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

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Date:   Wed, 30 Jun 2021 16:50:15 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     "Xu, Yanfei" <yanfei.xu@...driver.com>
Cc:     Waiman Long <longman@...hat.com>, Ingo Molnar <mingo@...hat.com>,
        Will Deacon <will@...nel.org>,
        Boqun Feng <boqun.feng@...il.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] locking/mutex: Reduce chance of setting HANDOFF bit on
 unlocked mutex

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().