linux-ext4 - Re: [PATCH] percpu_counter: Fix __percpu_counter

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Date:	Wed, 10 Dec 2008 08:12:42 +0100
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Eric Dumazet <dada1@...mosbay.com>
Cc:	Theodore Tso <tytso@....edu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux kernel <linux-kernel@...r.kernel.org>,
	"David S. Miller" <davem@...emloft.net>,
	Mingming Cao <cmm@...ibm.com>, linux-ext4@...r.kernel.org
Subject: Re: [PATCH] percpu_counter: Fix __percpu_counter_sum()

On Wed, 2008-12-10 at 06:09 +0100, Eric Dumazet wrote:
> Now percpu_counter_sum() is 'fixed', what about "percpu_counter_add()" ?
> 
> void __percpu_counter_add(struct percpu_counter *fbc, s64 amount, s32 batch)
> {
>         s64 count;
>         s32 *pcount;
>         int cpu = get_cpu();
> 
>         pcount = per_cpu_ptr(fbc->counters, cpu);
>         count = *pcount + amount;
>         if (count >= batch || count <= -batch) {
>                 spin_lock(&fbc->lock);
>                 fbc->count += count;
>                 *pcount = 0;
>                 spin_unlock(&fbc->lock);
>         } else {
>                 *pcount = count;
>         }
>         put_cpu();
> }
> 
> 
> If I read this well, this is not IRQ safe.
> 
> get_cpu() only disables preemption IMHO
> 
> For nr_files, nr_dentry, nr_inodes, it should not be a problem.
> 
> But for network counters (only in net-next-2.6) 
> and lib/proportions.c, we have a problem ?

Non of percpu_counter if irqsafe, for lib/proportions I disabled irqs by
hand when needed - I don't think we ought to bother with local_t, esp as
it basically sucks chunks on anything !x86.


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