Messages in this thread | | | Subject | Re: [tip: locking/urgent] locking/ww_mutex: Treat ww_mutex_lock() like a trylock | From | Waiman Long <> | Date | Wed, 17 Mar 2021 11:35:12 -0400 |
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On 3/17/21 10:03 AM, Waiman Long wrote: > On 3/17/21 9:31 AM, Peter Zijlstra wrote: >> On Wed, Mar 17, 2021 at 02:12:41PM +0100, Peter Zijlstra wrote: >>> On Wed, Mar 17, 2021 at 12:38:21PM -0000, tip-bot2 for Waiman Long >>> wrote: >>>> + /* >>>> + * Treat as trylock for ww_mutex. >>>> + */ >>>> + mutex_acquire_nest(&lock->dep_map, subclass, !!ww_ctx, >>>> nest_lock, ip); >>> I'm confused... why isn't nest_lock working here? >>> >>> For ww_mutex, we're supposed to have ctx->dep_map as a nest_lock, and >>> all lock acquisitions under a nest lock should be fine. Afaict the >>> above >>> is just plain wrong. >> To clarify: >> >> mutex_lock(&A); ww_mutex_lock(&B, ctx); >> ww_mutex_lock(&B, ctx); mutex_lock(&A); >> >> should still very much be a deadlock, but your 'fix' makes it not report >> that. >> >> Only order within the ww_ctx can be ignored, and that's exactly what >> nest_lock should be doing. >> > I will take a deeper look into why that is the case.
From reading the source code, nest_lock check is done in check_deadlock() so that it won't complain. However, nest_lock isn't considered in check_noncircular() which causes the splat to come out. Maybe we should add a check for nest_lock there. I will fiddle with the code to see if it can address the issue.
Cheers, Longman
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