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

[Peter Zijlstra <peterz@infradead.org>]

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();
		}