[PATCH v3] virtio_blk: use synchronous quiesce in virtblk_freeze_priv()

[Cong Zhang <cong.zhang@oss.qualcomm.com>]

blk_mq_run_work_fn() can call blk_mq_sched_dispatch_requests() through
blk_mq_run_dispatch_ops(). The dispatcher checks QUEUE_FLAG_QUIESCED
while the flag is clear. It can then be preempted before it gets a
request. At that time, it has no queue usage reference, so
blk_mq_freeze_queue() does not wait for it.

virtblk_freeze_priv() then calls blk_mq_quiesce_queue_nowait() and
blk_mq_unfreeze_queue(). A new request can enter the queue. The old
dispatcher can run again, use its old flag check, get the new request,
and call virtio_queue_rq(). At the same time, virtblk_freeze_priv() can
reset the device, delete the virtqueues, and free vblk->vqs. The old
dispatcher can then use a deleted virtqueue or a NULL vblk->vqs, and the
kernel can crash.

Fix this by using blk_mq_quiesce_queue(), which waits for dispatch code
that was already running. Keep the existing freeze and unfreeze order.
The queue remains quiesced until virtblk_restore_priv() calls
blk_mq_unquiesce_queue().

Fixes: 7678abee0867 ("virtio-blk: don't keep queue frozen during system suspend")
Cc: stable@vger.kernel.org
Acked-by: Jason Wang <jasowangio@gmail.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Cong Zhang <cong.zhang@oss.qualcomm.com>
---
Changes in v3:
- Move the race description into the commit log.
- Remove the blank line between tags.
- Add Cc: stable@vger.kernel.org to the commit trailers.
- Link to v2: https://lore.kernel.org/20260912-virtblk_sync_quiesce-v2-1-09a001549b72@oss.qualcomm.com

Changes in v2:
- Cc stable@vger.kernel.org.
- Link to v1: https://lore.kernel.org/20260911-virtblk_sync_quiesce-v1-1-a883f8f31258@oss.qualcomm.com
---
 drivers/block/virtio_blk.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/block/virtio_blk.c b/drivers/block/virtio_blk.c
index 32bf3ba07a9d..7a570591e040 100644
--- a/drivers/block/virtio_blk.c
+++ b/drivers/block/virtio_blk.c
@@ -1595,7 +1595,7 @@ static int virtblk_freeze_priv(struct virtio_device *vdev)
 
 	/* Ensure no requests in virtqueues before deleting vqs. */
 	memflags = blk_mq_freeze_queue(q);
-	blk_mq_quiesce_queue_nowait(q);
+	blk_mq_quiesce_queue(q);
 	blk_mq_unfreeze_queue(q, memflags);
 
 	/* Ensure we don't receive any more interrupts */

---
base-commit: 50d05c7c76c96b90462f24debacca971d2e86713
change-id: 20260910-virtblk_sync_quiesce-9b2c88d45416

Best regards,
--  
Cong Zhang <cong.zhang@oss.qualcomm.com>