[PATCH] block: Fix potential deadlock in blk_ia_range_sysfs_show()

[PATCH] block: Fix potential deadlock in blk_ia_range_sysfs_show()

[Damien Le Moal]

When being read, a sysfs attribute is already protected against removal
with the kobject node active reference counter. As a result, in
blk_ia_range_sysfs_show(), there is no need to take the queue sysfs
lock when reading the value of a range attribute. Using the queue sysfs
lock in this function creates a potential deadlock situation with the
disk removal, something that a lockdep signals with a splat when the
device is removed:

[  760.703551]  Possible unsafe locking scenario:
[  760.703551]
[  760.703554]        CPU0                    CPU1
[  760.703556]        ----                    ----
[  760.703558]   lock(&q->sysfs_lock);
[  760.703565]                                lock(kn->active#385);
[  760.703573]                                lock(&q->sysfs_lock);
[  760.703579]   lock(kn->active#385);
[  760.703587]
[  760.703587]  *** DEADLOCK ***

Solve this by removing the mutex_lock()/mutex_unlock() calls from
blk_ia_range_sysfs_show().

Fixes: a2247f19ee1c ("block: Add independent access ranges support")
Cc: stable@vger.kernel.org
Signed-off-by: Damien Le Moal <damien.lemoal@opensource.wdc.com>
---
 block/blk-ia-ranges.c | 7 +------
 1 file changed, 1 insertion(+), 6 deletions(-)

diff --git a/block/blk-ia-ranges.c b/block/blk-ia-ranges.c
index 18c68d8b9138..56ed48d2954e 100644
--- a/block/blk-ia-ranges.c
+++ b/block/blk-ia-ranges.c
@@ -54,13 +54,8 @@ static ssize_t blk_ia_range_sysfs_show(struct kobject *kobj,
 		container_of(attr, struct blk_ia_range_sysfs_entry, attr);
 	struct blk_independent_access_range *iar =
 		container_of(kobj, struct blk_independent_access_range, kobj);
-	ssize_t ret;
 
-	mutex_lock(&iar->queue->sysfs_lock);
-	ret = entry->show(iar, buf);
-	mutex_unlock(&iar->queue->sysfs_lock);
-
-	return ret;
+	return entry->show(iar, buf);
 }
 
 static const struct sysfs_ops blk_ia_range_sysfs_ops = {
-- 
2.36.1

Re: [PATCH] block: Fix potential deadlock in blk_ia_range_sysfs_show()

[Jens Axboe]

On Fri, 3 Jun 2022 11:19:05 +0900, Damien Le Moal wrote:
> When being read, a sysfs attribute is already protected against removal
> with the kobject node active reference counter. As a result, in
> blk_ia_range_sysfs_show(), there is no need to take the queue sysfs
> lock when reading the value of a range attribute. Using the queue sysfs
> lock in this function creates a potential deadlock situation with the
> disk removal, something that a lockdep signals with a splat when the
> device is removed:
> 
> [...]

Applied, thanks!

[1/1] block: Fix potential deadlock in blk_ia_range_sysfs_show()
      commit: 41e46b3c2aa24f755b2ae9ec4ce931ba5f0d8532

Best regards,
-- 
Jens Axboe