Re: [PATCH] blk-mq-debugfs: Fix warning about possible deadlock

[Mohamed Khalfella <mkhalfella@purestorage.com>]

On Sun 2026-02-15 19:06:21 +0530, Nilay Shroff wrote:
> 
> 
> On 2/14/26 4:15 AM, Mohamed Khalfella wrote:
> > Commit 65d466b62984 ("blk-mq-debugfs: warn about possible deadlock")
> > added WARN_ONCE_ON() to debugfs_create_files() to prevent potential
> > deadlock if the queue is frozen. We hit this warning with the stacktrace
> > below
> > 
> > WARNING: block/blk-mq-debugfs.c:620 at debugfs_create_files+0x9c/0x1d0
> > Workqueue: nvme-wq nvme_tcp_reconnect_ctrl_work [nvme_tcp]
> > RIP: 0010:debugfs_create_files+0x9c/0x1d0
> > Call Trace:
> >  <TASK>
> >  blk_mq_debugfs_register_hctx+0x186/0x320
> >  blk_mq_debugfs_register_hctxs+0x80/0xa0
> >  blk_mq_update_nr_hw_queues+0xc8f/0xd50
> >  nvme_tcp_setup_ctrl+0xcfa/0xda0 [nvme_tcp]
> >  nvme_tcp_reconnect_ctrl_work+0x51/0x180 [nvme_tcp]
> >  process_scheduled_works+0x840/0xd80
> >  worker_thread+0x36d/0x4a0
> >  kthread+0x366/0x380
> >  ret_from_fork+0x7b/0x670
> >  ret_from_fork_asm+0x1a/0x30
> >  </TASK>
> > 
> > Here __blk_mq_update_nr_hw_queues() did set PF_MEMALLOC_NOIO flag before
> > calling blk_mq_debugfs_register_hctxs() and this should guarantee the
> > described deadlock will not happen. Warn only if the queue is frozen and
> > NOIO flag is not set.
> > 
> > Fixes: 65d466b62984 ("blk-mq-debugfs: warn about possible deadlock")
> > Signed-off-by: Mohamed Khalfella <mkhalfella@purestorage.com>
> > ---
> >  block/blk-mq-debugfs.c | 4 +++-
> >  1 file changed, 3 insertions(+), 1 deletion(-)
> > 
> > diff --git a/block/blk-mq-debugfs.c b/block/blk-mq-debugfs.c
> > index faeaa1fc86a7..ec6670dc0a3d 100644
> > --- a/block/blk-mq-debugfs.c
> > +++ b/block/blk-mq-debugfs.c
> > @@ -612,12 +612,14 @@ static void debugfs_create_files(struct request_queue *q, struct dentry *parent,
> >  				 void *data,
> >  				 const struct blk_mq_debugfs_attr *attr)
> >  {
> > +	unsigned int pflags = READ_ONCE(current->flags);
> > +
> >  	lockdep_assert_held(&q->debugfs_mutex);
> >  	/*
> >  	 * Creating new debugfs entries with queue freezed has the risk of
> >  	 * deadlock.
> >  	 */
> > -	WARN_ON_ONCE(q->mq_freeze_depth != 0);
> > +	WARN_ON_ONCE((q->mq_freeze_depth != 0) && !(pflags & PF_MEMALLOC_NOIO));
> >  	/*
> >  	 * debugfs_mutex should not be nested under other locks that can be
> >  	 * grabbed while queue is frozen.
> > 
> > base-commit: cd7a5651db263b5384aef1950898e5e889425134
> 
> This issue is already being addressed here:
> https://lore.kernel.org/all/20260214054350.2322436-1-yukuai@fnnas.com/

Got it. Thank you.