From: Eric Dumazet Subject: Re: [PATCH] percpu_counter: Fix __percpu_counter_sum() Date: Sun, 07 Dec 2008 19:00:32 +0100 Message-ID: <493C0F40.7040304@cosmosbay.com> References: <4936D287.6090206@cosmosbay.com> <4936EB04.8000609@cosmosbay.com> <20081206202233.3b74febc.akpm@linux-foundation.org> <493BCF60.1080409@cosmosbay.com> <20081207092854.f6bcbfae.akpm@linux-foundation.org> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: QUOTED-PRINTABLE Cc: linux kernel , "David S. Miller" , Peter Zijlstra , Mingming Cao , "Theodore Ts'o" , linux-ext4@vger.kernel.org To: Andrew Morton Return-path: Received: from gw1.cosmosbay.com ([86.65.150.130]:56696 "EHLO gw1.cosmosbay.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751066AbYLGSGE convert rfc822-to-8bit (ORCPT ); Sun, 7 Dec 2008 13:06:04 -0500 In-Reply-To: <20081207092854.f6bcbfae.akpm@linux-foundation.org> Sender: linux-ext4-owner@vger.kernel.org List-ID: Andrew Morton a =E9crit : > On Sun, 07 Dec 2008 14:28:00 +0100 Eric Dumazet = wrote: >=20 >> Andrew Morton a __crit : >>> On Wed, 03 Dec 2008 21:24:36 +0100 Eric Dumazet 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_coun= ter *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 pa= tch. >>> >>> 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 go= t my >>> head around this correctly - I may not have done. >>> >> >> Hum... e8ced39d5e8911c662d4d69a342b9d053eaaac4e is probably followin= g >> the wrong path, but I see the intent. Even in the 'nr_files' case, i= t could >> help to reduce excessive calls to percpu_counter_sum() >> >=20 > We should fix this in 2.6.28 - right now percpu_counter_sum() is subt= ly > corrupting the counter's value. >=20 > I sent two revert patches which I hope to merge into 2.6.28. Could y= ou > guys please read/review/maybe-test them? Your revert patches have the same effect than my first patch : No write= s in percpu_counter_sum() I am lost here Andrew... -- To unsubscribe from this list: send the line "unsubscribe linux-ext4" i= n the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html