[PATCH 0/4] block: fix race between adding wbt and normal IO

[PATCH 0/4] block: fix race between adding wbt and normal IO

[Ming Lei]

Hello,

Yi reported several kernel panics on:

[16687.001777] Unable to handle kernel NULL pointer dereference at virtual address 0000000000000008
...
[16687.163549] pc : __rq_qos_track+0x38/0x60

or

[  997.690455] Unable to handle kernel NULL pointer dereference at virtual address 0000000000000020
...
[  997.850347] pc : __rq_qos_done+0x2c/0x50

Turns out it is caused by race between adding wbt and normal IO.

Fix the issue by moving wbt allocation/addition into blk_alloc_queue().


Ming Lei (4):
  block: split wbt_init() into two parts
  block: move wbt allocation into blk_alloc_queue
  block: reuse wbt_set_min_lat for setting wbt->min_lat_nsec
  block: mark queue init done at the end of blk_register_queue

 block/blk-core.c       |  6 +++++
 block/blk-mq-debugfs.c |  3 +++
 block/blk-sysfs.c      | 53 ++++++++++++++----------------------------
 block/blk-wbt.c        | 42 +++++++++++++++++++++++----------
 block/blk-wbt.h        | 14 +++++++----
 5 files changed, 66 insertions(+), 52 deletions(-)

Cc: Yi Zhang <yi.zhang@redhat.com>

-- 
2.29.2

[PATCH 1/4] block: split wbt_init() into two parts

[Ming Lei]

Split wbt_init() into wbt_alloc() and wbt_init(), and prepare for
moving wbt allocation into blk_alloc_queue().

Reported-by: Yi Zhang <yi.zhang@redhat.com>
Signed-off-by: Ming Lei <ming.lei@redhat.com>
---
 block/blk-wbt.c | 16 ++++++++++++++--
 1 file changed, 14 insertions(+), 2 deletions(-)

diff --git a/block/blk-wbt.c b/block/blk-wbt.c
index 42aed0160f86..efff1232446f 100644
--- a/block/blk-wbt.c
+++ b/block/blk-wbt.c
@@ -809,7 +809,7 @@ static struct rq_qos_ops wbt_rqos_ops = {
 #endif
 };
 
-int wbt_init(struct request_queue *q)
+static int wbt_alloc(struct request_queue *q)
 {
 	struct rq_wb *rwb;
 	int i;
@@ -832,7 +832,6 @@ int wbt_init(struct request_queue *q)
 	rwb->rqos.q = q;
 	rwb->last_comp = rwb->last_issue = jiffies;
 	rwb->win_nsec = RWB_WINDOW_NSEC;
-	rwb->enable_state = WBT_STATE_ON_DEFAULT;
 	rwb->wc = 1;
 	rwb->rq_depth.default_depth = RWB_DEF_DEPTH;
 
@@ -842,6 +841,19 @@ int wbt_init(struct request_queue *q)
 	rq_qos_add(q, &rwb->rqos);
 	blk_stat_add_callback(q, rwb->cb);
 
+	return 0;
+}
+
+int wbt_init(struct request_queue *q)
+{
+	int ret = wbt_alloc(q);
+	struct rq_wb *rwb;
+
+	if (ret)
+		return ret;
+
+	rwb = RQWB(wbt_rq_qos(q));
+	rwb->enable_state = WBT_STATE_ON_DEFAULT;
 	rwb->min_lat_nsec = wbt_default_latency_nsec(q);
 
 	wbt_queue_depth_changed(&rwb->rqos);
-- 
2.29.2

Re: [PATCH 1/4] block: split wbt_init() into two parts

[Bart Van Assche]

On 5/25/21 1:04 AM, Ming Lei wrote:
> +int wbt_init(struct request_queue *q)
> +{
> +	int ret = wbt_alloc(q);
> +	struct rq_wb *rwb;
> +
> +	if (ret)
> +		return ret;

A coding style nit: please move the 'ret = wbt_alloc(q)' assignment to a
line of its own since wbt_alloc() allocates memory. Otherwise this patch
looks good to me.

Thanks,

Bart.

[PATCH 2/4] block: move wbt allocation into blk_alloc_queue

[Ming Lei]

wbt_init() calls wbt_alloc() which adds allocated wbt instance into
q->rq_qos. This way is very dangerous because q->rq_qos is accessed in
IO fast path.

So far wbt_init() is called in the two code paths:

1) blk_register_queue(), when the block device has been exposed to
usespace, so IO may come when adding wbt into q->rq_qos

2) sysfs attribute store, in which normal IO is definitely allowed

Move wbt allocation into blk_alloc_queue() for avoiding to add wbt
instance dynamically to q->rq_qos. And before calling wbt_init(), the
wbt is disabled, so functionally it works as expected.

Reported-by: Yi Zhang <yi.zhang@redhat.com>
Signed-off-by: Ming Lei <ming.lei@redhat.com>
---
 block/blk-core.c       |  6 ++++++
 block/blk-mq-debugfs.c |  3 +++
 block/blk-sysfs.c      |  8 --------
 block/blk-wbt.c        | 21 ++++++---------------
 block/blk-wbt.h        | 10 +++++++---
 5 files changed, 22 insertions(+), 26 deletions(-)

diff --git a/block/blk-core.c b/block/blk-core.c
index 689aac2625d2..e7d9eac5d4c1 100644
--- a/block/blk-core.c
+++ b/block/blk-core.c
@@ -51,6 +51,7 @@
 #include "blk-mq-sched.h"
 #include "blk-pm.h"
 #include "blk-rq-qos.h"
+#include "blk-wbt.h"
 
 struct dentry *blk_debugfs_root;
 
@@ -579,12 +580,17 @@ struct request_queue *blk_alloc_queue(int node_id)
 	if (blkcg_init_queue(q))
 		goto fail_ref;
 
+	if (wbt_alloc(q))
+		goto fail_blkcg;
+
 	blk_queue_dma_alignment(q, 511);
 	blk_set_default_limits(&q->limits);
 	q->nr_requests = BLKDEV_MAX_RQ;
 
 	return q;
 
+fail_blkcg:
+	blkcg_exit_queue(q);
 fail_ref:
 	percpu_ref_exit(&q->q_usage_counter);
 fail_bdi:
diff --git a/block/blk-mq-debugfs.c b/block/blk-mq-debugfs.c
index 2a75bc7401df..c1790f313479 100644
--- a/block/blk-mq-debugfs.c
+++ b/block/blk-mq-debugfs.c
@@ -948,6 +948,9 @@ void blk_mq_debugfs_register_rqos(struct rq_qos *rqos)
 	struct request_queue *q = rqos->q;
 	const char *dir_name = rq_qos_id_to_name(rqos->id);
 
+	if (!q->debugfs_dir)
+		return;
+
 	if (rqos->debugfs_dir || !rqos->ops->debugfs_attrs)
 		return;
 
diff --git a/block/blk-sysfs.c b/block/blk-sysfs.c
index f89e2fc3963b..7fd4ffadcdfa 100644
--- a/block/blk-sysfs.c
+++ b/block/blk-sysfs.c
@@ -481,7 +481,6 @@ static ssize_t queue_wb_lat_show(struct request_queue *q, char *page)
 static ssize_t queue_wb_lat_store(struct request_queue *q, const char *page,
 				  size_t count)
 {
-	struct rq_qos *rqos;
 	ssize_t ret;
 	s64 val;
 
@@ -491,13 +490,6 @@ static ssize_t queue_wb_lat_store(struct request_queue *q, const char *page,
 	if (val < -1)
 		return -EINVAL;
 
-	rqos = wbt_rq_qos(q);
-	if (!rqos) {
-		ret = wbt_init(q);
-		if (ret)
-			return ret;
-	}
-
 	if (val == -1)
 		val = wbt_default_latency_nsec(q);
 	else if (val >= 0)
diff --git a/block/blk-wbt.c b/block/blk-wbt.c
index efff1232446f..5da48294e3e9 100644
--- a/block/blk-wbt.c
+++ b/block/blk-wbt.c
@@ -421,16 +421,14 @@ static void wbt_update_limits(struct rq_wb *rwb)
 u64 wbt_get_min_lat(struct request_queue *q)
 {
 	struct rq_qos *rqos = wbt_rq_qos(q);
-	if (!rqos)
-		return 0;
+
 	return RQWB(rqos)->min_lat_nsec;
 }
 
 void wbt_set_min_lat(struct request_queue *q, u64 val)
 {
 	struct rq_qos *rqos = wbt_rq_qos(q);
-	if (!rqos)
-		return;
+
 	RQWB(rqos)->min_lat_nsec = val;
 	RQWB(rqos)->enable_state = WBT_STATE_ON_MANUAL;
 	wbt_update_limits(RQWB(rqos));
@@ -637,7 +635,7 @@ void wbt_enable_default(struct request_queue *q)
 {
 	struct rq_qos *rqos = wbt_rq_qos(q);
 	/* Throttling already enabled? */
-	if (rqos)
+	if (rwb_enabled(RQWB(rqos)))
 		return;
 
 	/* Queue not registered? Maybe shutting down... */
@@ -809,7 +807,7 @@ static struct rq_qos_ops wbt_rqos_ops = {
 #endif
 };
 
-static int wbt_alloc(struct request_queue *q)
+int wbt_alloc(struct request_queue *q)
 {
 	struct rq_wb *rwb;
 	int i;
@@ -844,20 +842,13 @@ static int wbt_alloc(struct request_queue *q)
 	return 0;
 }
 
-int wbt_init(struct request_queue *q)
+void wbt_init(struct request_queue *q)
 {
-	int ret = wbt_alloc(q);
-	struct rq_wb *rwb;
-
-	if (ret)
-		return ret;
+	struct rq_wb *rwb = RQWB(wbt_rq_qos(q));
 
-	rwb = RQWB(wbt_rq_qos(q));
 	rwb->enable_state = WBT_STATE_ON_DEFAULT;
 	rwb->min_lat_nsec = wbt_default_latency_nsec(q);
 
 	wbt_queue_depth_changed(&rwb->rqos);
 	wbt_set_write_cache(q, test_bit(QUEUE_FLAG_WC, &q->queue_flags));
-
-	return 0;
 }
diff --git a/block/blk-wbt.h b/block/blk-wbt.h
index 16bdc85b8df9..de07963f3544 100644
--- a/block/blk-wbt.h
+++ b/block/blk-wbt.h
@@ -87,7 +87,8 @@ static inline unsigned int wbt_inflight(struct rq_wb *rwb)
 
 #ifdef CONFIG_BLK_WBT
 
-int wbt_init(struct request_queue *);
+int wbt_alloc(struct request_queue *q);
+void wbt_init(struct request_queue *);
 void wbt_disable_default(struct request_queue *);
 void wbt_enable_default(struct request_queue *);
 
@@ -103,9 +104,12 @@ u64 wbt_default_latency_nsec(struct request_queue *);
 static inline void wbt_track(struct request *rq, enum wbt_flags flags)
 {
 }
-static inline int wbt_init(struct request_queue *q)
+static inline int wbt_alloc(struct request_queue *q)
+{
+	return 0;
+}
+static inline void wbt_init(struct request_queue *q)
 {
-	return -EINVAL;
 }
 static inline void wbt_disable_default(struct request_queue *q)
 {
-- 
2.29.2

Re: [PATCH 2/4] block: move wbt allocation into blk_alloc_queue

[Bart Van Assche]

On 5/25/21 1:04 AM, Ming Lei wrote:
> wbt_init() calls wbt_alloc() which adds allocated wbt instance into
> q->rq_qos. This way is very dangerous because q->rq_qos is accessed in
> IO fast path.
> 
> So far wbt_init() is called in the two code paths:
> 
> 1) blk_register_queue(), when the block device has been exposed to
> usespace, so IO may come when adding wbt into q->rq_qos
> 
> 2) sysfs attribute store, in which normal IO is definitely allowed
> 
> Move wbt allocation into blk_alloc_queue() for avoiding to add wbt
> instance dynamically to q->rq_qos. And before calling wbt_init(), the
> wbt is disabled, so functionally it works as expected.

I don't like this change since it is not generic - it only helps the WBT
implementation.

All rq-qos policies call rq_qos_add() and all these policies take effect
before rq_qos_add() returns. Does the q->rq_qos list perhaps have to be
protected with RCU? Would that be sufficient to fix the crashes reported
in the cover letter?

Thanks,

Bart.

Re: [PATCH 2/4] block: move wbt allocation into blk_alloc_queue

[Ming Lei]

On Thu, Jun 03, 2021 at 05:44:00PM -0700, Bart Van Assche wrote:
> On 5/25/21 1:04 AM, Ming Lei wrote:
> > wbt_init() calls wbt_alloc() which adds allocated wbt instance into
> > q->rq_qos. This way is very dangerous because q->rq_qos is accessed in
> > IO fast path.
> > 
> > So far wbt_init() is called in the two code paths:
> > 
> > 1) blk_register_queue(), when the block device has been exposed to
> > usespace, so IO may come when adding wbt into q->rq_qos
> > 
> > 2) sysfs attribute store, in which normal IO is definitely allowed
> > 
> > Move wbt allocation into blk_alloc_queue() for avoiding to add wbt
> > instance dynamically to q->rq_qos. And before calling wbt_init(), the
> > wbt is disabled, so functionally it works as expected.
> 
> I don't like this change since it is not generic - it only helps the WBT
> implementation.

OK, actually except for wbt, the only one left is iocost which adds
rq_qos via cgroup attribute. 

> 
> All rq-qos policies call rq_qos_add() and all these policies take effect
> before rq_qos_add() returns. Does the q->rq_qos list perhaps have to be
> protected with RCU? Would that be sufficient to fix the crashes reported
> in the cover letter?

Freezing queue should be easier for providing the protection, and I will try
that approach.


Thanks,
Ming

[PATCH 3/4] block: reuse wbt_set_min_lat for setting wbt->min_lat_nsec

[Ming Lei]

We need to freeze queue when setting wbt->min_lat_nsec because
wbt enabled state can be changed, so reuse wbt_set_min_lat() for doing
that for users of wbt_init().

Meantime move wbt_enable_default() out of q->sysfs_lock, since we
don't need that lock for enabling wbt.

Reported-by: Yi Zhang <yi.zhang@redhat.com>
Signed-off-by: Ming Lei <ming.lei@redhat.com>
---
 block/blk-sysfs.c | 16 +++-------------
 block/blk-wbt.c   | 29 ++++++++++++++++++++++-------
 block/blk-wbt.h   |  4 ++--
 3 files changed, 27 insertions(+), 22 deletions(-)

diff --git a/block/blk-sysfs.c b/block/blk-sysfs.c
index 7fd4ffadcdfa..925043f926c5 100644
--- a/block/blk-sysfs.c
+++ b/block/blk-sysfs.c
@@ -498,18 +498,7 @@ static ssize_t queue_wb_lat_store(struct request_queue *q, const char *page,
 	if (wbt_get_min_lat(q) == val)
 		return count;
 
-	/*
-	 * Ensure that the queue is idled, in case the latency update
-	 * ends up either enabling or disabling wbt completely. We can't
-	 * have IO inflight if that happens.
-	 */
-	blk_mq_freeze_queue(q);
-	blk_mq_quiesce_queue(q);
-
-	wbt_set_min_lat(q, val);
-
-	blk_mq_unquiesce_queue(q);
-	blk_mq_unfreeze_queue(q);
+	wbt_set_min_lat(q, val, true);
 
 	return count;
 }
@@ -918,7 +907,6 @@ int blk_register_queue(struct gendisk *disk)
 	}
 
 	blk_queue_flag_set(QUEUE_FLAG_REGISTERED, q);
-	wbt_enable_default(q);
 	blk_throtl_register_queue(q);
 
 	/* Now everything is ready and send out KOBJ_ADD uevent */
@@ -927,6 +915,8 @@ int blk_register_queue(struct gendisk *disk)
 		kobject_uevent(&q->elevator->kobj, KOBJ_ADD);
 	mutex_unlock(&q->sysfs_lock);
 
+	wbt_enable_default(q);
+
 	ret = 0;
 unlock:
 	mutex_unlock(&q->sysfs_dir_lock);
diff --git a/block/blk-wbt.c b/block/blk-wbt.c
index 5da48294e3e9..5d2db197cce6 100644
--- a/block/blk-wbt.c
+++ b/block/blk-wbt.c
@@ -24,6 +24,7 @@
 #include <linux/slab.h>
 #include <linux/backing-dev.h>
 #include <linux/swap.h>
+#include <linux/blk-mq.h>
 
 #include "blk-wbt.h"
 #include "blk-rq-qos.h"
@@ -425,13 +426,27 @@ u64 wbt_get_min_lat(struct request_queue *q)
 	return RQWB(rqos)->min_lat_nsec;
 }
 
-void wbt_set_min_lat(struct request_queue *q, u64 val)
+void wbt_set_min_lat(struct request_queue *q, u64 val, bool manual)
 {
 	struct rq_qos *rqos = wbt_rq_qos(q);
 
+	/*
+	 * Ensure that the queue is idled, in case the latency update
+	 * ends up either enabling or disabling wbt completely. We can't
+	 * have IO inflight if that happens.
+	 */
+	blk_mq_freeze_queue(q);
+	blk_mq_quiesce_queue(q);
+
 	RQWB(rqos)->min_lat_nsec = val;
-	RQWB(rqos)->enable_state = WBT_STATE_ON_MANUAL;
+	if (manual)
+		RQWB(rqos)->enable_state = WBT_STATE_ON_MANUAL;
+	else
+		RQWB(rqos)->enable_state = WBT_STATE_ON_DEFAULT;
 	wbt_update_limits(RQWB(rqos));
+
+	blk_mq_unquiesce_queue(q);
+	blk_mq_unfreeze_queue(q);
 }
 
 
@@ -844,11 +859,11 @@ int wbt_alloc(struct request_queue *q)
 
 void wbt_init(struct request_queue *q)
 {
-	struct rq_wb *rwb = RQWB(wbt_rq_qos(q));
-
-	rwb->enable_state = WBT_STATE_ON_DEFAULT;
-	rwb->min_lat_nsec = wbt_default_latency_nsec(q);
+	struct rq_qos *rqos = wbt_rq_qos(q);
+	struct rq_wb *rwb = RQWB(rqos);
 
-	wbt_queue_depth_changed(&rwb->rqos);
+	rwb->rq_depth.queue_depth = blk_queue_depth(rqos->q);
 	wbt_set_write_cache(q, test_bit(QUEUE_FLAG_WC, &q->queue_flags));
+
+	wbt_set_min_lat(q, wbt_default_latency_nsec(q), false);
 }
diff --git a/block/blk-wbt.h b/block/blk-wbt.h
index de07963f3544..d9b643a55407 100644
--- a/block/blk-wbt.h
+++ b/block/blk-wbt.h
@@ -93,7 +93,7 @@ void wbt_disable_default(struct request_queue *);
 void wbt_enable_default(struct request_queue *);
 
 u64 wbt_get_min_lat(struct request_queue *q);
-void wbt_set_min_lat(struct request_queue *q, u64 val);
+void wbt_set_min_lat(struct request_queue *q, u64 val, bool manual);
 
 void wbt_set_write_cache(struct request_queue *, bool);
 
@@ -124,7 +124,7 @@ static inline u64 wbt_get_min_lat(struct request_queue *q)
 {
 	return 0;
 }
-static inline void wbt_set_min_lat(struct request_queue *q, u64 val)
+static inline void wbt_set_min_lat(struct request_queue *q, u64 val, bool manual)
 {
 }
 static inline u64 wbt_default_latency_nsec(struct request_queue *q)
-- 
2.29.2

[PATCH 4/4] block: mark queue init done at the end of blk_register_queue

[Ming Lei]

Mark queue init done when everything is done well in blk_register_queue(),
so that wbt_enable_default() can be run quickly without any RCU period
involved.

Also no any side effect by delaying to mark queue init done.

Reported-by: Yi Zhang <yi.zhang@redhat.com>
Signed-off-by: Ming Lei <ming.lei@redhat.com>
---
 block/blk-sysfs.c | 29 +++++++++++++++--------------
 1 file changed, 15 insertions(+), 14 deletions(-)

diff --git a/block/blk-sysfs.c b/block/blk-sysfs.c
index 925043f926c5..aef7bfd8e600 100644
--- a/block/blk-sysfs.c
+++ b/block/blk-sysfs.c
@@ -847,20 +847,6 @@ int blk_register_queue(struct gendisk *disk)
 		  "%s is registering an already registered queue\n",
 		  kobject_name(&dev->kobj));
 
-	/*
-	 * SCSI probing may synchronously create and destroy a lot of
-	 * request_queues for non-existent devices.  Shutting down a fully
-	 * functional queue takes measureable wallclock time as RCU grace
-	 * periods are involved.  To avoid excessive latency in these
-	 * cases, a request_queue starts out in a degraded mode which is
-	 * faster to shut down and is made fully functional here as
-	 * request_queues for non-existent devices never get registered.
-	 */
-	if (!blk_queue_init_done(q)) {
-		blk_queue_flag_set(QUEUE_FLAG_INIT_DONE, q);
-		percpu_ref_switch_to_percpu(&q->q_usage_counter);
-	}
-
 	blk_queue_update_readahead(q);
 
 	ret = blk_trace_init_sysfs(dev);
@@ -920,6 +906,21 @@ int blk_register_queue(struct gendisk *disk)
 	ret = 0;
 unlock:
 	mutex_unlock(&q->sysfs_dir_lock);
+
+	/*
+	 * SCSI probing may synchronously create and destroy a lot of
+	 * request_queues for non-existent devices.  Shutting down a fully
+	 * functional queue takes measureable wallclock time as RCU grace
+	 * periods are involved.  To avoid excessive latency in these
+	 * cases, a request_queue starts out in a degraded mode which is
+	 * faster to shut down and is made fully functional here as
+	 * request_queues for non-existent devices never get registered.
+	 */
+	if (!blk_queue_init_done(q)) {
+		blk_queue_flag_set(QUEUE_FLAG_INIT_DONE, q);
+		percpu_ref_switch_to_percpu(&q->q_usage_counter);
+	}
+
 	return ret;
 }
 EXPORT_SYMBOL_GPL(blk_register_queue);
-- 
2.29.2

Re: [PATCH 0/4] block: fix race between adding wbt and normal IO

[Ming Lei]

On Tue, May 25, 2021 at 04:04:38PM +0800, Ming Lei wrote:
> Hello,
> 
> Yi reported several kernel panics on:
> 
> [16687.001777] Unable to handle kernel NULL pointer dereference at virtual address 0000000000000008
> ...
> [16687.163549] pc : __rq_qos_track+0x38/0x60
> 
> or
> 
> [  997.690455] Unable to handle kernel NULL pointer dereference at virtual address 0000000000000020
> ...
> [  997.850347] pc : __rq_qos_done+0x2c/0x50
> 
> Turns out it is caused by race between adding wbt and normal IO.
> 
> Fix the issue by moving wbt allocation/addition into blk_alloc_queue().

Hello Guys,

Ping...


Thanks,
Ming