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Subject: Re: [PATCH] percpu: preemptless __per_cpu_counter_add
From: Eric Dumazet <eric.dumazet@gmail.com>
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Date: Thu, 28 Apr 2011 18:35:33 +0200
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To: Christoph Lameter <cl@linux.com>
Cc: Tejun Heo <tj@kernel.org>, Shaohua Li <shaohua.li@intel.com>, "akpm@linux-foundation.org" <akpm@linux-foundation.org>, "linux-mm@kvack.org" <linux-mm@kvack.org>

Le jeudi 28 avril 2011 A  11:17 -0500, Christoph Lameter a A(C)crit :
> On Thu, 28 Apr 2011, Eric Dumazet wrote:
> 
> > > If _sum() notices seqcount was changed too much, restart the loop.
> 
> This does not address the issue of cpus adding batch -1 while the
> loop is going on.


Yes, it does, I left the needed changes to write side as an exercice ;)


For example, based on current linux-2.6 code


void __percpu_counter_add(struct percpu_counter *fbc, s64 amount, s32 batch)
{
        s64 count;

        preempt_disable();
        count = __this_cpu_read(*fbc->counters) + amount;
        if (count >= batch || count <= -batch) {
                spin_lock(&fbc->lock);
		fbc->seqcount++;
                fbc->count += count;
                __this_cpu_write(*fbc->counters, 0);
                spin_unlock(&fbc->lock);
        } else {
                __this_cpu_write(*fbc->counters, count);
        }
        preempt_enable();
}


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