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.