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Date: Tue, 13 Oct 2015 20:23:38 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: Waiman Long <Waiman.Long@hpe.com>
Cc: Ingo Molnar <mingo@redhat.com>, Thomas Gleixner <tglx@linutronix.de>,
        "H. Peter Anvin" <hpa@zytor.com>, x86@kernel.org,
        linux-kernel@vger.kernel.org, Scott J Norton <scott.norton@hpe.com>,
        Douglas Hatch <doug.hatch@hpe.com>,
        Davidlohr Bueso <dave@stgolabs.net>
Subject: Re: [PATCH v7 4/5] locking/pvqspinlock: Allow 1 lock stealing attempt
Message-ID: <20151013182338.GW3816@twins.programming.kicks-ass.net>
References: <1442955044-43895-1-git-send-email-Waiman.Long@hpe.com>
 <1442955044-43895-5-git-send-email-Waiman.Long@hpe.com>
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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();
		}