Re: [PATCH] block: model freeze & enter queue as rwsem for supporting lockdep

[Jens Axboe <axboe@kernel.dk>]

On 10/17/24 7:35 PM, Ming Lei wrote:
> Recently we got several deadlock report[1][2][3] caused by blk_mq_freeze_queue
> and blk_enter_queue().
> 
> Turns out the two are just like one rwsem, so model them as rwsem for
> supporting lockdep:
> 
> 1) model blk_mq_freeze_queue() as down_write_trylock()
> - it is exclusive lock, so dependency with blk_enter_queue() is covered
> - it is trylock because blk_mq_freeze_queue() are allowed to run concurrently
> 
> 2) model blk_enter_queue() as down_read()
> - it is shared lock, so concurrent blk_enter_queue() are allowed
> - it is read lock, so dependency with blk_mq_freeze_queue() is modeled
> - blk_queue_exit() is often called from other contexts(such as irq), and
> it can't be annotated as rwsem_release(), so simply do it in
> blk_enter_queue(), this way still covered cases as many as possible
> 
> NVMe is the only subsystem which may call blk_mq_freeze_queue() and
> blk_mq_unfreeze_queue() from different context, so it is the only
> exception for the modeling. Add one tagset flag to exclude it from
> the lockdep support.
> 
> With lockdep support, such kind of reports may be reported asap and
> needn't wait until the real deadlock is triggered.

I think this is a great idea. We've had way too many issues in this
area, getting lockdep to grok it (and report issues) is the ideal way to
avoid that, and even find issues we haven't come across yet.

-- 
Jens Axboe