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

[Waiman Long]

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