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

[Peter Zijlstra <peterz@infradead.org>]

On Tue, Oct 22, 2024 at 08:18:05AM +0200, Christoph Hellwig wrote:
> 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.

So lockdep supports recursive reader state, but you're looking at
recursive exclusive state?

If you achieve this using an external nest count, then it is probably (I
haven't yet had morning juice) sufficient to use the regular exclusive
state on the outermost lock / unlock pair and simply ignore the inner
locks.