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

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Date:	Mon, 14 Sep 2015 15:57:49 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Waiman Long <Waiman.Long@....com>
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 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...
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