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

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Date:	Mon, 14 Sep 2015 15:02:52 -0400
From:	Waiman Long <waiman.long@....com>
To:	Peter Zijlstra <peterz@...radead.org>
CC:	Ingo Molnar <mingo@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
	linux-kernel@...r.kernel.org, Scott J Norton <scott.norton@...com>,
	Douglas Hatch <doug.hatch@...com>,
	Davidlohr Bueso <dave@...olabs.net>
Subject: Re: [PATCH v6 5/6] locking/pvqspinlock: Allow 1 lock stealing attempt

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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