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

[Christoph Hellwig <hch@lst.de>]

On Fri, Oct 18, 2024 at 09:35:42AM +0800, 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

Is this using the right terminology?  down_write and other locking
primitives obviously can run concurrently, the whole point is to
synchronize the code run inside the criticial section.

I think what you mean here is blk_mq_freeze_queue can be called more
than once due to a global recursion counter.

Not sure modelling it as a trylock is the right approach here,
I've added the lockdep maintainers if they have an idea.

> 
> 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.

rwsems have a non_owner variant for these kinds of uses cases,
we should do the same for blk_mq_freeze_queue to annoate the callsite
instead of a global flag.