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

[Ming Lei <ming.lei@redhat.com>]

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.

Yeah, looks we can just call lock_acquire for the outermost
freeze/unfreeze.

> 
> > 
> > 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.
 
Here it isn't real rwsem, and lockdep doesn't have non_owner variant
for rwsem_acquire() and rwsem_release().

Another corner case is blk_mark_disk_dead() in which freeze & unfreeze
may be run from different task contexts too.


thanks,
Ming