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

[Waiman Long]

On 09/14/2015 09:57 AM, Peter Zijlstra wrote:
> On Fri, Sep 11, 2015 at 02:37:37PM -0400, Waiman Long wrote:
> > +#define queued_spin_trylock(l)	pv_queued_spin_trylock_unfair(l)
> > +static inline bool pv_queued_spin_trylock_unfair(struct qspinlock *lock)
> > +{
> > +	struct __qspinlock *l = (void *)lock;
> > +
> > +	if (READ_ONCE(l->locked))
> > +		return 0;
> > +	/*
> > +	 * Wait a bit here to ensure that an actively spinning vCPU has a fair
> > +	 * chance of getting the lock.
> > +	 */
> > +	cpu_relax();
> > +
> > +	return cmpxchg(&l->locked, 0, _Q_LOCKED_VAL) == 0;
> > +}
> > +static inline int pvstat_trylock_unfair(struct qspinlock *lock)
> > +{
> > +	int ret = pv_queued_spin_trylock_unfair(lock);
> > +
> > +	if (ret)
> > +		pvstat_inc(pvstat_utrylock);
> > +	return ret;
> > +}
> > +#undef  queued_spin_trylock
> > +#define queued_spin_trylock(l)	pvstat_trylock_unfair(l)
> These aren't actually ever used...

The pvstat_trylock_unfair() is within the CONFIG_QUEUED_LOCK_STAT block. 
It will only be activated when the config parameter is set. Otherwise, 
pv_queued_spin_trylock_unfair() will be used without any counting.

It is used provide count of how many unfair trylock has successfully got 
the lock.

Cheers,
Longman
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