linux-kernel - Re: [PATCH v9 05/19] qspinlock: Optimize for smaller NR

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Date:	Thu, 17 Apr 2014 17:58:44 +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>,
	"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 v9 05/19] qspinlock: Optimize for smaller NR_CPUS

On Thu, Apr 17, 2014 at 11:03:57AM -0400, Waiman Long wrote:
> +static __always_inline void
> +clear_pending_set_locked(struct qspinlock *lock, u32 val)
> +{
> +	struct __qspinlock *l = (void *)lock;
> +
> +	ACCESS_ONCE(l->locked_pending) = 1;
> +}

> @@ -157,8 +251,13 @@ static inline int trylock_pending(struct qspinlock *lock, u32 *pval)
>  	 * we're pending, wait for the owner to go away.
>  	 *
>  	 * *,1,1 -> *,1,0
> +	 *
> +	 * this wait loop must be a load-acquire such that we match the
> +	 * store-release that clears the locked bit and create lock
> +	 * sequentiality; this because not all try_clear_pending_set_locked()
> +	 * implementations imply full barriers.

You renamed the function referred in the above comment.

>  	 */
> -	while ((val = atomic_read(&lock->val)) & _Q_LOCKED_MASK)
> +	while ((val = smp_load_acquire(&lock->val.counter)) & _Q_LOCKED_MASK)
>  		arch_mutex_cpu_relax();
>  
>  	/*
--
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