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

[Peter Zijlstra]

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.