Re: [tip: locking/urgent] locking/ww_mutex: Treat ww_mutex_lock() like a trylock

Peter Zijlstra
2021-03-17T13:32:05Z

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.