Re: [PATCH v6 5/6] locking/pvqspinlock: Allow 1 lock stealing attempt

[Waiman Long]

On 09/14/2015 03:15 PM, Waiman Long wrote:
> On 09/14/2015 10:00 AM, Peter Zijlstra wrote:
> > On Fri, Sep 11, 2015 at 02:37:37PM -0400, Waiman Long wrote:
> > > This patch allows one attempt for the lock waiter to steal the lock
> > > when entering the PV slowpath.  This helps to reduce the performance
> > > penalty caused by lock waiter preemption while not having much of
> > > the downsides of a real unfair lock.
> > > @@ -415,8 +458,12 @@ static void pv_wait_head(struct qspinlock 
> > > *lock, struct mcs_spinlock *node)
> > >
> > >       for (;; waitcnt++) {
> > >           for (loop = SPIN_THRESHOLD; loop; loop--) {
> > > -            if (!READ_ONCE(l->locked))
> > > -                return;
> > > +            /*
> > > +             * Try to acquire the lock when it is free.
> > > +             */
> > > +            if (!READ_ONCE(l->locked)&&
> > > +               (cmpxchg(&l->locked, 0, _Q_LOCKED_VAL) == 0))
> > > +                goto gotlock;
> > >               cpu_relax();
> > >           }
> > This isn't _once_, this is once per 'wakeup'. And note that interrupts
> > unrelated to the kick can equally wake the vCPU up.
>
> Oh! There is a minor bug that I shouldn't need to have a second 
> READ_ONCE() call here.

Oh! I misread the diff, the code was OK.

Cheers,
Longman
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