From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752668Ab3FFOhA (ORCPT ); Thu, 6 Jun 2013 10:37:00 -0400 Received: from mail-pd0-f171.google.com ([209.85.192.171]:41839 "EHLO mail-pd0-f171.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752554Ab3FFOg6 (ORCPT ); Thu, 6 Jun 2013 10:36:58 -0400 Message-ID: <51B09E83.9040606@gmail.com> Date: Thu, 06 Jun 2013 22:36:51 +0800 From: Jiang Liu User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:17.0) Gecko/20130404 Thunderbird/17.0.5 MIME-Version: 1.0 To: Jerome Marchand CC: Greg Kroah-Hartman , Nitin Gupta , Minchan Kim , Yijing Wang , Jiang Liu , devel@driverdev.osuosl.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH v2 10/10] zram: use atomic64_xxx() to replace zram_stat64_xxx() References: <1370361968-8764-1-git-send-email-jiang.liu@huawei.com> <1370361968-8764-10-git-send-email-jiang.liu@huawei.com> <51AF28C4.3040405@redhat.com> <51AF659F.9080205@gmail.com> <51B0584F.6080609@redhat.com> In-Reply-To: <51B0584F.6080609@redhat.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu 06 Jun 2013 05:37:19 PM CST, Jerome Marchand wrote: > On 06/05/2013 06:21 PM, Jiang Liu wrote: >> On Wed 05 Jun 2013 08:02:12 PM CST, Jerome Marchand wrote: >>> On 06/04/2013 06:06 PM, Jiang Liu wrote: >>>> Use atomic64_xxx() to replace open-coded zram_stat64_xxx(). >>>> Some architectures have native support of atomic64 operations, >>>> so we can get rid of the spin_lock() in zram_stat64_xxx(). >>>> On the other hand, for platforms use generic version of atomic64 >>>> implement, it may cause an extra save/restore of the interrupt >>>> flag. So it's a tradeoff. >>>> >>>> Signed-off-by: Jiang Liu >>> >>> Before optimizing stats, I'd like to make sure that they're correct. >>> What makes 64 bits fields so different that they need atomicity while >>> 32 bits wouldn't? Actually all of them save compr_size only increase, >>> which would make a race less critical than for 32 bits fields that all >>> can go up and down (if a decrement overwrites a increment, the counter >>> can wrap around zero). >>> >>> Jerome >>> >> Hi Jerome, >> I'm not sure about the design decision, but I could give a >> guess here. >> 1) All 32-bit counters are only modified by >> zram_bvec_write()/zram_page_free() >> and is/should be protected by down_write(&zram->lock). > > Good point! > >> 2) __zram_make_request() modifies some 64-bit counters without >> protection. >> 3) zram_bvec_write() modifies some 64-bit counters and it's protected >> with >> down_read(&zram->lock). > > I assume you mean down_write(). Actually I mean "zram_bvec_read()" instead of "zram_bvec_write()". Read side is protected by down_read(&zram->lock). Regards! Gerry > >> 4) It's always safe for sysfs handler to read 32bit counters. >> 5) It's unsafe for sysfs handler to read 64bit counters on 32bit >> platforms. > > I was unaware of that. > >> >> So it does work with current design, but very hard to understand. >> Suggest to use atomic_t for 32bit counters too for maintainability, >> though may be a little slower. >> Any suggestion? > > If atomic counter aren't necessary, no need to use them, but a comment > in zram_stats definition would be nice. Could you add one in your next > version of this patch? Sure! > > Thanks > Jerome > >> Regards! >> Gerry >> >