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

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Date:	Sun, 07 Dec 2008 19:00:32 +0100
From:	Eric Dumazet <dada1@...mosbay.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
CC:	linux kernel <linux-kernel@...r.kernel.org>,
	"David S. Miller" <davem@...emloft.net>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Mingming Cao <cmm@...ibm.com>, "Theodore Ts'o" <tytso@....edu>,
	linux-ext4@...r.kernel.org
Subject: Re: [PATCH] percpu_counter: Fix __percpu_counter_sum()

Andrew Morton a écrit :
> On Sun, 07 Dec 2008 14:28:00 +0100 Eric Dumazet <dada1@...mosbay.com> wrote:
> 
>> Andrew Morton a __crit :
>>> On Wed, 03 Dec 2008 21:24:36 +0100 Eric Dumazet <dada1@...mosbay.com> wrote:
>>>
>>>> Eric Dumazet a __crit :
>>>>
>>>> 1) __percpu_counter_sum() is buggy, it should not write
>>>> on per_cpu_ptr(fbc->counters, cpu), or another cpu
>>>> could get its changes lost.
>>>>
>>>> __percpu_counter_sum should be read only (const struct percpu_counter *fbc),
>>>> and no locking needed.
>>> No, we can't do this - it will break ext4.
>>>
>>> Take a closer look at 1f7c14c62ce63805f9574664a6c6de3633d4a354 and at
>>> e8ced39d5e8911c662d4d69a342b9d053eaaac4e.
>>>
>>> I suggest that what we do is to revert both those changes.  We can
>>> worry about the possibly-unneeded spin_lock later, in a separate patch.
>>>
>>> It should have been a separate patch anyway.  It's conceptually
>>> unrelated and is not a bugfix, but it was mixed in with a bugfix.
>>>
>>> Mingming, this needs urgent consideration, please.  Note that I had to
>>> make additional changes to ext4 due to the subsequent introduction of
>>> the dirty_blocks counter.
>>>
>>>
>>> Please read the below changelogs carefully and check that I have got my
>>> head around this correctly - I may not have done.
>>>
>>
>> Hum... e8ced39d5e8911c662d4d69a342b9d053eaaac4e is probably following
>> the wrong path, but I see the intent. Even in the 'nr_files' case, it could
>> help to reduce excessive calls to percpu_counter_sum()
>>
> 
> We should fix this in 2.6.28 - right now percpu_counter_sum() is subtly
> corrupting the counter's value.
> 
> I sent two revert patches which I hope to merge into 2.6.28.  Could you
> guys please read/review/maybe-test them?

Your revert patches have the same effect than my first patch : No writes
in percpu_counter_sum()

I am lost here Andrew...



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