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.
