linux-kernel - Re: [PATCH v7 4/5] locking/pvqspinlock: Allow 1 lock stealing attempt

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Message-ID: <20151013182338.GW3816@twins.programming.kicks-ass.net>
Date:	Tue, 13 Oct 2015 20:23:38 +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 v7 4/5] locking/pvqspinlock: Allow 1 lock stealing attempt

On Tue, Sep 22, 2015 at 04:50:43PM -0400, Waiman Long wrote:
>  	for (;; waitcnt++) {
> +		loop = SPIN_THRESHOLD;
> +		while (loop) {
> +			/*
> +			 * Spin until the lock is free
> +			 */
> +			for (; loop && READ_ONCE(l->locked); loop--)
> +				cpu_relax();
> +			/*
> +			 * Seeing the lock is free, this queue head vCPU is
> +			 * the rightful next owner of the lock. However, the
> +			 * lock may have just been stolen by another task which
> +			 * has entered the slowpath. So we need to use atomic
> +			 * operation to make sure that we really get the lock.
> +			 * Otherwise, we have to wait again.
> +			 */
> +			if (cmpxchg(&l->locked, 0, _Q_LOCKED_VAL) == 0)
> +				goto gotlock;
>  		}

		for (loop = SPIN_THRESHOLD; loop; --loop) {
			if (!READ_ONCE(l->locked) &&
			    cmpxchg(&l->locked, 0, _Q_LOCKED_VA) == 0)
				goto gotlock;

			cpu_relax();
		}

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