* [bpf PATCH 0/2] xdp devmap improvements cleanup
@ 2020-01-08 21:34 John Fastabend
2020-01-08 21:34 ` [bpf PATCH 1/2] bpf: xdp, update devmap comments to reflect napi/rcu usage John Fastabend
` (2 more replies)
0 siblings, 3 replies; 11+ messages in thread
From: John Fastabend @ 2020-01-08 21:34 UTC (permalink / raw)
To: bjorn.topel, bpf, toke; +Cc: netdev, john.fastabend, ast, daniel
Couple cleanup patches to recently posted series[0] from Bjorn to
cleanup and optimize the devmap usage. Patches have commit ids
the cleanup applies to.
[0] https://www.spinics.net/lists/netdev/msg620639.html
---
John Fastabend (2):
bpf: xdp, update devmap comments to reflect napi/rcu usage
bpf: xdp, remove no longer required rcu_read_{un}lock()
kernel/bpf/devmap.c | 23 +++++++++++------------
1 file changed, 11 insertions(+), 12 deletions(-)
--
Signature
^ permalink raw reply [flat|nested] 11+ messages in thread
* [bpf PATCH 1/2] bpf: xdp, update devmap comments to reflect napi/rcu usage
2020-01-08 21:34 [bpf PATCH 0/2] xdp devmap improvements cleanup John Fastabend
@ 2020-01-08 21:34 ` John Fastabend
2020-01-09 1:20 ` Song Liu
2020-01-08 21:35 ` [bpf PATCH 2/2] bpf: xdp, remove no longer required rcu_read_{un}lock() John Fastabend
2020-01-09 6:09 ` [bpf PATCH 0/2] xdp devmap improvements cleanup Björn Töpel
2 siblings, 1 reply; 11+ messages in thread
From: John Fastabend @ 2020-01-08 21:34 UTC (permalink / raw)
To: bjorn.topel, bpf, toke; +Cc: netdev, john.fastabend, ast, daniel
Now that we rely on synchronize_rcu and call_rcu waiting to
exit perempt-disable regions (NAPI) lets update the comments
to reflect this.
Fixes: 0536b85239b84 ("xdp: Simplify devmap cleanup")
Signed-off-by: John Fastabend <john.fastabend@gmail.com>
---
kernel/bpf/devmap.c | 21 +++++++++++----------
1 file changed, 11 insertions(+), 10 deletions(-)
diff --git a/kernel/bpf/devmap.c b/kernel/bpf/devmap.c
index da9c832..f0bf525 100644
--- a/kernel/bpf/devmap.c
+++ b/kernel/bpf/devmap.c
@@ -193,10 +193,12 @@ static void dev_map_free(struct bpf_map *map)
/* At this point bpf_prog->aux->refcnt == 0 and this map->refcnt == 0,
* so the programs (can be more than one that used this map) were
- * disconnected from events. Wait for outstanding critical sections in
- * these programs to complete. The rcu critical section only guarantees
- * no further reads against netdev_map. It does __not__ ensure pending
- * flush operations (if any) are complete.
+ * disconnected from events. The following synchronize_rcu() guarantees
+ * both rcu read critical sections complete and waits for
+ * preempt-disable regions (NAPI being the relavent context here) so we
+ * are certain there will be no further reads against the netdev_map and
+ * all flush operations are complete. Flush operations can only be done
+ * from NAPI context for this reason.
*/
spin_lock(&dev_map_lock);
@@ -498,12 +500,11 @@ static int dev_map_delete_elem(struct bpf_map *map, void *key)
return -EINVAL;
/* Use call_rcu() here to ensure any rcu critical sections have
- * completed, but this does not guarantee a flush has happened
- * yet. Because driver side rcu_read_lock/unlock only protects the
- * running XDP program. However, for pending flush operations the
- * dev and ctx are stored in another per cpu map. 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.
+ * 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);
if (old_dev)
^ permalink raw reply related [flat|nested] 11+ messages in thread
* [bpf PATCH 2/2] bpf: xdp, remove no longer required rcu_read_{un}lock()
2020-01-08 21:34 [bpf PATCH 0/2] xdp devmap improvements cleanup John Fastabend
2020-01-08 21:34 ` [bpf PATCH 1/2] bpf: xdp, update devmap comments to reflect napi/rcu usage John Fastabend
@ 2020-01-08 21:35 ` John Fastabend
2020-01-09 1:21 ` Song Liu
2020-01-09 6:01 ` Toshiaki Makita
2020-01-09 6:09 ` [bpf PATCH 0/2] xdp devmap improvements cleanup Björn Töpel
2 siblings, 2 replies; 11+ messages in thread
From: John Fastabend @ 2020-01-08 21:35 UTC (permalink / raw)
To: bjorn.topel, bpf, toke; +Cc: netdev, john.fastabend, ast, daniel
Now that we depend on rcu_call() and synchronize_rcu() to also wait
for preempt_disabled region to complete the rcu read critical section
in __dev_map_flush() is no longer relevant.
These originally ensured the map reference was safe while a map was
also being free'd. But flush by new rules can only be called from
preempt-disabled NAPI context. The synchronize_rcu from the map free
path and the rcu_call from the delete path will ensure the reference
here is safe. So lets remove the rcu_read_lock and rcu_read_unlock
pair to avoid any confusion around how this is being protected.
If the rcu_read_lock was required it would mean errors in the above
logic and the original patch would also be wrong.
Fixes: 0536b85239b84 ("xdp: Simplify devmap cleanup")
Signed-off-by: John Fastabend <john.fastabend@gmail.com>
---
kernel/bpf/devmap.c | 2 --
1 file changed, 2 deletions(-)
diff --git a/kernel/bpf/devmap.c b/kernel/bpf/devmap.c
index f0bf525..0129d4a 100644
--- a/kernel/bpf/devmap.c
+++ b/kernel/bpf/devmap.c
@@ -378,10 +378,8 @@ void __dev_map_flush(void)
struct list_head *flush_list = this_cpu_ptr(&dev_map_flush_list);
struct xdp_bulk_queue *bq, *tmp;
- rcu_read_lock();
list_for_each_entry_safe(bq, tmp, flush_list, flush_node)
bq_xmit_all(bq, XDP_XMIT_FLUSH);
- rcu_read_unlock();
}
/* rcu_read_lock (from syscall and BPF contexts) ensures that if a delete and/or
^ permalink raw reply related [flat|nested] 11+ messages in thread
* Re: [bpf PATCH 1/2] bpf: xdp, update devmap comments to reflect napi/rcu usage
2020-01-08 21:34 ` [bpf PATCH 1/2] bpf: xdp, update devmap comments to reflect napi/rcu usage John Fastabend
@ 2020-01-09 1:20 ` Song Liu
0 siblings, 0 replies; 11+ messages in thread
From: Song Liu @ 2020-01-09 1:20 UTC (permalink / raw)
To: John Fastabend
Cc: Björn Töpel, bpf, Toke Høiland-Jørgensen,
Networking, Alexei Starovoitov, Daniel Borkmann
On Wed, Jan 8, 2020 at 1:36 PM John Fastabend <john.fastabend@gmail.com> wrote:
>
> Now that we rely on synchronize_rcu and call_rcu waiting to
> exit perempt-disable regions (NAPI) lets update the comments
> to reflect this.
>
> Fixes: 0536b85239b84 ("xdp: Simplify devmap cleanup")
> Signed-off-by: John Fastabend <john.fastabend@gmail.com>
Acked-by: Song Liu <songliubraving@fb.com>
^ permalink raw reply [flat|nested] 11+ messages in thread
* Re: [bpf PATCH 2/2] bpf: xdp, remove no longer required rcu_read_{un}lock()
2020-01-08 21:35 ` [bpf PATCH 2/2] bpf: xdp, remove no longer required rcu_read_{un}lock() John Fastabend
@ 2020-01-09 1:21 ` Song Liu
2020-01-09 6:01 ` Toshiaki Makita
1 sibling, 0 replies; 11+ messages in thread
From: Song Liu @ 2020-01-09 1:21 UTC (permalink / raw)
To: John Fastabend
Cc: Björn Töpel, bpf, Toke Høiland-Jørgensen,
Networking, Alexei Starovoitov, Daniel Borkmann
On Wed, Jan 8, 2020 at 1:36 PM John Fastabend <john.fastabend@gmail.com> wrote:
>
> Now that we depend on rcu_call() and synchronize_rcu() to also wait
> for preempt_disabled region to complete the rcu read critical section
> in __dev_map_flush() is no longer relevant.
>
> These originally ensured the map reference was safe while a map was
> also being free'd. But flush by new rules can only be called from
> preempt-disabled NAPI context. The synchronize_rcu from the map free
> path and the rcu_call from the delete path will ensure the reference
> here is safe. So lets remove the rcu_read_lock and rcu_read_unlock
> pair to avoid any confusion around how this is being protected.
>
> If the rcu_read_lock was required it would mean errors in the above
> logic and the original patch would also be wrong.
>
> Fixes: 0536b85239b84 ("xdp: Simplify devmap cleanup")
> Signed-off-by: John Fastabend <john.fastabend@gmail.com>
Acked-by: Song Liu <songliubraving@fb.com>
^ permalink raw reply [flat|nested] 11+ messages in thread
* Re: [bpf PATCH 2/2] bpf: xdp, remove no longer required rcu_read_{un}lock()
2020-01-08 21:35 ` [bpf PATCH 2/2] bpf: xdp, remove no longer required rcu_read_{un}lock() John Fastabend
2020-01-09 1:21 ` Song Liu
@ 2020-01-09 6:01 ` Toshiaki Makita
2020-01-09 6:34 ` Toshiaki Makita
2020-01-09 6:35 ` John Fastabend
1 sibling, 2 replies; 11+ messages in thread
From: Toshiaki Makita @ 2020-01-09 6:01 UTC (permalink / raw)
To: John Fastabend, bjorn.topel, bpf, toke; +Cc: netdev, ast, daniel
On 2020/01/09 6:35, John Fastabend wrote:
> Now that we depend on rcu_call() and synchronize_rcu() to also wait
> for preempt_disabled region to complete the rcu read critical section
> in __dev_map_flush() is no longer relevant.
>
> These originally ensured the map reference was safe while a map was
> also being free'd. But flush by new rules can only be called from
> preempt-disabled NAPI context. The synchronize_rcu from the map free
> path and the rcu_call from the delete path will ensure the reference
> here is safe. So lets remove the rcu_read_lock and rcu_read_unlock
> pair to avoid any confusion around how this is being protected.
>
> If the rcu_read_lock was required it would mean errors in the above
> logic and the original patch would also be wrong.
>
> Fixes: 0536b85239b84 ("xdp: Simplify devmap cleanup")
> Signed-off-by: John Fastabend <john.fastabend@gmail.com>
> ---
> kernel/bpf/devmap.c | 2 --
> 1 file changed, 2 deletions(-)
>
> diff --git a/kernel/bpf/devmap.c b/kernel/bpf/devmap.c
> index f0bf525..0129d4a 100644
> --- a/kernel/bpf/devmap.c
> +++ b/kernel/bpf/devmap.c
> @@ -378,10 +378,8 @@ void __dev_map_flush(void)
> struct list_head *flush_list = this_cpu_ptr(&dev_map_flush_list);
> struct xdp_bulk_queue *bq, *tmp;
>
> - rcu_read_lock();
> list_for_each_entry_safe(bq, tmp, flush_list, flush_node)
> bq_xmit_all(bq, XDP_XMIT_FLUSH);
> - rcu_read_unlock();
I introduced this lock because some drivers have assumption that
.ndo_xdp_xmit() is called under RCU. (commit 86723c864063)
Maybe devmap deletion logic does not need this anymore, but is it
OK to drivers?
Toshiaki Makita
^ permalink raw reply [flat|nested] 11+ messages in thread
* Re: [bpf PATCH 0/2] xdp devmap improvements cleanup
2020-01-08 21:34 [bpf PATCH 0/2] xdp devmap improvements cleanup John Fastabend
2020-01-08 21:34 ` [bpf PATCH 1/2] bpf: xdp, update devmap comments to reflect napi/rcu usage John Fastabend
2020-01-08 21:35 ` [bpf PATCH 2/2] bpf: xdp, remove no longer required rcu_read_{un}lock() John Fastabend
@ 2020-01-09 6:09 ` Björn Töpel
2 siblings, 0 replies; 11+ messages in thread
From: Björn Töpel @ 2020-01-09 6:09 UTC (permalink / raw)
To: John Fastabend
Cc: bpf, Toke Høiland-Jørgensen, Netdev,
Alexei Starovoitov, Daniel Borkmann
On Wed, 8 Jan 2020 at 22:34, John Fastabend <john.fastabend@gmail.com> wrote:
>
> Couple cleanup patches to recently posted series[0] from Bjorn to
> cleanup and optimize the devmap usage. Patches have commit ids
> the cleanup applies to.
>
> [0] https://www.spinics.net/lists/netdev/msg620639.html
>
> ---
>
> John Fastabend (2):
> bpf: xdp, update devmap comments to reflect napi/rcu usage
> bpf: xdp, remove no longer required rcu_read_{un}lock()
>
Thanks for the clean-up, John!
For the series:
Acked-by: Björn Töpel <bjorn.topel@intel.com>
>
> kernel/bpf/devmap.c | 23 +++++++++++------------
> 1 file changed, 11 insertions(+), 12 deletions(-)
>
> --
> Signature
^ permalink raw reply [flat|nested] 11+ messages in thread
* Re: [bpf PATCH 2/2] bpf: xdp, remove no longer required rcu_read_{un}lock()
2020-01-09 6:01 ` Toshiaki Makita
@ 2020-01-09 6:34 ` Toshiaki Makita
2020-01-09 6:35 ` John Fastabend
1 sibling, 0 replies; 11+ messages in thread
From: Toshiaki Makita @ 2020-01-09 6:34 UTC (permalink / raw)
To: John Fastabend, bjorn.topel, bpf, toke; +Cc: netdev, ast, daniel
On 2020/01/09 15:01, Toshiaki Makita wrote:
> On 2020/01/09 6:35, John Fastabend wrote:
>> Now that we depend on rcu_call() and synchronize_rcu() to also wait
>> for preempt_disabled region to complete the rcu read critical section
>> in __dev_map_flush() is no longer relevant.
>>
>> These originally ensured the map reference was safe while a map was
>> also being free'd. But flush by new rules can only be called from
>> preempt-disabled NAPI context. The synchronize_rcu from the map free
>> path and the rcu_call from the delete path will ensure the reference
>> here is safe. So lets remove the rcu_read_lock and rcu_read_unlock
>> pair to avoid any confusion around how this is being protected.
>>
>> If the rcu_read_lock was required it would mean errors in the above
>> logic and the original patch would also be wrong.
>>
>> Fixes: 0536b85239b84 ("xdp: Simplify devmap cleanup")
>> Signed-off-by: John Fastabend <john.fastabend@gmail.com>
>> ---
>> kernel/bpf/devmap.c | 2 --
>> 1 file changed, 2 deletions(-)
>>
>> diff --git a/kernel/bpf/devmap.c b/kernel/bpf/devmap.c
>> index f0bf525..0129d4a 100644
>> --- a/kernel/bpf/devmap.c
>> +++ b/kernel/bpf/devmap.c
>> @@ -378,10 +378,8 @@ void __dev_map_flush(void)
>> struct list_head *flush_list = this_cpu_ptr(&dev_map_flush_list);
>> struct xdp_bulk_queue *bq, *tmp;
>> - rcu_read_lock();
>> list_for_each_entry_safe(bq, tmp, flush_list, flush_node)
>> bq_xmit_all(bq, XDP_XMIT_FLUSH);
>> - rcu_read_unlock();
>
> I introduced this lock because some drivers have assumption that
> .ndo_xdp_xmit() is called under RCU. (commit 86723c864063)
>
> Maybe devmap deletion logic does not need this anymore, but is it
> OK to drivers?
More references for the driver problem:
- Lockdep splat in virtio_net: https://lists.openwall.net/netdev/2019/04/24/282
- Discussion for fix: https://lists.openwall.net/netdev/2019/04/25/234
Toshiaki Makita
^ permalink raw reply [flat|nested] 11+ messages in thread
* Re: [bpf PATCH 2/2] bpf: xdp, remove no longer required rcu_read_{un}lock()
2020-01-09 6:01 ` Toshiaki Makita
2020-01-09 6:34 ` Toshiaki Makita
@ 2020-01-09 6:35 ` John Fastabend
2020-01-09 6:57 ` Toshiaki Makita
1 sibling, 1 reply; 11+ messages in thread
From: John Fastabend @ 2020-01-09 6:35 UTC (permalink / raw)
To: Toshiaki Makita, John Fastabend, bjorn.topel, bpf, toke
Cc: netdev, ast, daniel
Toshiaki Makita wrote:
> On 2020/01/09 6:35, John Fastabend wrote:
> > Now that we depend on rcu_call() and synchronize_rcu() to also wait
> > for preempt_disabled region to complete the rcu read critical section
> > in __dev_map_flush() is no longer relevant.
> >
> > These originally ensured the map reference was safe while a map was
> > also being free'd. But flush by new rules can only be called from
> > preempt-disabled NAPI context. The synchronize_rcu from the map free
> > path and the rcu_call from the delete path will ensure the reference
> > here is safe. So lets remove the rcu_read_lock and rcu_read_unlock
> > pair to avoid any confusion around how this is being protected.
> >
> > If the rcu_read_lock was required it would mean errors in the above
> > logic and the original patch would also be wrong.
> >
> > Fixes: 0536b85239b84 ("xdp: Simplify devmap cleanup")
> > Signed-off-by: John Fastabend <john.fastabend@gmail.com>
> > ---
> > kernel/bpf/devmap.c | 2 --
> > 1 file changed, 2 deletions(-)
> >
> > diff --git a/kernel/bpf/devmap.c b/kernel/bpf/devmap.c
> > index f0bf525..0129d4a 100644
> > --- a/kernel/bpf/devmap.c
> > +++ b/kernel/bpf/devmap.c
> > @@ -378,10 +378,8 @@ void __dev_map_flush(void)
> > struct list_head *flush_list = this_cpu_ptr(&dev_map_flush_list);
> > struct xdp_bulk_queue *bq, *tmp;
> >
> > - rcu_read_lock();
> > list_for_each_entry_safe(bq, tmp, flush_list, flush_node)
> > bq_xmit_all(bq, XDP_XMIT_FLUSH);
> > - rcu_read_unlock();
>
> I introduced this lock because some drivers have assumption that
> .ndo_xdp_xmit() is called under RCU. (commit 86723c864063)
>
> Maybe devmap deletion logic does not need this anymore, but is it
> OK to drivers?
Ah OK thanks for catching this. So its a strange requirement from
virto_net to need read_lock like this. Quickly scanned the drivers
and seems its the only one.
I think the best path forward is to fix virtio_net so it doesn't
need rcu_read_lock() here then the locking is much cleaner IMO.
I'll send a v2 and either move the xdp enabled check (the piece
using the rcu_read_lock) into a bitmask flag or push the
rcu_read_lock() into virtio_net so its clear that this is a detail
of virtio_net and not a general thing. FWIW I don't think the
rcu_read_lock is actually needed in the virtio_net case anymore
either but pretty sure the rcu_dereference will cause an rcu
splat. Maybe there is another annotation we can use. I'll dig
into it tomorrow. Thanks
>
> Toshiaki Makita
^ permalink raw reply [flat|nested] 11+ messages in thread
* Re: [bpf PATCH 2/2] bpf: xdp, remove no longer required rcu_read_{un}lock()
2020-01-09 6:35 ` John Fastabend
@ 2020-01-09 6:57 ` Toshiaki Makita
2020-01-09 14:58 ` John Fastabend
0 siblings, 1 reply; 11+ messages in thread
From: Toshiaki Makita @ 2020-01-09 6:57 UTC (permalink / raw)
To: John Fastabend, bjorn.topel, bpf, toke; +Cc: netdev, ast, daniel
On 2020/01/09 15:35, John Fastabend wrote:
> Toshiaki Makita wrote:
>> On 2020/01/09 6:35, John Fastabend wrote:
>>> Now that we depend on rcu_call() and synchronize_rcu() to also wait
>>> for preempt_disabled region to complete the rcu read critical section
>>> in __dev_map_flush() is no longer relevant.
>>>
>>> These originally ensured the map reference was safe while a map was
>>> also being free'd. But flush by new rules can only be called from
>>> preempt-disabled NAPI context. The synchronize_rcu from the map free
>>> path and the rcu_call from the delete path will ensure the reference
>>> here is safe. So lets remove the rcu_read_lock and rcu_read_unlock
>>> pair to avoid any confusion around how this is being protected.
>>>
>>> If the rcu_read_lock was required it would mean errors in the above
>>> logic and the original patch would also be wrong.
>>>
>>> Fixes: 0536b85239b84 ("xdp: Simplify devmap cleanup")
>>> Signed-off-by: John Fastabend <john.fastabend@gmail.com>
>>> ---
>>> kernel/bpf/devmap.c | 2 --
>>> 1 file changed, 2 deletions(-)
>>>
>>> diff --git a/kernel/bpf/devmap.c b/kernel/bpf/devmap.c
>>> index f0bf525..0129d4a 100644
>>> --- a/kernel/bpf/devmap.c
>>> +++ b/kernel/bpf/devmap.c
>>> @@ -378,10 +378,8 @@ void __dev_map_flush(void)
>>> struct list_head *flush_list = this_cpu_ptr(&dev_map_flush_list);
>>> struct xdp_bulk_queue *bq, *tmp;
>>>
>>> - rcu_read_lock();
>>> list_for_each_entry_safe(bq, tmp, flush_list, flush_node)
>>> bq_xmit_all(bq, XDP_XMIT_FLUSH);
>>> - rcu_read_unlock();
>>
>> I introduced this lock because some drivers have assumption that
>> .ndo_xdp_xmit() is called under RCU. (commit 86723c864063)
>>
>> Maybe devmap deletion logic does not need this anymore, but is it
>> OK to drivers?
>
> Ah OK thanks for catching this. So its a strange requirement from
> virto_net to need read_lock like this. Quickly scanned the drivers
> and seems its the only one.
>
> I think the best path forward is to fix virtio_net so it doesn't
> need rcu_read_lock() here then the locking is much cleaner IMO.
Actually veth is calling rcu_dereference in .ndo_xdp_xmit() so it needs
the same treatment. In the reference I sent in another mail, Jesper
said mlx5 also has some RCU dependency.
> I'll send a v2 and either move the xdp enabled check (the piece
> using the rcu_read_lock) into a bitmask flag or push the
> rcu_read_lock() into virtio_net so its clear that this is a detail
> of virtio_net and not a general thing. FWIW I don't think the
> rcu_read_lock is actually needed in the virtio_net case anymore
> either but pretty sure the rcu_dereference will cause an rcu
> splat. Maybe there is another annotation we can use. I'll dig
> into it tomorrow. Thanks
I'm thinking we can just move the rcu_lock to wrap around only ndo_xdp_xmit.
But as you suggest if we can identify all drivers which depends on RCU and move the
rcu_lock into the drivers (or remove the dependency) it's better.
Toshiaki Makita
^ permalink raw reply [flat|nested] 11+ messages in thread
* Re: [bpf PATCH 2/2] bpf: xdp, remove no longer required rcu_read_{un}lock()
2020-01-09 6:57 ` Toshiaki Makita
@ 2020-01-09 14:58 ` John Fastabend
0 siblings, 0 replies; 11+ messages in thread
From: John Fastabend @ 2020-01-09 14:58 UTC (permalink / raw)
To: Toshiaki Makita, John Fastabend, bjorn.topel, bpf, toke, jbrouer
Cc: netdev, ast, daniel
Toshiaki Makita wrote:
> On 2020/01/09 15:35, John Fastabend wrote:
> > Toshiaki Makita wrote:
> >> On 2020/01/09 6:35, John Fastabend wrote:
> >>> Now that we depend on rcu_call() and synchronize_rcu() to also wait
> >>> for preempt_disabled region to complete the rcu read critical section
> >>> in __dev_map_flush() is no longer relevant.
> >>>
> >>> These originally ensured the map reference was safe while a map was
> >>> also being free'd. But flush by new rules can only be called from
> >>> preempt-disabled NAPI context. The synchronize_rcu from the map free
> >>> path and the rcu_call from the delete path will ensure the reference
> >>> here is safe. So lets remove the rcu_read_lock and rcu_read_unlock
> >>> pair to avoid any confusion around how this is being protected.
> >>>
> >>> If the rcu_read_lock was required it would mean errors in the above
> >>> logic and the original patch would also be wrong.
> >>>
> >>> Fixes: 0536b85239b84 ("xdp: Simplify devmap cleanup")
> >>> Signed-off-by: John Fastabend <john.fastabend@gmail.com>
> >>> ---
> >>> kernel/bpf/devmap.c | 2 --
> >>> 1 file changed, 2 deletions(-)
> >>>
> >>> diff --git a/kernel/bpf/devmap.c b/kernel/bpf/devmap.c
> >>> index f0bf525..0129d4a 100644
> >>> --- a/kernel/bpf/devmap.c
> >>> +++ b/kernel/bpf/devmap.c
> >>> @@ -378,10 +378,8 @@ void __dev_map_flush(void)
> >>> struct list_head *flush_list = this_cpu_ptr(&dev_map_flush_list);
> >>> struct xdp_bulk_queue *bq, *tmp;
> >>>
> >>> - rcu_read_lock();
> >>> list_for_each_entry_safe(bq, tmp, flush_list, flush_node)
> >>> bq_xmit_all(bq, XDP_XMIT_FLUSH);
> >>> - rcu_read_unlock();
> >>
> >> I introduced this lock because some drivers have assumption that
> >> .ndo_xdp_xmit() is called under RCU. (commit 86723c864063)
> >>
> >> Maybe devmap deletion logic does not need this anymore, but is it
> >> OK to drivers?
> >
> > Ah OK thanks for catching this. So its a strange requirement from
> > virto_net to need read_lock like this. Quickly scanned the drivers
> > and seems its the only one.
> >
> > I think the best path forward is to fix virtio_net so it doesn't
> > need rcu_read_lock() here then the locking is much cleaner IMO.
>
> Actually veth is calling rcu_dereference in .ndo_xdp_xmit() so it needs
> the same treatment. In the reference I sent in another mail, Jesper
> said mlx5 also has some RCU dependency.
So veth, virtio and tun seem to need rcu_read_lock/unlock because
they use an rcu_dereference() in the xdp_xmit path. I'll audit the
rest today.
@Jesper, recall why mlx5 would require rcu_read_lock()/unlock()
pair? I just looked at mlx5_xdp_xmit and I'm not seeing a
rcu_dereference in there so if it is required I would want
to understand why.
>
> > I'll send a v2 and either move the xdp enabled check (the piece
> > using the rcu_read_lock) into a bitmask flag or push the
> > rcu_read_lock() into virtio_net so its clear that this is a detail
> > of virtio_net and not a general thing. FWIW I don't think the
> > rcu_read_lock is actually needed in the virtio_net case anymore
> > either but pretty sure the rcu_dereference will cause an rcu
> > splat. Maybe there is another annotation we can use. I'll dig
> > into it tomorrow. Thanks
>
> I'm thinking we can just move the rcu_lock to wrap around only ndo_xdp_xmit.
> But as you suggest if we can identify all drivers which depends on RCU and move the
> rcu_lock into the drivers (or remove the dependency) it's better.
I think we are working in bpf-next tree here so it would be best
to identify the minimal set of drivers that require the read_lock
and push that into the driver. I prefer these things are a precise
so its easy to understand when reading the code. Otherwise its
really not clear without grepping through the code or walking
the git history why we wrapped this in a rcu_read_lock/unlock.
At minimum we want a comment but that feels like a big hammer
that is not needed.
Most drivers should not care about the rcu_read_lock it seems
to just be special cases in the software devices where this happens.
veth for example is dereferencing the peer netdev. tun is dereference
the tun_file. virtio_net usage seems to be arbitrary to me and
is simply used to decide if xdp is enabled but we can do that
in a cleaner way.
I'll put a v2 together today and send it out for review.
^ permalink raw reply [flat|nested] 11+ messages in thread
end of thread, other threads:[~2020-01-09 14:58 UTC | newest]
Thread overview: 11+ messages (download: mbox.gz / follow: Atom feed)
-- links below jump to the message on this page --
2020-01-08 21:34 [bpf PATCH 0/2] xdp devmap improvements cleanup John Fastabend
2020-01-08 21:34 ` [bpf PATCH 1/2] bpf: xdp, update devmap comments to reflect napi/rcu usage John Fastabend
2020-01-09 1:20 ` Song Liu
2020-01-08 21:35 ` [bpf PATCH 2/2] bpf: xdp, remove no longer required rcu_read_{un}lock() John Fastabend
2020-01-09 1:21 ` Song Liu
2020-01-09 6:01 ` Toshiaki Makita
2020-01-09 6:34 ` Toshiaki Makita
2020-01-09 6:35 ` John Fastabend
2020-01-09 6:57 ` Toshiaki Makita
2020-01-09 14:58 ` John Fastabend
2020-01-09 6:09 ` [bpf PATCH 0/2] xdp devmap improvements cleanup Björn Töpel
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