[PATCH] loop: Fix recently introduced lock inversion

[PATCH] loop: Fix recently introduced lock inversion

[Bart Van Assche]

All block driver code except loop_set_dio() calls
queue_limits_start_update() before it freezes the request queue. Make
loop_set_dio() follow this convention. This patch fixes the following
lockdep complaint:

======================================================
WARNING: possible circular locking dependency detected
7.2.0-rc5-dbg #11 Not tainted
------------------------------------------------------
losetup/2924 is trying to acquire lock:
ffff88816c76da68 (&q->limits_lock){+.+.}-{4:4}, at: loop_set_dio+0x318/0x720 [loop]

but task is already holding lock:
ffff88816c76d430 (&q->q_usage_counter(io)#24){++++}-{0:0}, at: blk_mq_freeze_queue_nomemsave+0x1a/0x30

which lock already depends on the new lock.

Cc: Keith Busch <kbusch@kernel.org>
Fixes: 6c8dec275ccc ("loop: set dma_alignment from the backing file for direct I/O")
Reported-by: syzbot+cc0de396bac84da51919@syzkaller.appspotmail.com
Closes: https://lore.kernel.org/linux-block/6a7d5368.d5f0ebe7.22d851.0013.GAE@google.com/
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
---
 drivers/block/loop.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/block/loop.c b/drivers/block/loop.c
index 8639fa34b847..6f12976035b0 100644
--- a/drivers/block/loop.c
+++ b/drivers/block/loop.c
@@ -1452,12 +1452,12 @@ static int loop_set_dio(struct loop_device *lo, unsigned long arg)
 		vfs_fsync(lo->lo_backing_file, 0);
 	}
 
+	lim = queue_limits_start_update(lo->lo_queue);
 	memflags = blk_mq_freeze_queue(lo->lo_queue);
 	if (use_dio)
 		lo->lo_flags |= LO_FLAGS_DIRECT_IO;
 	else
 		lo->lo_flags &= ~LO_FLAGS_DIRECT_IO;
-	lim = queue_limits_start_update(lo->lo_queue);
 	loop_set_dma_limit(lo, &lim);
 	queue_limits_commit_update(lo->lo_queue, &lim);
 	blk_mq_unfreeze_queue(lo->lo_queue, memflags);

Re: [PATCH] loop: Fix recently introduced lock inversion

[Keith Busch]

On Thu, Aug 13, 2026 at 04:14:20PM +0000, Bart Van Assche wrote:
> All block driver code except loop_set_dio() calls
> queue_limits_start_update() before it freezes the request queue. Make
> loop_set_dio() follow this convention. This patch fixes the following
> lockdep complaint:
> 
> ======================================================
> WARNING: possible circular locking dependency detected
> 7.2.0-rc5-dbg #11 Not tainted
> ------------------------------------------------------
> losetup/2924 is trying to acquire lock:
> ffff88816c76da68 (&q->limits_lock){+.+.}-{4:4}, at: loop_set_dio+0x318/0x720 [loop]
> 
> but task is already holding lock:
> ffff88816c76d430 (&q->q_usage_counter(io)#24){++++}-{0:0}, at: blk_mq_freeze_queue_nomemsave+0x1a/0x30
> 
> which lock already depends on the new lock.

Thanks, looks good.

Reviewed-by: Keith Busch <kbusch@kernel.org>