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

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Message-ID: <20151013195627.GY3816@twins.programming.kicks-ass.net>
Date:	Tue, 13 Oct 2015 21:56:27 +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:

> +gotlock:
>  	/*
> +	 * We now have the lock. We need to either clear the tail code or
> +	 * notify the next one in queue as the new queue head.
>  	 */
> +	old = atomic_read(&lock->val);
> +	while ((old & _Q_TAIL_MASK) == tail) {
> +		int val;
> +		int new = old & ~_Q_TAIL_MASK;
> +
> +		/*
> +		 * We are the only one in the queue, so clear the tail code
> +		 * and return.
> +		 */
> +		val = atomic_cmpxchg(&lock->val, old, new);
> +		if (old == val)
> +			goto done;
> +		old = val;
> +	}
> +

This i need to think about a wee bit; its almost the same...


So the below is exactly duplicated from the normal slowpath, so why
don't you keep that there?

It would get you something like:

	if (pv_wait_head_or_steal(..))
		goto stolen;


stolen:
> +	/*
> +	 * contended path; wait for next, release.
> +	 */
> +	while (!(next = READ_ONCE(node->next)))
> +		cpu_relax();
> +
> +	arch_mcs_spin_unlock_contended(&next->locked);
> +	pv_kick_node(lock, next);

release:
	...
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