Re: [PATCH] locking/mutex: Reduce chance of setting HANDOFF bit on unlocked mutex

[Peter Zijlstra]

On Wed, Jun 30, 2021 at 10:43:37PM +0800, Xu, Yanfei wrote:
> > +static inline struct task_struct *__mutex_trylock_or_owner(struct mutex *lock, bool handoff)
> >   {
> >          unsigned long owner, curr = (unsigned long)current;
> > 
> >          owner = atomic_long_read(&lock->owner);
> >          for (;;) { /* must loop, can race against a flag */
> > +               unsigned long flags = __owner_flags(owner);
> >                  unsigned long task = owner & ~MUTEX_FLAGS;
> > 
> >                  if (task) {
> > +                       if (flags & MUTEX_FLAG_PICKUP) {
> > 
> > +                               if (task != curr)
> > +                                       break;
> > +
> > +                               flags &= ~MUTEX_FLAG_PICKUP;
> > +
> 
> Hmm.. Should we also clear HANDOFF bit here? I don't find where it is
> cleared.

Should already be gone; see __mutex_handoff().