linux-kernel - Re: [patch v3 3/3] percpu_counter: use atomic64 for counter in SMP

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Message-ID: <20110415041506.GC29512@mtj.dyndns.org>
Date:	Fri, 15 Apr 2011 13:15:06 +0900
From:	Tejun Heo <tj@...nel.org>
To:	shaohua.li@...el.com
Cc:	linux-kernel@...r.kernel.org, akpm@...ux-foundation.org,
	cl@...ux.com, eric.dumazet@...il.com
Subject: Re: [patch v3 3/3] percpu_counter: use atomic64 for counter in SMP

Hello,

On Thu, Apr 14, 2011 at 10:04:50AM +0800, shaohua.li@...el.com wrote:
> Index: linux/lib/percpu_counter.c
> ===================================================================
> --- linux.orig/lib/percpu_counter.c	2011-04-14 09:53:04.000000000 +0800
> +++ linux/lib/percpu_counter.c	2011-04-14 10:01:29.000000000 +0800
> @@ -59,13 +59,17 @@ void percpu_counter_set(struct percpu_co
>  {
>  	int cpu;
>  
> -	spin_lock(&fbc->lock);
> +	/*
> +	 * Don't really need to disable preempt here, just make sure there is
> +	 * no big latency because of preemption
> +	 */
> +	preempt_disable();
>  	for_each_possible_cpu(cpu) {
>  		s32 *pcount = per_cpu_ptr(fbc->counters, cpu);
>  		*pcount = 0;
>  	}
> -	fbc->count = amount;
> -	spin_unlock(&fbc->lock);
> +	atomic64_set(&fbc->count, amount);
> +	preempt_enable();

Disabling preemption here doesn't make any sense.
percpu_counter_set() inherently requires its users to guarantee that
no other user is modifying the percpu counter.

Thanks.

-- 
tejun
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