) {
@@ -496,7 +493,6 @@ ice_run_xdp_zc(struct ice_ring *rx_ring, struct xdp_buff *xdp)
 		break;
 	}
 
-	rcu_read_unlock();
 	return result;
 }
 
diff --git a/drivers/net/ethernet/intel/igb/igb_main.c b/drivers/net/ethernet/intel/igb/igb_main.c
index 038a9fd1af44..0b68d589218a 100644
--- a/drivers/net/ethernet/intel/igb/igb_main.c
+++ b/drivers/net/ethernet/intel/igb/igb_main.c
@@ -8387,7 +8387,6 @@ static struct sk_buff *igb_run_xdp(struct igb_adapter *adapter,
 	struct bpf_prog *xdp_prog;
 	u32 act;
 
-	rcu_read_lock();
 	xdp_prog = READ_ONCE(rx_ring->xdp_prog);
 
 	if (!xdp_prog)
@@ -8395,6 +8394,9 @@ static struct sk_buff *igb_run_xdp(struct igb_adapter *adapter,
 
 	prefetchw(xdp->data_hard_start); /* xdp_frame write */
 
+	/* This code is invoked within a single NAPI poll cycle and thus under
+	 * local_bh_disable(), which provides the needed RCU protection.
+	 */
 	act = bpf_prog_run_xdp(xdp_prog, xdp);
 	switch (act) {
 	case XDP_PASS:
@@ -8420,7 +8422,6 @@ static struct sk_buff *igb_run_xdp(struct igb_adapter *adapter,
 		break;
 	}
 xdp_out:
-	rcu_read_unlock();
 	return ERR_PTR(-result);
 }
 
diff --git a/drivers/net/ethernet/intel/igc/igc_main.c b/drivers/net/ethernet/intel/igc/igc_main.c
index ea998d2defa4..333057ce60c7 100644
--- a/drivers/net/ethernet/intel/igc/igc_main.c
+++ b/drivers/net/ethernet/intel/igc/igc_main.c
@@ -2175,18 +2175,18 @@ static struct sk_buff *igc_xdp_run_prog(struct igc_adapter *adapter,
 	struct bpf_prog *prog;
 	int res;
 
-	rcu_read_lock();
-
 	prog = READ_ONCE(adapter->xdp_prog);
 	if (!prog) {
 		res = IGC_XDP_PASS;
-		goto unlock;
+		goto out;
 	}
 
+	/* This code is invoked within a single NAPI poll cycle and thus under
+	 * local_bh_disable(), which provides the needed RCU protection.
+	 */
 	res = __igc_xdp_run_prog(adapter, prog, xdp);
 
-unlock:
-	rcu_read_unlock();
+out:
 	return ERR_PTR(-res);
 }
 
diff --git a/drivers/net/ethernet/intel/ixgbe/ixgbe_main.c b/drivers/net/ethernet/intel/ixgbe/ixgbe_main.c
index c5ec17d19c59..9cebe7894111 100644
--- a/drivers/net/ethernet/intel/ixgbe/ixgbe_main.c
+++ b/drivers/net/ethernet/intel/ixgbe/ixgbe_main.c
@@ -2199,7 +2199,6 @@ static struct sk_buff *ixgbe_run_xdp(struct ixgbe_adapter *adapter,
 	struct xdp_frame *xdpf;
 	u32 act;
 
-	rcu_read_lock();
 	xdp_prog = READ_ONCE(rx_ring->xdp_prog);
 
 	if (!xdp_prog)
@@ -2207,6 +2206,9 @@ static struct sk_buff *ixgbe_run_xdp(struct ixgbe_adapter *adapter,
 
 	prefetchw(xdp->data_hard_start); /* xdp_frame write */
 
+	/* This code is invoked within a single NAPI poll cycle and thus under
+	 * local_bh_disable(), which provides the needed RCU protection.
+	 */
 	act = bpf_prog_run_xdp(xdp_prog, xdp);
 	switch (act) {
 	case XDP_PASS:
@@ -2237,7 +2239,6 @@ static struct sk_buff *ixgbe_run_xdp(struct ixgbe_adapter *adapter,
 		break;
 	}
 xdp_out:
-	rcu_read_unlock();
 	return ERR_PTR(-result);
 }
 
diff --git a/drivers/net/ethernet/intel/ixgbe/ixgbe_xsk.c b/drivers/net/ethernet/intel/ixgbe/ixgbe_xsk.c
index 91ad5b902673..4a075e667082 100644
--- a/drivers/net/ethernet/intel/ixgbe/ixgbe_xsk.c
+++ b/drivers/net/ethernet/intel/ixgbe/ixgbe_xsk.c
@@ -100,15 +100,15 @@ static int ixgbe_run_xdp_zc(struct ixgbe_adapter *adapter,
 	struct xdp_frame *xdpf;
 	u32 act;
 
-	rcu_read_lock();
+	/* This code is invoked within a single NAPI poll cycle and thus under
+	 * local_bh_disable(), which provides the needed RCU protection.
+	 */
 	xdp_prog = READ_ONCE(rx_ring->xdp_prog);
 	act = bpf_prog_run_xdp(xdp_prog, xdp);
 
 	if (likely(act == XDP_REDIRECT)) {
 		err = xdp_do_redirect(rx_ring->netdev, xdp, xdp_prog);
-		result = !err ? IXGBE_XDP_REDIR : IXGBE_XDP_CONSUMED;
-		rcu_read_unlock();
-		return result;
+		return !err ? IXGBE_XDP_REDIR : IXGBE_XDP_CONSUMED;
 	}
 
 	switch (act) {
@@ -132,7 +132,6 @@ static int ixgbe_run_xdp_zc(struct ixgbe_adapter *adapter,
 		result = IXGBE_XDP_CONSUMED;
 		break;
 	}
-	rcu_read_unlock();
 	return result;
 }
 
diff --git a/drivers/net/ethernet/intel/ixgbevf/ixgbevf_main.c b/drivers/net/ethernet/intel/ixgbevf/ixgbevf_main.c
index ba2ed8a43d2d..ab05ebc3d350 100644
--- a/drivers/net/ethernet/intel/ixgbevf/ixgbevf_main.c
+++ b/drivers/net/ethernet/intel/ixgbevf/ixgbevf_main.c
@@ -1054,12 +1054,14 @@ static struct sk_buff *ixgbevf_run_xdp(struct ixgbevf_adapter *adapter,
 	struct bpf_prog *xdp_prog;
 	u32 act;
 
-	rcu_read_lock();
 	xdp_prog = READ_ONCE(rx_ring->xdp_prog);
 
 	if (!xdp_prog)
 		goto xdp_out;
 
+	/* This code is invoked within a single NAPI poll cycle and thus under
+	 * local_bh_disable(), which provides the needed RCU protection.
+	 */
 	act = bpf_prog_run_xdp(xdp_prog, xdp);
 	switch (act) {
 	case XDP_PASS:
@@ -1079,7 +1081,6 @@ static struct sk_buff *ixgbevf_run_xdp(struct ixgbevf_adapter *adapter,
 		break;
 	}
 xdp_out:
-	rcu_read_unlock();
 	return ERR_PTR(-result);
 }
 
-- 
2.32.0

[Intel-wired-lan] [PATCH bpf-next v3 08/16] net: intel: remove rcu_read_lock() around XDP program invocation

[Toke =?unknown-8bit?q?H=C3=B8iland-J=C3=B8rgensen?=]

The Intel drivers all have rcu_read_lock()/rcu_read_unlock() pairs around
XDP program invocations. However, the actual lifetime of the objects
referred by the XDP program invocation is longer, all the way through to
the call to xdp_do_flush(), making the scope of the rcu_read_lock() too
small. This turns out to be harmless because it all happens in a single
NAPI poll cycle (and thus under local_bh_disable()), but it makes the
rcu_read_lock() misleading.

Rather than extend the scope of the rcu_read_lock(), just get rid of it
entirely. With the addition of RCU annotations to the XDP_REDIRECT map
types that take bh execution into account, lockdep even understands this to
be safe, so there's really no reason to keep it around.

Cc: Jesse Brandeburg <jesse.brandeburg@intel.com>
Cc: Tony Nguyen <anthony.l.nguyen@intel.com>
Cc: intel-wired-lan at lists.osuosl.org
Tested-by: Jesper Dangaard Brouer <brouer@redhat.com> # i40e
Signed-off-by: Toke H?iland-J?rgensen <toke@redhat.com>
---
 drivers/net/ethernet/intel/i40e/i40e_txrx.c       |  5 +++--
 drivers/net/ethernet/intel/i40e/i40e_xsk.c        | 11 +++++------
 drivers/net/ethernet/intel/ice/ice_txrx.c         |  6 +-----
 drivers/net/ethernet/intel/ice/ice_xsk.c          |  6 +-----
 drivers/net/ethernet/intel/igb/igb_main.c         |  5 +++--
 drivers/net/ethernet/intel/igc/igc_main.c         | 10 +++++-----
 drivers/net/ethernet/intel/ixgbe/ixgbe_main.c     |  5 +++--
 drivers/net/ethernet/intel/ixgbe/ixgbe_xsk.c      |  9 ++++-----
 drivers/net/ethernet/intel/ixgbevf/ixgbevf_main.c |  5 +++--
 9 files changed, 28 insertions(+), 34 deletions(-)

diff --git a/drivers/net/ethernet/intel/i40e/i40e_txrx.c b/drivers/net/ethernet/intel/i40e/i40e_txrx.c
index de70c16ef619..7fc5bdb5cd9e 100644
--- a/drivers/net/ethernet/intel/i40e/i40e_txrx.c
+++ b/drivers/net/ethernet/intel/i40e/i40e_txrx.c
@@ -2298,7 +2298,6 @@ static int i40e_run_xdp(struct i40e_ring *rx_ring, struct xdp_buff *xdp)
 	struct bpf_prog *xdp_prog;
 	u32 act;
 
-	rcu_read_lock();
 	xdp_prog = READ_ONCE(rx_ring->xdp_prog);
 
 	if (!xdp_prog)
@@ -2306,6 +2305,9 @@ static int i40e_run_xdp(struct i40e_ring *rx_ring, struct xdp_buff *xdp)
 
 	prefetchw(xdp->data_hard_start); /* xdp_frame write */
 
+	/* This code is invoked within a single NAPI poll cycle and thus under
+	 * local_bh_disable(), which provides the needed RCU protection.
+	 */
 	act = bpf_prog_run_xdp(xdp_prog, xdp);
 	switch (act) {
 	case XDP_PASS:
@@ -2329,7 +2331,6 @@ static int i40e_run_xdp(struct i40e_ring *rx_ring, struct xdp_buff *xdp)
 		break;
 	}
 xdp_out:
-	rcu_read_unlock();
 	return result;
 }
 
diff --git a/drivers/net/ethernet/intel/i40e/i40e_xsk.c b/drivers/net/ethernet/intel/i40e/i40e_xsk.c
index 46d884417c63..a5982cd112be 100644
--- a/drivers/net/ethernet/intel/i40e/i40e_xsk.c
+++ b/drivers/net/ethernet/intel/i40e/i40e_xsk.c
@@ -153,8 +153,10 @@ static int i40e_run_xdp_zc(struct i40e_ring *rx_ring, struct xdp_buff *xdp)
 	struct bpf_prog *xdp_prog;
 	u32 act;
 
-	rcu_read_lock();
-	/* NB! xdp_prog will always be !NULL, due to the fact that
+	/* This code is invoked within a single NAPI poll cycle and thus under
+	 * local_bh_disable(), which provides the needed RCU protection.
+	 *
+	 * NB! xdp_prog will always be !NULL, due to the fact that
 	 * this path is enabled by setting an XDP program.
 	 */
 	xdp_prog = READ_ONCE(rx_ring->xdp_prog);
@@ -162,9 +164,7 @@ static int i40e_run_xdp_zc(struct i40e_ring *rx_ring, struct xdp_buff *xdp)
 
 	if (likely(act == XDP_REDIRECT)) {
 		err = xdp_do_redirect(rx_ring->netdev, xdp, xdp_prog);
-		result = !err ? I40E_XDP_REDIR : I40E_XDP_CONSUMED;
-		rcu_read_unlock();
-		return result;
+		return !err ? I40E_XDP_REDIR : I40E_XDP_CONSUMED;
 	}
 
 	switch (act) {
@@ -184,7 +184,6 @@ static int i40e_run_xdp_zc(struct i40e_ring *rx_ring, struct xdp_buff *xdp)
 		result = I40E_XDP_CONSUMED;
 		break;
 	}
-	rcu_read_unlock();
 	return result;
 }
 
diff --git a/drivers/net/ethernet/intel/ice/ice_txrx.c b/drivers/net/ethernet/intel/ice/ice_txrx.c
index e2b4b29ea207..1a311e91fb6d 100644
--- a/drivers/net/ethernet/intel/ice/ice_txrx.c
+++ b/drivers/net/ethernet/intel/ice/ice_txrx.c
@@ -1129,15 +1129,11 @@ int ice_clean_rx_irq(struct ice_ring *rx_ring, int budget)
 		xdp.frame_sz = ice_rx_frame_truesize(rx_ring, size);
 #endif
 
-		rcu_read_lock();
 		xdp_prog = READ_ONCE(rx_ring->xdp_prog);
-		if (!xdp_prog) {
-			rcu_read_unlock();
+		if (!xdp_prog)
 			goto construct_skb;
-		}
 
 		xdp_res = ice_run_xdp(rx_ring, &xdp, xdp_prog);
-		rcu_read_unlock();
 		if (!xdp_res)
 			goto construct_skb;
 		if (xdp_res & (ICE_XDP_TX | ICE_XDP_REDIR)) {
diff --git a/drivers/net/ethernet/intel/ice/ice_xsk.c b/drivers/net/ethernet/intel/ice/ice_xsk.c
index faa7b8d96adb..d6da377f5ac3 100644
--- a/drivers/net/ethernet/intel/ice/ice_xsk.c
+++ b/drivers/net/ethernet/intel/ice/ice_xsk.c
@@ -463,7 +463,6 @@ ice_run_xdp_zc(struct ice_ring *rx_ring, struct xdp_buff *xdp)
 	struct ice_ring *xdp_ring;
 	u32 act;
 
-	rcu_read_lock();
 	/* ZC patch is enabled only when XDP program is set,
 	 * so here it can not be NULL
 	 */
@@ -473,9 +472,7 @@ ice_run_xdp_zc(struct ice_ring *rx_ring, struct xdp_buff *xdp)
 
 	if (likely(act == XDP_REDIRECT)) {
 		err = xdp_do_redirect(rx_ring->netdev, xdp, xdp_prog);
-		result = !err ? ICE_XDP_REDIR : ICE_XDP_CONSUMED;
-		rcu_read_unlock();
-		return result;
+		return !err ? ICE_XDP_REDIR : ICE_XDP_CONSUMED;
 	}
 
 	switch (act) {
@@ -496,7 +493,6 @@ ice_run_xdp_zc(struct ice_ring *rx_ring, struct xdp_buff *xdp)
 		break;
 	}
 
-	rcu_read_unlock();
 	return result;
 }
 
diff --git a/drivers/net/ethernet/intel/igb/igb_main.c b/drivers/net/ethernet/intel/igb/igb_main.c
index 038a9fd1af44..0b68d589218a 100644
--- a/drivers/net/ethernet/intel/igb/igb_main.c
+++ b/drivers/net/ethernet/intel/igb/igb_main.c
@@ -8387,7 +8387,6 @@ static struct sk_buff *igb_run_xdp(struct igb_adapter *adapter,
 	struct bpf_prog *xdp_prog;
 	u32 act;
 
-	rcu_read_lock();
 	xdp_prog = READ_ONCE(rx_ring->xdp_prog);
 
 	if (!xdp_prog)
@@ -8395,6 +8394,9 @@ static struct sk_buff *igb_run_xdp(struct igb_adapter *adapter,
 
 	prefetchw(xdp->data_hard_start); /* xdp_frame write */
 
+	/* This code is invoked within a single NAPI poll cycle and thus under
+	 * local_bh_disable(), which provides the needed RCU protection.
+	 */
 	act = bpf_prog_run_xdp(xdp_prog, xdp);
 	switch (act) {
 	case XDP_PASS:
@@ -8420,7 +8422,6 @@ static struct sk_buff *igb_run_xdp(struct igb_adapter *adapter,
 		break;
 	}
 xdp_out:
-	rcu_read_unlock();
 	return ERR_PTR(-result);
 }
 
diff --git a/drivers/net/ethernet/intel/igc/igc_main.c b/drivers/net/ethernet/intel/igc/igc_main.c
index ea998d2defa4..333057ce60c7 100644
--- a/drivers/net/ethernet/intel/igc/igc_main.c
+++ b/drivers/net/ethernet/intel/igc/igc_main.c
@@ -2175,18 +2175,18 @@ static struct sk_buff *igc_xdp_run_prog(struct igc_adapter *adapter,
 	struct bpf_prog *prog;
 	int res;
 
-	rcu_read_lock();
-
 	prog = READ_ONCE(adapter->xdp_prog);
 	if (!prog) {
 		res = IGC_XDP_PASS;
-		goto unlock;
+		goto out;
 	}
 
+	/* This code is invoked within a single NAPI poll cycle and thus under
+	 * local_bh_disable(), which provides the needed RCU protection.
+	 */
 	res = __igc_xdp_run_prog(adapter, prog, xdp);
 
-unlock:
-	rcu_read_unlock();
+out:
 	return ERR_PTR(-res);
 }
 
diff --git a/drivers/net/ethernet/intel/ixgbe/ixgbe_main.c b/drivers/net/ethernet/intel/ixgbe/ixgbe_main.c
index c5ec17d19c59..9cebe7894111 100644
--- a/drivers/net/ethernet/intel/ixgbe/ixgbe_main.c
+++ b/drivers/net/ethernet/intel/ixgbe/ixgbe_main.c
@@ -2199,7 +2199,6 @@ static struct sk_buff *ixgbe_run_xdp(struct ixgbe_adapter *adapter,
 	struct xdp_frame *xdpf;
 	u32 act;
 
-	rcu_read_lock();
 	xdp_prog = READ_ONCE(rx_ring->xdp_prog);
 
 	if (!xdp_prog)
@@ -2207,6 +2206,9 @@ static struct sk_buff *ixgbe_run_xdp(struct ixgbe_adapter *adapter,
 
 	prefetchw(xdp->data_hard_start); /* xdp_frame write */
 
+	/* This code is invoked within a single NAPI poll cycle and thus under
+	 * local_bh_disable(), which provides the needed RCU protection.
+	 */
 	act = bpf_prog_run_xdp(xdp_prog, xdp);
 	switch (act) {
 	case XDP_PASS:
@@ -2237,7 +2239,6 @@ static struct sk_buff *ixgbe_run_xdp(struct ixgbe_adapter *adapter,
 		break;
 	}
 xdp_out:
-	rcu_read_unlock();
 	return ERR_PTR(-result);
 }
 
diff --git a/drivers/net/ethernet/intel/ixgbe/ixgbe_xsk.c b/drivers/net/ethernet/intel/ixgbe/ixgbe_xsk.c
index 91ad5b902673..4a075e667082 100644
--- a/drivers/net/ethernet/intel/ixgbe/ixgbe_xsk.c
+++ b/drivers/net/ethernet/intel/ixgbe/ixgbe_xsk.c
@@ -100,15 +100,15 @@ static int ixgbe_run_xdp_zc(struct ixgbe_adapter *adapter,
 	struct xdp_frame *xdpf;
 	u32 act;
 
-	rcu_read_lock();
+	/* This code is invoked within a single NAPI poll cycle and thus under
+	 * local_bh_disable(), which provides the needed RCU protection.
+	 */
 	xdp_prog = READ_ONCE(rx_ring->xdp_prog);
 	act = bpf_prog_run_xdp(xdp_prog, xdp);
 
 	if (likely(act == XDP_REDIRECT)) {
 		err = xdp_do_redirect(rx_ring->netdev, xdp, xdp_prog);
-		result = !err ? IXGBE_XDP_REDIR : IXGBE_XDP_CONSUMED;
-		rcu_read_unlock();
-		return result;
+		return !err ? IXGBE_XDP_REDIR : IXGBE_XDP_CONSUMED;
 	}
 
 	switch (act) {
@@ -132,7 +132,6 @@ static int ixgbe_run_xdp_zc(struct ixgbe_adapter *adapter,
 		result = IXGBE_XDP_CONSUMED;
 		break;
 	}
-	rcu_read_unlock();
 	return result;
 }
 
diff --git a/drivers/net/ethernet/intel/ixgbevf/ixgbevf_main.c b/drivers/net/ethernet/intel/ixgbevf/ixgbevf_main.c
index ba2ed8a43d2d..ab05ebc3d350 100644
--- a/drivers/net/ethernet/intel/ixgbevf/ixgbevf_main.c
+++ b/drivers/net/ethernet/intel/ixgbevf/ixgbevf_main.c
@@ -1054,12 +1054,14 @@ static struct sk_buff *ixgbevf_run_xdp(struct ixgbevf_adapter *adapter,
 	struct bpf_prog *xdp_prog;
 	u32 act;
 
-	rcu_read_lock();
 	xdp_prog = READ_ONCE(rx_ring->xdp_prog);
 
 	if (!xdp_prog)
 		goto xdp_out;
 
+	/* This code is invoked within a single NAPI poll cycle and thus under
+	 * local_bh_disable(), which provides the needed RCU protection.
+	 */
 	act = bpf_prog_run_xdp(xdp_prog, xdp);
 	switch (act) {
 	case XDP_PASS:
@@ -1079,7 +1081,6 @@ static struct sk_buff *ixgbevf_run_xdp(struct ixgbevf_adapter *adapter,
 		break;
 	}
 xdp_out:
-	rcu_read_unlock();
 	return ERR_PTR(-result);
 }
 
-- 
2.32.0

[PATCH bpf-next v3 09/16] marvell: remove rcu_read_lock() around XDP program invocation

[Toke Høiland-Jørgensen]

The mvneta and mvpp2 drivers have rcu_read_lock()/rcu_read_unlock() pairs
around XDP program invocations. However, the actual lifetime of the objects
referred by the XDP program invocation is longer, all the way through to
the call to xdp_do_flush(), making the scope of the rcu_read_lock() too
small. This turns out to be harmless because it all happens in a single
NAPI poll cycle (and thus under local_bh_disable()), but it makes the
rcu_read_lock() misleading.

Rather than extend the scope of the rcu_read_lock(), just get rid of it
entirely. With the addition of RCU annotations to the XDP_REDIRECT map
types that take bh execution into account, lockdep even understands this to
be safe, so there's really no reason to keep it around.

Cc: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Cc: Marcin Wojtas <mw@semihalf.com>
Cc: Russell King <linux@armlinux.org.uk>
Signed-off-by: Toke Høiland-Jørgensen <toke@redhat.com>
---
 drivers/net/ethernet/marvell/mvneta.c           | 6 ++++--
 drivers/net/ethernet/marvell/mvpp2/mvpp2_main.c | 8 ++++----
 2 files changed, 8 insertions(+), 6 deletions(-)

diff --git a/drivers/net/ethernet/marvell/mvneta.c b/drivers/net/ethernet/marvell/mvneta.c
index 7d5cd9bc6c99..c2e9cbebc8d1 100644
--- a/drivers/net/ethernet/marvell/mvneta.c
+++ b/drivers/net/ethernet/marvell/mvneta.c
@@ -2370,7 +2370,6 @@ static int mvneta_rx_swbm(struct napi_struct *napi,
 	/* Get number of received packets */
 	rx_todo = mvneta_rxq_busy_desc_num_get(pp, rxq);
 
-	rcu_read_lock();
 	xdp_prog = READ_ONCE(pp->xdp_prog);
 
 	/* Fairness NAPI loop */
@@ -2421,6 +2420,10 @@ static int mvneta_rx_swbm(struct napi_struct *napi,
 			goto next;
 		}
 
+		/* This code is invoked within a single NAPI poll cycle and thus
+		 * under local_bh_disable(), which provides the needed RCU
+		 * protection.
+		 */
 		if (xdp_prog &&
 		    mvneta_run_xdp(pp, rxq, xdp_prog, &xdp_buf, frame_sz, &ps))
 			goto next;
@@ -2448,7 +2451,6 @@ static int mvneta_rx_swbm(struct napi_struct *napi,
 		xdp_buf.data_hard_start = NULL;
 		sinfo.nr_frags = 0;
 	}
-	rcu_read_unlock();
 
 	if (xdp_buf.data_hard_start)
 		mvneta_xdp_put_buff(pp, rxq, &xdp_buf, &sinfo, -1);
diff --git a/drivers/net/ethernet/marvell/mvpp2/mvpp2_main.c b/drivers/net/ethernet/marvell/mvpp2/mvpp2_main.c
index b2259bf1d299..658db1720826 100644
--- a/drivers/net/ethernet/marvell/mvpp2/mvpp2_main.c
+++ b/drivers/net/ethernet/marvell/mvpp2/mvpp2_main.c
@@ -3852,8 +3852,6 @@ static int mvpp2_rx(struct mvpp2_port *port, struct napi_struct *napi,
 	int rx_done = 0;
 	u32 xdp_ret = 0;
 
-	rcu_read_lock();
-
 	xdp_prog = READ_ONCE(port->xdp_prog);
 
 	/* Get number of received packets and clamp the to-do */
@@ -3925,6 +3923,10 @@ static int mvpp2_rx(struct mvpp2_port *port, struct napi_struct *napi,
 					 MVPP2_MH_SIZE + MVPP2_SKB_HEADROOM,
 					 rx_bytes, false);
 
+			/* This code is invoked within a single NAPI poll cycle
+			 * and thus under local_bh_disable(), which provides the
+			 * needed RCU protection.
+			 */
 			ret = mvpp2_run_xdp(port, xdp_prog, &xdp, pp, &ps);
 
 			if (ret) {
@@ -3988,8 +3990,6 @@ static int mvpp2_rx(struct mvpp2_port *port, struct napi_struct *napi,
 		mvpp2_bm_pool_put(port, pool, dma_addr, phys_addr);
 	}
 
-	rcu_read_unlock();
-
 	if (xdp_ret & MVPP2_XDP_REDIR)
 		xdp_do_flush_map();
 
-- 
2.32.0

[PATCH bpf-next v3 10/16] mlx4: remove rcu_read_lock() around XDP program invocation

[Toke Høiland-Jørgensen]

The mlx4 driver has rcu_read_lock()/rcu_read_unlock() pairs around XDP
program invocations. However, the actual lifetime of the objects referred
by the XDP program invocation is longer, all the way through to the call to
xdp_do_flush(), making the scope of the rcu_read_lock() too small. This
turns out to be harmless because it all happens in a single NAPI poll
cycle (and thus under local_bh_disable()), but it makes the rcu_read_lock()
misleading.

Rather than extend the scope of the rcu_read_lock(), just get rid of it
entirely. With the addition of RCU annotations to the XDP_REDIRECT map
types that take bh execution into account, lockdep even understands this to
be safe, so there's really no reason to keep it around. Also switch the RCU
dereferences in the driver loop itself to the _bh variants.

Cc: Tariq Toukan <tariqt@nvidia.com>
Reviewed-by: Tariq Toukan <tariqt@nvidia.com>
Signed-off-by: Toke Høiland-Jørgensen <toke@redhat.com>
---
 drivers/net/ethernet/mellanox/mlx4/en_rx.c | 8 ++------
 1 file changed, 2 insertions(+), 6 deletions(-)

diff --git a/drivers/net/ethernet/mellanox/mlx4/en_rx.c b/drivers/net/ethernet/mellanox/mlx4/en_rx.c
index e35e4d7ef4d1..3f08c14d0441 100644
--- a/drivers/net/ethernet/mellanox/mlx4/en_rx.c
+++ b/drivers/net/ethernet/mellanox/mlx4/en_rx.c
@@ -679,9 +679,7 @@ int mlx4_en_process_rx_cq(struct net_device *dev, struct mlx4_en_cq *cq, int bud
 
 	ring = priv->rx_ring[cq_ring];
 
-	/* Protect accesses to: ring->xdp_prog, priv->mac_hash list */
-	rcu_read_lock();
-	xdp_prog = rcu_dereference(ring->xdp_prog);
+	xdp_prog = rcu_dereference_bh(ring->xdp_prog);
 	xdp_init_buff(&xdp, priv->frag_info[0].frag_stride, &ring->xdp_rxq);
 	doorbell_pending = false;
 
@@ -744,7 +742,7 @@ int mlx4_en_process_rx_cq(struct net_device *dev, struct mlx4_en_cq *cq, int bud
 				/* Drop the packet, since HW loopback-ed it */
 				mac_hash = ethh->h_source[MLX4_EN_MAC_HASH_IDX];
 				bucket = &priv->mac_hash[mac_hash];
-				hlist_for_each_entry_rcu(entry, bucket, hlist) {
+				hlist_for_each_entry_rcu_bh(entry, bucket, hlist) {
 					if (ether_addr_equal_64bits(entry->mac,
 								    ethh->h_source))
 						goto next;
@@ -899,8 +897,6 @@ int mlx4_en_process_rx_cq(struct net_device *dev, struct mlx4_en_cq *cq, int bud
 			break;
 	}
 
-	rcu_read_unlock();
-
 	if (likely(polled)) {
 		if (doorbell_pending) {
 			priv->tx_cq[TX_XDP][cq_ring]->xdp_busy = true;
-- 
2.32.0

[PATCH bpf-next v3 11/16] nfp: remove rcu_read_lock() around XDP program invocation

[Toke Høiland-Jørgensen]

The nfp driver has rcu_read_lock()/rcu_read_unlock() pairs around XDP
program invocations. However, the actual lifetime of the objects referred
by the XDP program invocation is longer, all the way through to the call to
xdp_do_flush(), making the scope of the rcu_read_lock() too small.

While this is not actually an issue for the nfp driver because it doesn't
support XDP_REDIRECT (and thus doesn't call xdp_do_flush()), the
rcu_read_lock() is still unneeded. And With the addition of RCU annotations
to the XDP_REDIRECT map types that take bh execution into account, lockdep
even understands this to be safe, so there's really no reason to keep it
around.

Cc: Simon Horman <simon.horman@netronome.com>
Cc: oss-drivers@netronome.com
Reviewed-by: Simon Horman <simon.horman@netronome.com>
Signed-off-by: Toke Høiland-Jørgensen <toke@redhat.com>
---
 drivers/net/ethernet/netronome/nfp/nfp_net_common.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/drivers/net/ethernet/netronome/nfp/nfp_net_common.c b/drivers/net/ethernet/netronome/nfp/nfp_net_common.c
index eeb30680b4dc..a3d59abed6ae 100644
--- a/drivers/net/ethernet/netronome/nfp/nfp_net_common.c
+++ b/drivers/net/ethernet/netronome/nfp/nfp_net_common.c
@@ -1819,7 +1819,6 @@ static int nfp_net_rx(struct nfp_net_rx_ring *rx_ring, int budget)
 	struct xdp_buff xdp;
 	int idx;
 
-	rcu_read_lock();
 	xdp_prog = READ_ONCE(dp->xdp_prog);
 	true_bufsz = xdp_prog ? PAGE_SIZE : dp->fl_bufsz;
 	xdp_init_buff(&xdp, PAGE_SIZE - NFP_NET_RX_BUF_HEADROOM,
@@ -1919,6 +1918,10 @@ static int nfp_net_rx(struct nfp_net_rx_ring *rx_ring, int budget)
 					 pkt_off - NFP_NET_RX_BUF_HEADROOM,
 					 pkt_len, true);
 
+			/* This code is invoked within a single NAPI poll cycle
+			 * and thus under local_bh_disable(), which provides the
+			 * needed RCU protection.
+			 */
 			act = bpf_prog_run_xdp(xdp_prog, &xdp);
 
 			pkt_len = xdp.data_end - xdp.data;
@@ -2036,7 +2039,6 @@ static int nfp_net_rx(struct nfp_net_rx_ring *rx_ring, int budget)
 			if (!nfp_net_xdp_complete(tx_ring))
 				pkts_polled = budget;
 	}
-	rcu_read_unlock();
 
 	return pkts_polled;
 }
-- 
2.32.0

[PATCH bpf-next v3 12/16] qede: remove rcu_read_lock() around XDP program invocation

[Toke Høiland-Jørgensen]

The qede driver has rcu_read_lock()/rcu_read_unlock() pairs around XDP
program invocations. However, the actual lifetime of the objects referred
by the XDP program invocation is longer, all the way through to the call to
xdp_do_flush(), making the scope of the rcu_read_lock() too small. This
turns out to be harmless because it all happens in a single NAPI poll
cycle (and thus under local_bh_disable()), but it makes the rcu_read_lock()
misleading.

Rather than extend the scope of the rcu_read_lock(), just get rid of it
entirely. With the addition of RCU annotations to the XDP_REDIRECT map
types that take bh execution into account, lockdep even understands this to
be safe, so there's really no reason to keep it around.

Cc: Ariel Elior <aelior@marvell.com>
Cc: GR-everest-linux-l2@marvell.com
Signed-off-by: Toke Høiland-Jørgensen <toke@redhat.com>
---
 drivers/net/ethernet/qlogic/qede/qede_fp.c | 7 ++-----
 1 file changed, 2 insertions(+), 5 deletions(-)

diff --git a/drivers/net/ethernet/qlogic/qede/qede_fp.c b/drivers/net/ethernet/qlogic/qede/qede_fp.c
index 8e150dd4f899..d806ab925cee 100644
--- a/drivers/net/ethernet/qlogic/qede/qede_fp.c
+++ b/drivers/net/ethernet/qlogic/qede/qede_fp.c
@@ -1089,13 +1089,10 @@ static bool qede_rx_xdp(struct qede_dev *edev,
 	xdp_prepare_buff(&xdp, page_address(bd->data), *data_offset,
 			 *len, false);
 
-	/* Queues always have a full reset currently, so for the time
-	 * being until there's atomic program replace just mark read
-	 * side for map helpers.
+	/* This code is invoked within a single NAPI poll cycle and thus under
+	 * local_bh_disable(), which provides the needed RCU protection.
 	 */
-	rcu_read_lock();
 	act = bpf_prog_run_xdp(prog, &xdp);
-	rcu_read_unlock();
 
 	/* Recalculate, as XDP might have changed the headers */
 	*data_offset = xdp.data - xdp.data_hard_start;
-- 
2.32.0

[PATCH bpf-next v3 13/16] sfc: remove rcu_read_lock() around XDP program invocation

[Toke Høiland-Jørgensen]

The sfc driver has rcu_read_lock()/rcu_read_unlock() pairs around XDP
program invocations. However, the actual lifetime of the objects referred
by the XDP program invocation is longer, all the way through to the call to
xdp_do_flush(), making the scope of the rcu_read_lock() too small. This
turns out to be harmless because it all happens in a single NAPI poll
cycle (and thus under local_bh_disable()), but it makes the rcu_read_lock()
misleading.

Rather than extend the scope of the rcu_read_lock(), just get rid of it
entirely. With the addition of RCU annotations to the XDP_REDIRECT map
types that take bh execution into account, lockdep even understands this to
be safe, so there's really no reason to keep it around.

Cc: Edward Cree <ecree.xilinx@gmail.com>
Cc: Martin Habets <habetsm.xilinx@gmail.com>
Acked-by: Edward Cree <ecree.xilinx@gmail.com>
Signed-off-by: Toke Høiland-Jørgensen <toke@redhat.com>
---
 drivers/net/ethernet/sfc/rx.c | 12 +++++-------
 1 file changed, 5 insertions(+), 7 deletions(-)

diff --git a/drivers/net/ethernet/sfc/rx.c b/drivers/net/ethernet/sfc/rx.c
index 17b8119c48e5..3e5b88ab42db 100644
--- a/drivers/net/ethernet/sfc/rx.c
+++ b/drivers/net/ethernet/sfc/rx.c
@@ -260,18 +260,14 @@ static bool efx_do_xdp(struct efx_nic *efx, struct efx_channel *channel,
 	s16 offset;
 	int err;
 
-	rcu_read_lock();
-	xdp_prog = rcu_dereference(efx->xdp_prog);
-	if (!xdp_prog) {
-		rcu_read_unlock();
+	xdp_prog = rcu_dereference_bh(efx->xdp_prog);
+	if (!xdp_prog)
 		return true;
-	}
 
 	rx_queue = efx_channel_get_rx_queue(channel);
 
 	if (unlikely(channel->rx_pkt_n_frags > 1)) {
 		/* We can't do XDP on fragmented packets - drop. */
-		rcu_read_unlock();
 		efx_free_rx_buffers(rx_queue, rx_buf,
 				    channel->rx_pkt_n_frags);
 		if (net_ratelimit())
@@ -295,8 +291,10 @@ static bool efx_do_xdp(struct efx_nic *efx, struct efx_channel *channel,
 	xdp_prepare_buff(&xdp, *ehp - EFX_XDP_HEADROOM, EFX_XDP_HEADROOM,
 			 rx_buf->len, false);
 
+	/* This code is invoked within a single NAPI poll cycle and thus under
+	 * local_bh_disable(), which provides the needed RCU protection.
+	 */
 	xdp_act = bpf_prog_run_xdp(xdp_prog, &xdp);
-	rcu_read_unlock();
 
 	offset = (u8 *)xdp.data - *ehp;
 
-- 
2.32.0

[PATCH bpf-next v3 14/16] netsec: remove rcu_read_lock() around XDP program invocation

[Toke Høiland-Jørgensen]

The netsec driver has a rcu_read_lock()/rcu_read_unlock() pair around the
full RX loop, covering everything up to and including xdp_do_flush(). This
is actually the correct behaviour, but because it all happens in a single
NAPI poll cycle (and thus under local_bh_disable()), it is also technically
redundant.

With the addition of RCU annotations to the XDP_REDIRECT map types that
take bh execution into account, lockdep even understands this to be safe,
so there's really no reason to keep the rcu_read_lock() around anymore, so
let's just remove it.

Cc: Jassi Brar <jaswinder.singh@linaro.org>
Cc: Ilias Apalodimas <ilias.apalodimas@linaro.org>
Acked-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
Signed-off-by: Toke Høiland-Jørgensen <toke@redhat.com>
---
 drivers/net/ethernet/socionext/netsec.c | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/drivers/net/ethernet/socionext/netsec.c b/drivers/net/ethernet/socionext/netsec.c
index dfc85cc68173..e07b7c870046 100644
--- a/drivers/net/ethernet/socionext/netsec.c
+++ b/drivers/net/ethernet/socionext/netsec.c
@@ -958,7 +958,6 @@ static int netsec_process_rx(struct netsec_priv *priv, int budget)
 
 	xdp_init_buff(&xdp, PAGE_SIZE, &dring->xdp_rxq);
 
-	rcu_read_lock();
 	xdp_prog = READ_ONCE(priv->xdp_prog);
 	dma_dir = page_pool_get_dma_dir(dring->page_pool);
 
@@ -1019,6 +1018,10 @@ static int netsec_process_rx(struct netsec_priv *priv, int budget)
 				 pkt_len, false);
 
 		if (xdp_prog) {
+			/* This code is invoked within a single NAPI poll cycle
+			 * and thus under local_bh_disable(), which provides the
+			 * needed RCU protection.
+			 */
 			xdp_result = netsec_run_xdp(priv, xdp_prog, &xdp);
 			if (xdp_result != NETSEC_XDP_PASS) {
 				xdp_act |= xdp_result;
@@ -1069,8 +1072,6 @@ static int netsec_process_rx(struct netsec_priv *priv, int budget)
 	}
 	netsec_finalize_xdp_rx(priv, xdp_act, xdp_xmit);
 
-	rcu_read_unlock();
-
 	return done;
 }
 
-- 
2.32.0

[PATCH bpf-next v3 15/16] stmmac: remove rcu_read_lock() around XDP program invocation

[Toke Høiland-Jørgensen]

The stmmac driver has rcu_read_lock()/rcu_read_unlock() pairs around XDP
program invocations. However, the actual lifetime of the objects referred
by the XDP program invocation is longer, all the way through to the call to
xdp_do_flush(), making the scope of the rcu_read_lock() too small. This
turns out to be harmless because it all happens in a single NAPI poll
cycle (and thus under local_bh_disable()), but it makes the rcu_read_lock()
misleading.

Rather than extend the scope of the rcu_read_lock(), just get rid of it
entirely. With the addition of RCU annotations to the XDP_REDIRECT map
types that take bh execution into account, lockdep even understands this to
be safe, so there's really no reason to keep it around.

Cc: Giuseppe Cavallaro <peppe.cavallaro@st.com>
Cc: Alexandre Torgue <alexandre.torgue@foss.st.com>
Cc: Jose Abreu <joabreu@synopsys.com>
Signed-off-by: Toke Høiland-Jørgensen <toke@redhat.com>
---
 drivers/net/ethernet/stmicro/stmmac/stmmac_main.c | 13 +++++--------
 1 file changed, 5 insertions(+), 8 deletions(-)

diff --git a/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c b/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c
index bf9fe25fed69..5dcc8a42abf9 100644
--- a/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c
+++ b/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c
@@ -4654,7 +4654,6 @@ static int stmmac_xdp_xmit_back(struct stmmac_priv *priv,
 	return res;
 }
 
-/* This function assumes rcu_read_lock() is held by the caller. */
 static int __stmmac_xdp_run_prog(struct stmmac_priv *priv,
 				 struct bpf_prog *prog,
 				 struct xdp_buff *xdp)
@@ -4662,6 +4661,9 @@ static int __stmmac_xdp_run_prog(struct stmmac_priv *priv,
 	u32 act;
 	int res;
 
+	/* This code is invoked within a single NAPI poll cycle and thus under
+	 * local_bh_disable(), which provides the needed RCU protection.
+	 */
 	act = bpf_prog_run_xdp(prog, xdp);
 	switch (act) {
 	case XDP_PASS:
@@ -4696,17 +4698,14 @@ static struct sk_buff *stmmac_xdp_run_prog(struct stmmac_priv *priv,
 	struct bpf_prog *prog;
 	int res;
 
-	rcu_read_lock();
-
 	prog = READ_ONCE(priv->xdp_prog);
 	if (!prog) {
 		res = STMMAC_XDP_PASS;
-		goto unlock;
+		goto out;
 	}
 
 	res = __stmmac_xdp_run_prog(priv, prog, xdp);
-unlock:
-	rcu_read_unlock();
+out:
 	return ERR_PTR(-res);
 }
 
@@ -4976,10 +4975,8 @@ static int stmmac_rx_zc(struct stmmac_priv *priv, int limit, u32 queue)
 		buf->xdp->data_end = buf->xdp->data + buf1_len;
 		xsk_buff_dma_sync_for_cpu(buf->xdp, rx_q->xsk_pool);
 
-		rcu_read_lock();
 		prog = READ_ONCE(priv->xdp_prog);
 		res = __stmmac_xdp_run_prog(priv, prog, buf->xdp);
-		rcu_read_unlock();
 
 		switch (res) {
 		case STMMAC_XDP_PASS:
-- 
2.32.0

[PATCH bpf-next v3 16/16] net: ti: remove rcu_read_lock() around XDP program invocation

[Toke Høiland-Jørgensen]

The cpsw driver has rcu_read_lock()/rcu_read_unlock() pairs around XDP
program invocations. However, the actual lifetime of the objects referred
by the XDP program invocation is longer, all the way through to the call to
xdp_do_flush(), making the scope of the rcu_read_lock() too small. This
turns out to be harmless because it all happens in a single NAPI poll
cycle (and thus under local_bh_disable()), but it makes the rcu_read_lock()
misleading.

Rather than extend the scope of the rcu_read_lock(), just get rid of it
entirely. With the addition of RCU annotations to the XDP_REDIRECT map
types that take bh execution into account, lockdep even understands this to
be safe, so there's really no reason to keep it around.

Cc: Grygorii Strashko <grygorii.strashko@ti.com>
Cc: linux-omap@vger.kernel.org
Tested-by: Grygorii Strashko <grygorii.strashko@ti.com>
Reviewed-by: Grygorii Strashko <grygorii.strashko@ti.com>
Signed-off-by: Toke Høiland-Jørgensen <toke@redhat.com>
---
 drivers/net/ethernet/ti/cpsw_priv.c | 13 +++++--------
 1 file changed, 5 insertions(+), 8 deletions(-)

diff --git a/drivers/net/ethernet/ti/cpsw_priv.c b/drivers/net/ethernet/ti/cpsw_priv.c
index 5862f0a4a975..25fa7304a542 100644
--- a/drivers/net/ethernet/ti/cpsw_priv.c
+++ b/drivers/net/ethernet/ti/cpsw_priv.c
@@ -1328,14 +1328,13 @@ int cpsw_run_xdp(struct cpsw_priv *priv, int ch, struct xdp_buff *xdp,
 	struct bpf_prog *prog;
 	u32 act;
 
-	rcu_read_lock();
-
 	prog = READ_ONCE(priv->xdp_prog);
-	if (!prog) {
-		ret = CPSW_XDP_PASS;
-		goto out;
-	}
+	if (!prog)
+		return CPSW_XDP_PASS;
 
+	/* This code is invoked within a single NAPI poll cycle and thus under
+	 * local_bh_disable(), which provides the needed RCU protection.
+	 */
 	act = bpf_prog_run_xdp(prog, xdp);
 	/* XDP prog might have changed packet data and boundaries */
 	*len = xdp->data_end - xdp->data;
@@ -1378,10 +1377,8 @@ int cpsw_run_xdp(struct cpsw_priv *priv, int ch, struct xdp_buff *xdp,
 	ndev->stats.rx_bytes += *len;
 	ndev->stats.rx_packets++;
 out:
-	rcu_read_unlock();
 	return ret;
 drop:
-	rcu_read_unlock();
 	page_pool_recycle_direct(cpsw->page_pool[ch], page);
 	return ret;
 }
-- 
2.32.0

Re: [PATCH bpf-next v3 03/16] xdp: add proper __rcu annotations to redirect map entries

[Martin KaFai Lau]

On Thu, Jun 17, 2021 at 11:27:35PM +0200, Toke Høiland-Jørgensen wrote:
> XDP_REDIRECT works by a three-step process: the bpf_redirect() and
> bpf_redirect_map() helpers will lookup the target of the redirect and store
> it (along with some other metadata) in a per-CPU struct bpf_redirect_info.
> Next, when the program returns the XDP_REDIRECT return code, the driver
> will call xdp_do_redirect() which will use the information thus stored to
> actually enqueue the frame into a bulk queue structure (that differs
> slightly by map type, but shares the same principle). Finally, before
> exiting its NAPI poll loop, the driver will call xdp_do_flush(), which will
> flush all the different bulk queues, thus completing the redirect.
> 
> Pointers to the map entries will be kept around for this whole sequence of
> steps, protected by RCU. However, there is no top-level rcu_read_lock() in
> the core code; instead drivers add their own rcu_read_lock() around the XDP
> portions of the code, but somewhat inconsistently as Martin discovered[0].
> However, things still work because everything happens inside a single NAPI
> poll sequence, which means it's between a pair of calls to
> local_bh_disable()/local_bh_enable(). So Paul suggested[1] that we could
> document this intention by using rcu_dereference_check() with
> rcu_read_lock_bh_held() as a second parameter, thus allowing sparse and
> lockdep to verify that everything is done correctly.
> 
> This patch does just that: we add an __rcu annotation to the map entry
> pointers and remove the various comments explaining the NAPI poll assurance
> strewn through devmap.c in favour of a longer explanation in filter.c. The
> goal is to have one coherent documentation of the entire flow, and rely on
> the RCU annotations as a "standard" way of communicating the flow in the
> map code (which can additionally be understood by sparse and lockdep).
> 
> The RCU annotation replacements result in a fairly straight-forward
> replacement where READ_ONCE() becomes rcu_dereference_check(), WRITE_ONCE()
> becomes rcu_assign_pointer() and xchg() and cmpxchg() gets wrapped in the
> proper constructs to cast the pointer back and forth between __rcu and
> __kernel address space (for the benefit of sparse). The one complication is
> that xskmap has a few constructions where double-pointers are passed back
> and forth; these simply all gain __rcu annotations, and only the final
> reference/dereference to the inner-most pointer gets changed.
> 
> With this, everything can be run through sparse without eliciting
> complaints, and lockdep can verify correctness even without the use of
> rcu_read_lock() in the drivers. Subsequent patches will clean these up from
> the drivers.
> 
> [0] https://lore.kernel.org/bpf/20210415173551.7ma4slcbqeyiba2r@kafai-mbp.dhcp.thefacebook.com/
> [1] https://lore.kernel.org/bpf/20210419165837.GA975577@paulmck-ThinkPad-P17-Gen-1/
> 
> Signed-off-by: Toke Høiland-Jørgensen <toke@redhat.com>
Acked-by: Martin KaFai Lau <kafai@fb.com>

Re: [PATCH bpf-next v3 15/16] stmmac: remove rcu_read_lock() around XDP program invocation

[Wong Vee Khee]

On Thu, Jun 17, 2021 at 11:27:47PM +0200, Toke Høiland-Jørgensen wrote:
> The stmmac driver has rcu_read_lock()/rcu_read_unlock() pairs around XDP
> program invocations. However, the actual lifetime of the objects referred
> by the XDP program invocation is longer, all the way through to the call to
> xdp_do_flush(), making the scope of the rcu_read_lock() too small. This
> turns out to be harmless because it all happens in a single NAPI poll
> cycle (and thus under local_bh_disable()), but it makes the rcu_read_lock()
> misleading.
> 
> Rather than extend the scope of the rcu_read_lock(), just get rid of it
> entirely. With the addition of RCU annotations to the XDP_REDIRECT map
> types that take bh execution into account, lockdep even understands this to
> be safe, so there's really no reason to keep it around.
> 
> Cc: Giuseppe Cavallaro <peppe.cavallaro@st.com>
> Cc: Alexandre Torgue <alexandre.torgue@foss.st.com>
> Cc: Jose Abreu <joabreu@synopsys.com>
> Signed-off-by: Toke Høiland-Jørgensen <toke@redhat.com>

Acked-by: Wong Vee Khee <vee.khee.wong@linux.intel.com>
Tested-by: Song, Yoong Siang <yoong.siang.song@intel.com>

> ---
>  drivers/net/ethernet/stmicro/stmmac/stmmac_main.c | 13 +++++--------
>  1 file changed, 5 insertions(+), 8 deletions(-)

Re: [PATCH bpf-next v3 03/16] xdp: add proper __rcu annotations to redirect map entries

[Daniel Borkmann]

On 6/17/21 11:27 PM, Toke Høiland-Jørgensen wrote:
> XDP_REDIRECT works by a three-step process: the bpf_redirect() and
> bpf_redirect_map() helpers will lookup the target of the redirect and store
> it (along with some other metadata) in a per-CPU struct bpf_redirect_info.
> Next, when the program returns the XDP_REDIRECT return code, the driver
> will call xdp_do_redirect() which will use the information thus stored to
> actually enqueue the frame into a bulk queue structure (that differs
> slightly by map type, but shares the same principle). Finally, before
> exiting its NAPI poll loop, the driver will call xdp_do_flush(), which will
> flush all the different bulk queues, thus completing the redirect.
> 
> Pointers to the map entries will be kept around for this whole sequence of
> steps, protected by RCU. However, there is no top-level rcu_read_lock() in
> the core code; instead drivers add their own rcu_read_lock() around the XDP
> portions of the code, but somewhat inconsistently as Martin discovered[0].
> However, things still work because everything happens inside a single NAPI
> poll sequence, which means it's between a pair of calls to
> local_bh_disable()/local_bh_enable(). So Paul suggested[1] that we could
> document this intention by using rcu_dereference_check() with
> rcu_read_lock_bh_held() as a second parameter, thus allowing sparse and
> lockdep to verify that everything is done correctly.
> 
> This patch does just that: we add an __rcu annotation to the map entry
> pointers and remove the various comments explaining the NAPI poll assurance
> strewn through devmap.c in favour of a longer explanation in filter.c. The
> goal is to have one coherent documentation of the entire flow, and rely on
> the RCU annotations as a "standard" way of communicating the flow in the
> map code (which can additionally be understood by sparse and lockdep).
> 
> The RCU annotation replacements result in a fairly straight-forward
> replacement where READ_ONCE() becomes rcu_dereference_check(), WRITE_ONCE()
> becomes rcu_assign_pointer() and xchg() and cmpxchg() gets wrapped in the
> proper constructs to cast the pointer back and forth between __rcu and
> __kernel address space (for the benefit of sparse). The one complication is
> that xskmap has a few constructions where double-pointers are passed back
> and forth; these simply all gain __rcu annotations, and only the final
> reference/dereference to the inner-most pointer gets changed.
> 
> With this, everything can be run through sparse without eliciting
> complaints, and lockdep can verify correctness even without the use of
> rcu_read_lock() in the drivers. Subsequent patches will clean these up from
> the drivers.
> 
> [0] https://lore.kernel.org/bpf/20210415173551.7ma4slcbqeyiba2r@kafai-mbp.dhcp.thefacebook.com/
> [1] https://lore.kernel.org/bpf/20210419165837.GA975577@paulmck-ThinkPad-P17-Gen-1/
> 
> Signed-off-by: Toke Høiland-Jørgensen <toke@redhat.com>
> ---
>   include/net/xdp_sock.h |  2 +-
>   kernel/bpf/cpumap.c    | 13 +++++++----
>   kernel/bpf/devmap.c    | 49 ++++++++++++++++++------------------------
>   net/core/filter.c      | 28 ++++++++++++++++++++++++
>   net/xdp/xsk.c          |  4 ++--
>   net/xdp/xsk.h          |  4 ++--
>   net/xdp/xskmap.c       | 29 ++++++++++++++-----------
>   7 files changed, 80 insertions(+), 49 deletions(-)
[...]
>   						 __dev_map_entry_free);
> diff --git a/net/core/filter.c b/net/core/filter.c
> index caa88955562e..0b7db5c70385 100644
> --- a/net/core/filter.c
> +++ b/net/core/filter.c
> @@ -3922,6 +3922,34 @@ static const struct bpf_func_proto bpf_xdp_adjust_meta_proto = {
>   	.arg2_type	= ARG_ANYTHING,
>   };
>   
> +/* XDP_REDIRECT works by a three-step process, implemented in the functions
> + * below:
> + *
> + * 1. The bpf_redirect() and bpf_redirect_map() helpers will lookup the target
> + *    of the redirect and store it (along with some other metadata) in a per-CPU
> + *    struct bpf_redirect_info.
> + *
> + * 2. When the program returns the XDP_REDIRECT return code, the driver will
> + *    call xdp_do_redirect() which will use the information in struct
> + *    bpf_redirect_info to actually enqueue the frame into a map type-specific
> + *    bulk queue structure.
> + *
> + * 3. Before exiting its NAPI poll loop, the driver will call xdp_do_flush(),
> + *    which will flush all the different bulk queues, thus completing the
> + *    redirect.
> + *
> + * Pointers to the map entries will be kept around for this whole sequence of
> + * steps, protected by RCU. However, there is no top-level rcu_read_lock() in
> + * the core code; instead, the RCU protection relies on everything happening
> + * inside a single NAPI poll sequence, which means it's between a pair of calls
> + * to local_bh_disable()/local_bh_enable().
> + *
> + * The map entries are marked as __rcu and the map code makes sure to
> + * dereference those pointers with rcu_dereference_check() in a way that works
> + * for both sections that to hold an rcu_read_lock() and sections that are
> + * called from NAPI without a separate rcu_read_lock(). The code below does not
> + * use RCU annotations, but relies on those in the map code.

One more follow-up question related to tc BPF: given we do use rcu_read_lock_bh()
in case of sch_handle_egress(), could we also remove the rcu_read_lock() pair
from cls_bpf_classify() then?

It would also be great if this scenario in general could be placed under the
Documentation/RCU/whatisRCU.rst as an example, so we could refer to the official
doc on this, too, if Paul is good with this.

Could you also update the RCU comment in bpf_prog_run_xdp()? Or alternatively move all
the below driver comments in there as a single location?

   /* This code is invoked within a single NAPI poll cycle and thus under
    * local_bh_disable(), which provides the needed RCU protection.
    */

Thanks,
Daniel

Re: [PATCH bpf-next v3 03/16] xdp: add proper __rcu annotations to redirect map entries

[Toke Høiland-Jørgensen]

Daniel Borkmann <daniel@iogearbox.net> writes:

> On 6/17/21 11:27 PM, Toke Høiland-Jørgensen wrote:
>> XDP_REDIRECT works by a three-step process: the bpf_redirect() and
>> bpf_redirect_map() helpers will lookup the target of the redirect and store
>> it (along with some other metadata) in a per-CPU struct bpf_redirect_info.
>> Next, when the program returns the XDP_REDIRECT return code, the driver
>> will call xdp_do_redirect() which will use the information thus stored to
>> actually enqueue the frame into a bulk queue structure (that differs
>> slightly by map type, but shares the same principle). Finally, before
>> exiting its NAPI poll loop, the driver will call xdp_do_flush(), which will
>> flush all the different bulk queues, thus completing the redirect.
>> 
>> Pointers to the map entries will be kept around for this whole sequence of
>> steps, protected by RCU. However, there is no top-level rcu_read_lock() in
>> the core code; instead drivers add their own rcu_read_lock() around the XDP
>> portions of the code, but somewhat inconsistently as Martin discovered[0].
>> However, things still work because everything happens inside a single NAPI
>> poll sequence, which means it's between a pair of calls to
>> local_bh_disable()/local_bh_enable(). So Paul suggested[1] that we could
>> document this intention by using rcu_dereference_check() with
>> rcu_read_lock_bh_held() as a second parameter, thus allowing sparse and
>> lockdep to verify that everything is done correctly.
>> 
>> This patch does just that: we add an __rcu annotation to the map entry
>> pointers and remove the various comments explaining the NAPI poll assurance
>> strewn through devmap.c in favour of a longer explanation in filter.c. The
>> goal is to have one coherent documentation of the entire flow, and rely on
>> the RCU annotations as a "standard" way of communicating the flow in the
>> map code (which can additionally be understood by sparse and lockdep).
>> 
>> The RCU annotation replacements result in a fairly straight-forward
>> replacement where READ_ONCE() becomes rcu_dereference_check(), WRITE_ONCE()
>> becomes rcu_assign_pointer() and xchg() and cmpxchg() gets wrapped in the
>> proper constructs to cast the pointer back and forth between __rcu and
>> __kernel address space (for the benefit of sparse). The one complication is
>> that xskmap has a few constructions where double-pointers are passed back
>> and forth; these simply all gain __rcu annotations, and only the final
>> reference/dereference to the inner-most pointer gets changed.
>> 
>> With this, everything can be run through sparse without eliciting
>> complaints, and lockdep can verify correctness even without the use of
>> rcu_read_lock() in the drivers. Subsequent patches will clean these up from
>> the drivers.
>> 
>> [0] https://lore.kernel.org/bpf/20210415173551.7ma4slcbqeyiba2r@kafai-mbp.dhcp.thefacebook.com/
>> [1] https://lore.kernel.org/bpf/20210419165837.GA975577@paulmck-ThinkPad-P17-Gen-1/
>> 
>> Signed-off-by: Toke Høiland-Jørgensen <toke@redhat.com>
>> ---
>>   include/net/xdp_sock.h |  2 +-
>>   kernel/bpf/cpumap.c    | 13 +++++++----
>>   kernel/bpf/devmap.c    | 49 ++++++++++++++++++------------------------
>>   net/core/filter.c      | 28 ++++++++++++++++++++++++
>>   net/xdp/xsk.c          |  4 ++--
>>   net/xdp/xsk.h          |  4 ++--
>>   net/xdp/xskmap.c       | 29 ++++++++++++++-----------
>>   7 files changed, 80 insertions(+), 49 deletions(-)
> [...]
>>   						 __dev_map_entry_free);
>> diff --git a/net/core/filter.c b/net/core/filter.c
>> index caa88955562e..0b7db5c70385 100644
>> --- a/net/core/filter.c
>> +++ b/net/core/filter.c
>> @@ -3922,6 +3922,34 @@ static const struct bpf_func_proto bpf_xdp_adjust_meta_proto = {
>>   	.arg2_type	= ARG_ANYTHING,
>>   };
>>   
>> +/* XDP_REDIRECT works by a three-step process, implemented in the functions
>> + * below:
>> + *
>> + * 1. The bpf_redirect() and bpf_redirect_map() helpers will lookup the target
>> + *    of the redirect and store it (along with some other metadata) in a per-CPU
>> + *    struct bpf_redirect_info.
>> + *
>> + * 2. When the program returns the XDP_REDIRECT return code, the driver will
>> + *    call xdp_do_redirect() which will use the information in struct
>> + *    bpf_redirect_info to actually enqueue the frame into a map type-specific
>> + *    bulk queue structure.
>> + *
>> + * 3. Before exiting its NAPI poll loop, the driver will call xdp_do_flush(),
>> + *    which will flush all the different bulk queues, thus completing the
>> + *    redirect.
>> + *
>> + * Pointers to the map entries will be kept around for this whole sequence of
>> + * steps, protected by RCU. However, there is no top-level rcu_read_lock() in
>> + * the core code; instead, the RCU protection relies on everything happening
>> + * inside a single NAPI poll sequence, which means it's between a pair of calls
>> + * to local_bh_disable()/local_bh_enable().
>> + *
>> + * The map entries are marked as __rcu and the map code makes sure to
>> + * dereference those pointers with rcu_dereference_check() in a way that works
>> + * for both sections that to hold an rcu_read_lock() and sections that are
>> + * called from NAPI without a separate rcu_read_lock(). The code below does not
>> + * use RCU annotations, but relies on those in the map code.
>
> One more follow-up question related to tc BPF: given we do use rcu_read_lock_bh()
> in case of sch_handle_egress(), could we also remove the rcu_read_lock() pair
> from cls_bpf_classify() then?

I believe so, yeah. Patch 2 in this series should even make lockdep stop
complaining about it :)

I can add a patch removing the rcu_read_lock() from cls_bpf in the next
version.

> It would also be great if this scenario in general could be placed
> under the Documentation/RCU/whatisRCU.rst as an example, so we could
> refer to the official doc on this, too, if Paul is good with this.

I'll take a look and see if I can find a way to fit it in there...

> Could you also update the RCU comment in bpf_prog_run_xdp()? Or
> alternatively move all the below driver comments in there as a single
> location?
>
>    /* This code is invoked within a single NAPI poll cycle and thus under
>     * local_bh_disable(), which provides the needed RCU protection.
>     */

Sure, can do. And yeah, I do agree that moving the comment in there
makes more sense than scattering it over all the drivers, even if that
means I have to go back and edit all the drivers again :P

-Toke

Re: [PATCH bpf-next v3 03/16] xdp: add proper __rcu annotations to redirect map entries

[Daniel Borkmann]

On 6/21/21 11:39 PM, Toke Høiland-Jørgensen wrote:
> Daniel Borkmann <daniel@iogearbox.net> writes:
>> On 6/17/21 11:27 PM, Toke Høiland-Jørgensen wrote:
>>> XDP_REDIRECT works by a three-step process: the bpf_redirect() and
>>> bpf_redirect_map() helpers will lookup the target of the redirect and store
>>> it (along with some other metadata) in a per-CPU struct bpf_redirect_info.
>>> Next, when the program returns the XDP_REDIRECT return code, the driver
>>> will call xdp_do_redirect() which will use the information thus stored to
>>> actually enqueue the frame into a bulk queue structure (that differs
>>> slightly by map type, but shares the same principle). Finally, before
>>> exiting its NAPI poll loop, the driver will call xdp_do_flush(), which will
>>> flush all the different bulk queues, thus completing the redirect.
>>>
>>> Pointers to the map entries will be kept around for this whole sequence of
>>> steps, protected by RCU. However, there is no top-level rcu_read_lock() in
>>> the core code; instead drivers add their own rcu_read_lock() around the XDP
>>> portions of the code, but somewhat inconsistently as Martin discovered[0].
>>> However, things still work because everything happens inside a single NAPI
>>> poll sequence, which means it's between a pair of calls to
>>> local_bh_disable()/local_bh_enable(). So Paul suggested[1] that we could
>>> document this intention by using rcu_dereference_check() with
>>> rcu_read_lock_bh_held() as a second parameter, thus allowing sparse and
>>> lockdep to verify that everything is done correctly.
>>>
>>> This patch does just that: we add an __rcu annotation to the map entry
>>> pointers and remove the various comments explaining the NAPI poll assurance
>>> strewn through devmap.c in favour of a longer explanation in filter.c. The
>>> goal is to have one coherent documentation of the entire flow, and rely on
>>> the RCU annotations as a "standard" way of communicating the flow in the
>>> map code (which can additionally be understood by sparse and lockdep).
>>>
>>> The RCU annotation replacements result in a fairly straight-forward
>>> replacement where READ_ONCE() becomes rcu_dereference_check(), WRITE_ONCE()
>>> becomes rcu_assign_pointer() and xchg() and cmpxchg() gets wrapped in the
>>> proper constructs to cast the pointer back and forth between __rcu and
>>> __kernel address space (for the benefit of sparse). The one complication is
>>> that xskmap has a few constructions where double-pointers are passed back
>>> and forth; these simply all gain __rcu annotations, and only the final
>>> reference/dereference to the inner-most pointer gets changed.
>>>
>>> With this, everything can be run through sparse without eliciting
>>> complaints, and lockdep can verify correctness even without the use of
>>> rcu_read_lock() in the drivers. Subsequent patches will clean these up from
>>> the drivers.
>>>
>>> [0] https://lore.kernel.org/bpf/20210415173551.7ma4slcbqeyiba2r@kafai-mbp.dhcp.thefacebook.com/
>>> [1] https://lore.kernel.org/bpf/20210419165837.GA975577@paulmck-ThinkPad-P17-Gen-1/
>>>
>>> Signed-off-by: Toke Høiland-Jørgensen <toke@redhat.com>
>>> ---
>>>    include/net/xdp_sock.h |  2 +-
>>>    kernel/bpf/cpumap.c    | 13 +++++++----
>>>    kernel/bpf/devmap.c    | 49 ++++++++++++++++++------------------------
>>>    net/core/filter.c      | 28 ++++++++++++++++++++++++
>>>    net/xdp/xsk.c          |  4 ++--
>>>    net/xdp/xsk.h          |  4 ++--
>>>    net/xdp/xskmap.c       | 29 ++++++++++++++-----------
>>>    7 files changed, 80 insertions(+), 49 deletions(-)
>> [...]
>>>    						 __dev_map_entry_free);
>>> diff --git a/net/core/filter.c b/net/core/filter.c
>>> index caa88955562e..0b7db5c70385 100644
>>> --- a/net/core/filter.c
>>> +++ b/net/core/filter.c
>>> @@ -3922,6 +3922,34 @@ static const struct bpf_func_proto bpf_xdp_adjust_meta_proto = {
>>>    	.arg2_type	= ARG_ANYTHING,
>>>    };
>>>    
>>> +/* XDP_REDIRECT works by a three-step process, implemented in the functions
>>> + * below:
>>> + *
>>> + * 1. The bpf_redirect() and bpf_redirect_map() helpers will lookup the target
>>> + *    of the redirect and store it (along with some other metadata) in a per-CPU
>>> + *    struct bpf_redirect_info.
>>> + *
>>> + * 2. When the program returns the XDP_REDIRECT return code, the driver will
>>> + *    call xdp_do_redirect() which will use the information in struct
>>> + *    bpf_redirect_info to actually enqueue the frame into a map type-specific
>>> + *    bulk queue structure.
>>> + *
>>> + * 3. Before exiting its NAPI poll loop, the driver will call xdp_do_flush(),
>>> + *    which will flush all the different bulk queues, thus completing the
>>> + *    redirect.
>>> + *
>>> + * Pointers to the map entries will be kept around for this whole sequence of
>>> + * steps, protected by RCU. However, there is no top-level rcu_read_lock() in
>>> + * the core code; instead, the RCU protection relies on everything happening
>>> + * inside a single NAPI poll sequence, which means it's between a pair of calls
>>> + * to local_bh_disable()/local_bh_enable().
>>> + *
>>> + * The map entries are marked as __rcu and the map code makes sure to
>>> + * dereference those pointers with rcu_dereference_check() in a way that works
>>> + * for both sections that to hold an rcu_read_lock() and sections that are
>>> + * called from NAPI without a separate rcu_read_lock(). The code below does not
>>> + * use RCU annotations, but relies on those in the map code.
>>
>> One more follow-up question related to tc BPF: given we do use rcu_read_lock_bh()
>> in case of sch_handle_egress(), could we also remove the rcu_read_lock() pair
>> from cls_bpf_classify() then?
> 
> I believe so, yeah. Patch 2 in this series should even make lockdep stop
> complaining about it :)

Btw, I was wondering whether we should just get rid of all the WARN_ON_ONCE()s
from those map helpers given in most situations these are not triggered anyway
due to retpoline avoidance where verifier rewrites the calls to jump to the map
backend implementation directly. One alternative could be to have an extension
to the bpf prologue generation under CONFIG_DEBUG_LOCK_ALLOC and call the lockdep
checks from there, but it's probably not worth the effort. (In the trampoline
case we have those __bpf_prog_enter()/__bpf_prog_enter_sleepable() where the
latter in particular has asserts like might_fault(), fwiw.)

> I can add a patch removing the rcu_read_lock() from cls_bpf in the next
> version.
> 
>> It would also be great if this scenario in general could be placed
>> under the Documentation/RCU/whatisRCU.rst as an example, so we could
>> refer to the official doc on this, too, if Paul is good with this.
> 
> I'll take a look and see if I can find a way to fit it in there...
> 
>> Could you also update the RCU comment in bpf_prog_run_xdp()? Or
>> alternatively move all the below driver comments in there as a single
>> location?
>>
>>     /* This code is invoked within a single NAPI poll cycle and thus under
>>      * local_bh_disable(), which provides the needed RCU protection.
>>      */
> 
> Sure, can do. And yeah, I do agree that moving the comment in there
> makes more sense than scattering it over all the drivers, even if that
> means I have to go back and edit all the drivers again :P

Yeap, all of the above sounds good, thanks!

Re: [PATCH bpf-next v3 03/16] xdp: add proper __rcu annotations to redirect map entries

[Toke Høiland-Jørgensen]

Daniel Borkmann <daniel@iogearbox.net> writes:

> On 6/21/21 11:39 PM, Toke Høiland-Jørgensen wrote:
>> Daniel Borkmann <daniel@iogearbox.net> writes:
>>> On 6/17/21 11:27 PM, Toke Høiland-Jørgensen wrote:
>>>> XDP_REDIRECT works by a three-step process: the bpf_redirect() and
>>>> bpf_redirect_map() helpers will lookup the target of the redirect and store
>>>> it (along with some other metadata) in a per-CPU struct bpf_redirect_info.
>>>> Next, when the program returns the XDP_REDIRECT return code, the driver
>>>> will call xdp_do_redirect() which will use the information thus stored to
>>>> actually enqueue the frame into a bulk queue structure (that differs
>>>> slightly by map type, but shares the same principle). Finally, before
>>>> exiting its NAPI poll loop, the driver will call xdp_do_flush(), which will
>>>> flush all the different bulk queues, thus completing the redirect.
>>>>
>>>> Pointers to the map entries will be kept around for this whole sequence of
>>>> steps, protected by RCU. However, there is no top-level rcu_read_lock() in
>>>> the core code; instead drivers add their own rcu_read_lock() around the XDP
>>>> portions of the code, but somewhat inconsistently as Martin discovered[0].
>>>> However, things still work because everything happens inside a single NAPI
>>>> poll sequence, which means it's between a pair of calls to
>>>> local_bh_disable()/local_bh_enable(). So Paul suggested[1] that we could
>>>> document this intention by using rcu_dereference_check() with
>>>> rcu_read_lock_bh_held() as a second parameter, thus allowing sparse and
>>>> lockdep to verify that everything is done correctly.
>>>>
>>>> This patch does just that: we add an __rcu annotation to the map entry
>>>> pointers and remove the various comments explaining the NAPI poll assurance
>>>> strewn through devmap.c in favour of a longer explanation in filter.c. The
>>>> goal is to have one coherent documentation of the entire flow, and rely on
>>>> the RCU annotations as a "standard" way of communicating the flow in the
>>>> map code (which can additionally be understood by sparse and lockdep).
>>>>
>>>> The RCU annotation replacements result in a fairly straight-forward
>>>> replacement where READ_ONCE() becomes rcu_dereference_check(), WRITE_ONCE()
>>>> becomes rcu_assign_pointer() and xchg() and cmpxchg() gets wrapped in the
>>>> proper constructs to cast the pointer back and forth between __rcu and
>>>> __kernel address space (for the benefit of sparse). The one complication is
>>>> that xskmap has a few constructions where double-pointers are passed back
>>>> and forth; these simply all gain __rcu annotations, and only the final
>>>> reference/dereference to the inner-most pointer gets changed.
>>>>
>>>> With this, everything can be run through sparse without eliciting
>>>> complaints, and lockdep can verify correctness even without the use of
>>>> rcu_read_lock() in the drivers. Subsequent patches will clean these up from
>>>> the drivers.
>>>>
>>>> [0] https://lore.kernel.org/bpf/20210415173551.7ma4slcbqeyiba2r@kafai-mbp.dhcp.thefacebook.com/
>>>> [1] https://lore.kernel.org/bpf/20210419165837.GA975577@paulmck-ThinkPad-P17-Gen-1/
>>>>
>>>> Signed-off-by: Toke Høiland-Jørgensen <toke@redhat.com>
>>>> ---
>>>>    include/net/xdp_sock.h |  2 +-
>>>>    kernel/bpf/cpumap.c    | 13 +++++++----
>>>>    kernel/bpf/devmap.c    | 49 ++++++++++++++++++------------------------
>>>>    net/core/filter.c      | 28 ++++++++++++++++++++++++
>>>>    net/xdp/xsk.c          |  4 ++--
>>>>    net/xdp/xsk.h          |  4 ++--
>>>>    net/xdp/xskmap.c       | 29 ++++++++++++++-----------
>>>>    7 files changed, 80 insertions(+), 49 deletions(-)
>>> [...]
>>>>    						 __dev_map_entry_free);
>>>> diff --git a/net/core/filter.c b/net/core/filter.c
>>>> index caa88955562e..0b7db5c70385 100644
>>>> --- a/net/core/filter.c
>>>> +++ b/net/core/filter.c
>>>> @@ -3922,6 +3922,34 @@ static const struct bpf_func_proto bpf_xdp_adjust_meta_proto = {
>>>>    	.arg2_type	= ARG_ANYTHING,
>>>>    };
>>>>    
>>>> +/* XDP_REDIRECT works by a three-step process, implemented in the functions
>>>> + * below:
>>>> + *
>>>> + * 1. The bpf_redirect() and bpf_redirect_map() helpers will lookup the target
>>>> + *    of the redirect and store it (along with some other metadata) in a per-CPU
>>>> + *    struct bpf_redirect_info.
>>>> + *
>>>> + * 2. When the program returns the XDP_REDIRECT return code, the driver will
>>>> + *    call xdp_do_redirect() which will use the information in struct
>>>> + *    bpf_redirect_info to actually enqueue the frame into a map type-specific
>>>> + *    bulk queue structure.
>>>> + *
>>>> + * 3. Before exiting its NAPI poll loop, the driver will call xdp_do_flush(),
>>>> + *    which will flush all the different bulk queues, thus completing the
>>>> + *    redirect.
>>>> + *
>>>> + * Pointers to the map entries will be kept around for this whole sequence of
>>>> + * steps, protected by RCU. However, there is no top-level rcu_read_lock() in
>>>> + * the core code; instead, the RCU protection relies on everything happening
>>>> + * inside a single NAPI poll sequence, which means it's between a pair of calls
>>>> + * to local_bh_disable()/local_bh_enable().
>>>> + *
>>>> + * The map entries are marked as __rcu and the map code makes sure to
>>>> + * dereference those pointers with rcu_dereference_check() in a way that works
>>>> + * for both sections that to hold an rcu_read_lock() and sections that are
>>>> + * called from NAPI without a separate rcu_read_lock(). The code below does not
>>>> + * use RCU annotations, but relies on those in the map code.
>>>
>>> One more follow-up question related to tc BPF: given we do use rcu_read_lock_bh()
>>> in case of sch_handle_egress(), could we also remove the rcu_read_lock() pair
>>> from cls_bpf_classify() then?
>> 
>> I believe so, yeah. Patch 2 in this series should even make lockdep stop
>> complaining about it :)
>
> Btw, I was wondering whether we should just get rid of all the WARN_ON_ONCE()s
> from those map helpers given in most situations these are not triggered anyway
> due to retpoline avoidance where verifier rewrites the calls to jump to the map
> backend implementation directly. One alternative could be to have an extension
> to the bpf prologue generation under CONFIG_DEBUG_LOCK_ALLOC and call the lockdep
> checks from there, but it's probably not worth the effort. (In the trampoline
> case we have those __bpf_prog_enter()/__bpf_prog_enter_sleepable() where the
> latter in particular has asserts like might_fault(), fwiw.)

I agree that it's probably overkill to amend the prologue. No strong
opinion on whether removing the checks entirely is a good idea; I guess
they at least serve as documentation even if they're not actually called
that often?

>> I can add a patch removing the rcu_read_lock() from cls_bpf in the next
>> version.
>> 
>>> It would also be great if this scenario in general could be placed
>>> under the Documentation/RCU/whatisRCU.rst as an example, so we could
>>> refer to the official doc on this, too, if Paul is good with this.
>> 
>> I'll take a look and see if I can find a way to fit it in there...
>> 
>>> Could you also update the RCU comment in bpf_prog_run_xdp()? Or
>>> alternatively move all the below driver comments in there as a single
>>> location?
>>>
>>>     /* This code is invoked within a single NAPI poll cycle and thus under
>>>      * local_bh_disable(), which provides the needed RCU protection.
>>>      */
>> 
>> Sure, can do. And yeah, I do agree that moving the comment in there
>> makes more sense than scattering it over all the drivers, even if that
>> means I have to go back and edit all the drivers again :P
>
> Yeap, all of the above sounds good, thanks!

Cool :)

Re: [PATCH bpf-next v3 03/16] xdp: add proper __rcu annotations to redirect map entries

[Daniel Borkmann]

On 6/22/21 12:35 AM, Toke Høiland-Jørgensen wrote:
> Daniel Borkmann <daniel@iogearbox.net> writes:
>> On 6/21/21 11:39 PM, Toke Høiland-Jørgensen wrote:
>>> Daniel Borkmann <daniel@iogearbox.net> writes:
>>>> On 6/17/21 11:27 PM, Toke Høiland-Jørgensen wrote:
>>>>> XDP_REDIRECT works by a three-step process: the bpf_redirect() and
>>>>> bpf_redirect_map() helpers will lookup the target of the redirect and store
>>>>> it (along with some other metadata) in a per-CPU struct bpf_redirect_info.
>>>>> Next, when the program returns the XDP_REDIRECT return code, the driver
>>>>> will call xdp_do_redirect() which will use the information thus stored to
>>>>> actually enqueue the frame into a bulk queue structure (that differs
>>>>> slightly by map type, but shares the same principle). Finally, before
>>>>> exiting its NAPI poll loop, the driver will call xdp_do_flush(), which will
>>>>> flush all the different bulk queues, thus completing the redirect.
>>>>>
>>>>> Pointers to the map entries will be kept around for this whole sequence of
>>>>> steps, protected by RCU. However, there is no top-level rcu_read_lock() in
>>>>> the core code; instead drivers add their own rcu_read_lock() around the XDP
>>>>> portions of the code, but somewhat inconsistently as Martin discovered[0].
>>>>> However, things still work because everything happens inside a single NAPI
>>>>> poll sequence, which means it's between a pair of calls to
>>>>> local_bh_disable()/local_bh_enable(). So Paul suggested[1] that we could
>>>>> document this intention by using rcu_dereference_check() with
>>>>> rcu_read_lock_bh_held() as a second parameter, thus allowing sparse and
>>>>> lockdep to verify that everything is done correctly.
>>>>>
>>>>> This patch does just that: we add an __rcu annotation to the map entry
>>>>> pointers and remove the various comments explaining the NAPI poll assurance
>>>>> strewn through devmap.c in favour of a longer explanation in filter.c. The
>>>>> goal is to have one coherent documentation of the entire flow, and rely on
>>>>> the RCU annotations as a "standard" way of communicating the flow in the
>>>>> map code (which can additionally be understood by sparse and lockdep).
>>>>>
>>>>> The RCU annotation replacements result in a fairly straight-forward
>>>>> replacement where READ_ONCE() becomes rcu_dereference_check(), WRITE_ONCE()
>>>>> becomes rcu_assign_pointer() and xchg() and cmpxchg() gets wrapped in the
>>>>> proper constructs to cast the pointer back and forth between __rcu and
>>>>> __kernel address space (for the benefit of sparse). The one complication is
>>>>> that xskmap has a few constructions where double-pointers are passed back
>>>>> and forth; these simply all gain __rcu annotations, and only the final
>>>>> reference/dereference to the inner-most pointer gets changed.
>>>>>
>>>>> With this, everything can be run through sparse without eliciting
>>>>> complaints, and lockdep can verify correctness even without the use of
>>>>> rcu_read_lock() in the drivers. Subsequent patches will clean these up from
>>>>> the drivers.
>>>>>
>>>>> [0] https://lore.kernel.org/bpf/20210415173551.7ma4slcbqeyiba2r@kafai-mbp.dhcp.thefacebook.com/
>>>>> [1] https://lore.kernel.org/bpf/20210419165837.GA975577@paulmck-ThinkPad-P17-Gen-1/
>>>>>
>>>>> Signed-off-by: Toke Høiland-Jørgensen <toke@redhat.com>
>>>>> ---
>>>>>     include/net/xdp_sock.h |  2 +-
>>>>>     kernel/bpf/cpumap.c    | 13 +++++++----
>>>>>     kernel/bpf/devmap.c    | 49 ++++++++++++++++++------------------------
>>>>>     net/core/filter.c      | 28 ++++++++++++++++++++++++
>>>>>     net/xdp/xsk.c          |  4 ++--
>>>>>     net/xdp/xsk.h          |  4 ++--
>>>>>     net/xdp/xskmap.c       | 29 ++++++++++++++-----------
>>>>>     7 files changed, 80 insertions(+), 49 deletions(-)
>>>> [...]
>>>>>     						 __dev_map_entry_free);
>>>>> diff --git a/net/core/filter.c b/net/core/filter.c
>>>>> index caa88955562e..0b7db5c70385 100644
>>>>> --- a/net/core/filter.c
>>>>> +++ b/net/core/filter.c
>>>>> @@ -3922,6 +3922,34 @@ static const struct bpf_func_proto bpf_xdp_adjust_meta_proto = {
>>>>>     	.arg2_type	= ARG_ANYTHING,
>>>>>     };
>>>>>     
>>>>> +/* XDP_REDIRECT works by a three-step process, implemented in the functions
>>>>> + * below:
>>>>> + *
>>>>> + * 1. The bpf_redirect() and bpf_redirect_map() helpers will lookup the target
>>>>> + *    of the redirect and store it (along with some other metadata) in a per-CPU
>>>>> + *    struct bpf_redirect_info.
>>>>> + *
>>>>> + * 2. When the program returns the XDP_REDIRECT return code, the driver will
>>>>> + *    call xdp_do_redirect() which will use the information in struct
>>>>> + *    bpf_redirect_info to actually enqueue the frame into a map type-specific
>>>>> + *    bulk queue structure.
>>>>> + *
>>>>> + * 3. Before exiting its NAPI poll loop, the driver will call xdp_do_flush(),
>>>>> + *    which will flush all the different bulk queues, thus completing the
>>>>> + *    redirect.
>>>>> + *
>>>>> + * Pointers to the map entries will be kept around for this whole sequence of
>>>>> + * steps, protected by RCU. However, there is no top-level rcu_read_lock() in
>>>>> + * the core code; instead, the RCU protection relies on everything happening
>>>>> + * inside a single NAPI poll sequence, which means it's between a pair of calls
>>>>> + * to local_bh_disable()/local_bh_enable().
>>>>> + *
>>>>> + * The map entries are marked as __rcu and the map code makes sure to
>>>>> + * dereference those pointers with rcu_dereference_check() in a way that works
>>>>> + * for both sections that to hold an rcu_read_lock() and sections that are
>>>>> + * called from NAPI without a separate rcu_read_lock(). The code below does not
>>>>> + * use RCU annotations, but relies on those in the map code.
>>>>
>>>> One more follow-up question related to tc BPF: given we do use rcu_read_lock_bh()
>>>> in case of sch_handle_egress(), could we also remove the rcu_read_lock() pair
>>>> from cls_bpf_classify() then?
>>>
>>> I believe so, yeah. Patch 2 in this series should even make lockdep stop
>>> complaining about it :)
>>
>> Btw, I was wondering whether we should just get rid of all the WARN_ON_ONCE()s
>> from those map helpers given in most situations these are not triggered anyway
>> due to retpoline avoidance where verifier rewrites the calls to jump to the map
>> backend implementation directly. One alternative could be to have an extension
>> to the bpf prologue generation under CONFIG_DEBUG_LOCK_ALLOC and call the lockdep
>> checks from there, but it's probably not worth the effort. (In the trampoline
>> case we have those __bpf_prog_enter()/__bpf_prog_enter_sleepable() where the
>> latter in particular has asserts like might_fault(), fwiw.)
> 
> I agree that it's probably overkill to amend the prologue. No strong
> opinion on whether removing the checks entirely is a good idea; I guess
> they at least serve as documentation even if they're not actually called
> that often?

Ack, that's okay with me, and if we find a better solution, we can always change it
later on.

Thanks,
Daniel

Re: [PATCH bpf-next v3 03/16] xdp: add proper __rcu annotations to redirect map entries

[Toke Høiland-Jørgensen]

Toke Høiland-Jørgensen <toke@redhat.com> writes:

>> It would also be great if this scenario in general could be placed
>> under the Documentation/RCU/whatisRCU.rst as an example, so we could
>> refer to the official doc on this, too, if Paul is good with this.
>
> I'll take a look and see if I can find a way to fit it in there...

OK, I poked around in Documentation/RCU and decided that the most
natural place to put this was in checklist.rst which already talks about
local_bh_disable(), but a bit differently. Fixing that up to correspond
to what we've been discussing in this thread, and adding a mention of
XDP as a usage example, results in the patch below.

Paul, WDYT?

-Toke



diff --git a/Documentation/RCU/checklist.rst b/Documentation/RCU/checklist.rst
index 1030119294d0..e5bc93e8f9f5 100644
--- a/Documentation/RCU/checklist.rst
+++ b/Documentation/RCU/checklist.rst
@@ -226,12 +226,16 @@ over a rather long period of time, but improvements are always welcome!
 	broken kernels, and has even resulted in an exploitable security
 	issue.
 
-	One exception to this rule: rcu_read_lock() and rcu_read_unlock()
-	may be substituted for rcu_read_lock_bh() and rcu_read_unlock_bh()
-	in cases where local bottom halves are already known to be
-	disabled, for example, in irq or softirq context.  Commenting
-	such cases is a must, of course!  And the jury is still out on
-	whether the increased speed is worth it.
+	One exception to this rule: a pair of local_bh_disable() /
+	local_bh_enable() calls function like one big RCU read-side critical
+	section, so separate rcu_read_lock()s can be omitted in cases where
+	local bottom halves are already known to be disabled, for example, in
+	irq or softirq context. Commenting such cases is a must, of course!
+	One notable example of this usage is the XDP feature in networking,
+	which calls BPF programs from network-driver NAPI (softirq) context.
+	BPF relies heavily on RCU protection for its data structures, but
+	because the BPF program invocation happens entirely within a single
+	local_bh_disable() section in a NAPI poll cycle, this usage is safe.
 
 8.	Although synchronize_rcu() is slower than is call_rcu(), it
 	usually results in simpler code.  So, unless update performance is

Re: [PATCH bpf-next v3 03/16] xdp: add proper __rcu annotations to redirect map entries

[Paul E. McKenney]

On Tue, Jun 22, 2021 at 03:55:25PM +0200, Toke Høiland-Jørgensen wrote:
> Toke Høiland-Jørgensen <toke@redhat.com> writes:
> 
> >> It would also be great if this scenario in general could be placed
> >> under the Documentation/RCU/whatisRCU.rst as an example, so we could
> >> refer to the official doc on this, too, if Paul is good with this.
> >
> > I'll take a look and see if I can find a way to fit it in there...
> 
> OK, I poked around in Documentation/RCU and decided that the most
> natural place to put this was in checklist.rst which already talks about
> local_bh_disable(), but a bit differently. Fixing that up to correspond
> to what we've been discussing in this thread, and adding a mention of
> XDP as a usage example, results in the patch below.
> 
> Paul, WDYT?

I think that my original paragraph needed to have been updated back
when v4.20 came out.  And again when RCU Tasks Trace came out.  ;-)

So I did that updating, then approximated your patch on top of it,
as shown below.  Does this work for you?

							Thanx, Paul

------------------------------------------------------------------------

commit c6ef58907d22f4f327f1e9a637b50a5899aac450
Author: Toke Høiland-Jørgensen <toke@redhat.com>
Date:   Tue Jun 22 11:54:34 2021 -0700

    doc: Give XDP as example of non-obvious RCU reader/updater pairing
    
    This commit gives an example of non-obvious RCU reader/updater pairing
    in the guise of the XDP feature in networking, which calls BPF programs
    from network-driver NAPI (softirq) context.
    
    Signed-off-by: Toke Høiland-Jørgensen <toke@redhat.com>
    Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

diff --git a/Documentation/RCU/checklist.rst b/Documentation/RCU/checklist.rst
index 4df78f8bd700..f4545b7c9a63 100644
--- a/Documentation/RCU/checklist.rst
+++ b/Documentation/RCU/checklist.rst
@@ -236,8 +236,15 @@ over a rather long period of time, but improvements are always welcome!
 
 	Mixing things up will result in confusion and broken kernels, and
 	has even resulted in an exploitable security issue.  Therefore,
-	when using non-obvious pairs of primitives, commenting is of
-	course a must.
+	when using non-obvious pairs of primitives, commenting is
+	of course a must.  One example of non-obvious pairing is
+	the XDP feature in networking, which calls BPF programs from
+	network-driver NAPI (softirq) context.	BPF relies heavily on RCU
+	protection for its data structures, but because the BPF program
+	invocation happens entirely within a single local_bh_disable()
+	section in a NAPI poll cycle, this usage is safe.  The reason
+	that this usage is safe is that readers can use anything that
+	disables BH when updaters use call_rcu() or synchronize_rcu().
 
 8.	Although synchronize_rcu() is slower than is call_rcu(), it
 	usually results in simpler code.  So, unless update performance is

Re: [PATCH bpf-next v3 03/16] xdp: add proper __rcu annotations to redirect map entries

[Toke Høiland-Jørgensen]

"Paul E. McKenney" <paulmck@kernel.org> writes:

> On Tue, Jun 22, 2021 at 03:55:25PM +0200, Toke Høiland-Jørgensen wrote:
>> Toke Høiland-Jørgensen <toke@redhat.com> writes:
>> 
>> >> It would also be great if this scenario in general could be placed
>> >> under the Documentation/RCU/whatisRCU.rst as an example, so we could
>> >> refer to the official doc on this, too, if Paul is good with this.
>> >
>> > I'll take a look and see if I can find a way to fit it in there...
>> 
>> OK, I poked around in Documentation/RCU and decided that the most
>> natural place to put this was in checklist.rst which already talks about
>> local_bh_disable(), but a bit differently. Fixing that up to correspond
>> to what we've been discussing in this thread, and adding a mention of
>> XDP as a usage example, results in the patch below.
>> 
>> Paul, WDYT?
>
> I think that my original paragraph needed to have been updated back
> when v4.20 came out.  And again when RCU Tasks Trace came out.  ;-)
>
> So I did that updating, then approximated your patch on top of it,
> as shown below.  Does this work for you?

Yup, LGTM, thanks! Shall I just fold that version into the next version
of my series, or do you want to take it through your tree (I suppose
it's independent of the rest, so either way is fine by me)?

-Toke

Re: [PATCH bpf-next v3 03/16] xdp: add proper __rcu annotations to redirect map entries

[Paul E. McKenney]

On Tue, Jun 22, 2021 at 11:48:26PM +0200, Toke Høiland-Jørgensen wrote:
> "Paul E. McKenney" <paulmck@kernel.org> writes:
> 
> > On Tue, Jun 22, 2021 at 03:55:25PM +0200, Toke Høiland-Jørgensen wrote:
> >> Toke Høiland-Jørgensen <toke@redhat.com> writes:
> >> 
> >> >> It would also be great if this scenario in general could be placed
> >> >> under the Documentation/RCU/whatisRCU.rst as an example, so we could
> >> >> refer to the official doc on this, too, if Paul is good with this.
> >> >
> >> > I'll take a look and see if I can find a way to fit it in there...
> >> 
> >> OK, I poked around in Documentation/RCU and decided that the most
> >> natural place to put this was in checklist.rst which already talks about
> >> local_bh_disable(), but a bit differently. Fixing that up to correspond
> >> to what we've been discussing in this thread, and adding a mention of
> >> XDP as a usage example, results in the patch below.
> >> 
> >> Paul, WDYT?
> >
> > I think that my original paragraph needed to have been updated back
> > when v4.20 came out.  And again when RCU Tasks Trace came out.  ;-)
> >
> > So I did that updating, then approximated your patch on top of it,
> > as shown below.  Does this work for you?
> 
> Yup, LGTM, thanks! Shall I just fold that version into the next version
> of my series, or do you want to take it through your tree (I suppose
> it's independent of the rest, so either way is fine by me)?

I currently have the two here in -rcu, most likely for v5.15 (as in
the merge window after the upcoming one):

2b7cb9d95ba4 ("doc: Clarify and expand RCU updaters and corresponding readers")
c6ef58907d22 ("doc: Give XDP as example of non-obvious RCU reader/updater pairing")

I am happy taking it, but if you really would like to add it to your
series, please do take both.  ;-)

							Thanx, Paul

Re: [PATCH bpf-next v3 03/16] xdp: add proper __rcu annotations to redirect map entries

[Toke Høiland-Jørgensen]

"Paul E. McKenney" <paulmck@kernel.org> writes:

> On Tue, Jun 22, 2021 at 11:48:26PM +0200, Toke Høiland-Jørgensen wrote:
>> "Paul E. McKenney" <paulmck@kernel.org> writes:
>> 
>> > On Tue, Jun 22, 2021 at 03:55:25PM +0200, Toke Høiland-Jørgensen wrote:
>> >> Toke Høiland-Jørgensen <toke@redhat.com> writes:
>> >> 
>> >> >> It would also be great if this scenario in general could be placed
>> >> >> under the Documentation/RCU/whatisRCU.rst as an example, so we could
>> >> >> refer to the official doc on this, too, if Paul is good with this.
>> >> >
>> >> > I'll take a look and see if I can find a way to fit it in there...
>> >> 
>> >> OK, I poked around in Documentation/RCU and decided that the most
>> >> natural place to put this was in checklist.rst which already talks about
>> >> local_bh_disable(), but a bit differently. Fixing that up to correspond
>> >> to what we've been discussing in this thread, and adding a mention of
>> >> XDP as a usage example, results in the patch below.
>> >> 
>> >> Paul, WDYT?
>> >
>> > I think that my original paragraph needed to have been updated back
>> > when v4.20 came out.  And again when RCU Tasks Trace came out.  ;-)
>> >
>> > So I did that updating, then approximated your patch on top of it,
>> > as shown below.  Does this work for you?
>> 
>> Yup, LGTM, thanks! Shall I just fold that version into the next version
>> of my series, or do you want to take it through your tree (I suppose
>> it's independent of the rest, so either way is fine by me)?
>
> I currently have the two here in -rcu, most likely for v5.15 (as in
> the merge window after the upcoming one):
>
> 2b7cb9d95ba4 ("doc: Clarify and expand RCU updaters and corresponding readers")
> c6ef58907d22 ("doc: Give XDP as example of non-obvious RCU reader/updater pairing")
>
> I am happy taking it, but if you really would like to add it to your
> series, please do take both.  ;-)

Alright, I'll fold both in to v4 :)

-Toke