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

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Message-ID: <20241022061805.GA10573@lst.de>
Date: Tue, 22 Oct 2024 08:18:05 +0200
From: Christoph Hellwig <hch@....de>
To: Ming Lei <ming.lei@...hat.com>
Cc: Jens Axboe <axboe@...nel.dk>, linux-block@...r.kernel.org,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...hat.com>, Will Deacon <will@...nel.org>,
	Waiman Long <longman@...hat.com>, Boqun Feng <boqun.feng@...il.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] block: model freeze & enter queue as rwsem for
 supporting lockdep

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.