linux-kernel - Re: [PATCH v10 06/19] qspinlock: prolong the stay in the pending bit path

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Date:	Thu, 8 May 2014 20:58:54 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Waiman Long <Waiman.Long@...com>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>, linux-arch@...r.kernel.org,
	x86@...nel.org, linux-kernel@...r.kernel.org,
	virtualization@...ts.linux-foundation.org,
	xen-devel@...ts.xenproject.org, kvm@...r.kernel.org,
	Paolo Bonzini <paolo.bonzini@...il.com>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	Boris Ostrovsky <boris.ostrovsky@...cle.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Rik van Riel <riel@...hat.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Raghavendra K T <raghavendra.kt@...ux.vnet.ibm.com>,
	David Vrabel <david.vrabel@...rix.com>,
	Oleg Nesterov <oleg@...hat.com>,
	Gleb Natapov <gleb@...hat.com>,
	Scott J Norton <scott.norton@...com>,
	Chegu Vinod <chegu_vinod@...com>
Subject: Re: [PATCH v10 06/19] qspinlock: prolong the stay in the pending bit
 path

On Wed, May 07, 2014 at 11:01:34AM -0400, Waiman Long wrote:
> @@ -221,11 +222,37 @@ static inline int trylock_pending(struct qspinlock *lock, u32 *pval)
>  	 */
>  	for (;;) {
>  		/*
> -		 * If we observe any contention; queue.
> +		 * If we observe that the queue is not empty,
> +		 * return and be queued.
>  		 */
> -		if (val & ~_Q_LOCKED_MASK)
> +		if (val & _Q_TAIL_MASK)
>  			return 0;
>  
> +		if (val == (_Q_LOCKED_VAL|_Q_PENDING_VAL)) {
> +			/*
> +			 * If both the lock and pending bits are set, we wait
> +			 * a while to see if that either bit will be cleared.
> +			 * If that is no change, we return and be queued.
> +			 */
> +			if (!retry)
> +				return 0;
> +			retry--;
> +			cpu_relax();
> +			cpu_relax();
> +			*pval = val = atomic_read(&lock->val);
> +			continue;
> +		} else if (val == _Q_PENDING_VAL) {
> +			/*
> +			 * Pending bit is set, but not the lock bit.
> +			 * Assuming that the pending bit holder is going to
> +			 * set the lock bit and clear the pending bit soon,
> +			 * it is better to wait than to exit at this point.
> +			 */
> +			cpu_relax();
> +			*pval = val = atomic_read(&lock->val);
> +			continue;
> +		}

Didn't I give a much saner alternative to this mess last time?
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