There is a deadlock between sm_release and sm_cache_flush_work
which is a work item. The cancel_work_sync in sm_release will
not return until sm_cache_flush_work is finished. If we hold
mutex_lock and use cancel_work_sync to wait the work item to
finish, the work item also requires mutex_lock. As a result,
the sm_release will be blocked forever. The race condition is
shown below:

    (Thread 1)             |   (Thread 2)
sm_release                 |
  mutex_lock(&ftl->mutex)  | sm_cache_flush_work
                           |   mutex_lock(&ftl->mutex)
  cancel_work_sync         |   ...

This patch moves del_timer_sync and cancel_work_sync out of
mutex_lock in order to mitigate deadlock.

Fixes: 7d17c02a01a1 ("mtd: Add new SmartMedia/xD FTL")
Signed-off-by: Duoming Zhou <duoming@zju.edu.cn>
---
 drivers/mtd/sm_ftl.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/mtd/sm_ftl.c b/drivers/mtd/sm_ftl.c
index 0cff2cda1b5..7f955fade83 100644
--- a/drivers/mtd/sm_ftl.c
+++ b/drivers/mtd/sm_ftl.c
@@ -1111,9 +1111,9 @@ static void sm_release(struct mtd_blktrans_dev *dev)
 {
 	struct sm_ftl *ftl = dev->priv;
 
-	mutex_lock(&ftl->mutex);
 	del_timer_sync(&ftl->timer);
 	cancel_work_sync(&ftl->flush_work);
+	mutex_lock(&ftl->mutex);
 	sm_cache_flush(ftl);
 	mutex_unlock(&ftl->mutex);
 }
-- 
2.17.1

On Tue, 2022-05-24 at 04:48:41 UTC, Duoming Zhou wrote:
> There is a deadlock between sm_release and sm_cache_flush_work
> which is a work item. The cancel_work_sync in sm_release will
> not return until sm_cache_flush_work is finished. If we hold
> mutex_lock and use cancel_work_sync to wait the work item to
> finish, the work item also requires mutex_lock. As a result,
> the sm_release will be blocked forever. The race condition is
> shown below:
> 
>     (Thread 1)             |   (Thread 2)
> sm_release                 |
>   mutex_lock(&ftl->mutex)  | sm_cache_flush_work
>                            |   mutex_lock(&ftl->mutex)
>   cancel_work_sync         |   ...
> 
> This patch moves del_timer_sync and cancel_work_sync out of
> mutex_lock in order to mitigate deadlock.
> 
> Fixes: 7d17c02a01a1 ("mtd: Add new SmartMedia/xD FTL")
> Signed-off-by: Duoming Zhou <duoming@zju.edu.cn>

Applied to https://git.kernel.org/pub/scm/linux/kernel/git/mtd/linux.git mtd/next, thanks.

Miquel