Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752646Ab3FFP4x (ORCPT ); Thu, 6 Jun 2013 11:56:53 -0400 Received: from mail-pb0-f45.google.com ([209.85.160.45]:63429 "EHLO mail-pb0-f45.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751353Ab3FFP4w (ORCPT ); Thu, 6 Jun 2013 11:56:52 -0400 Message-ID: <51B0B13E.8030305@gmail.com> Date: Thu, 06 Jun 2013 23:56:46 +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> <51B09E83.9040606@gmail.com> <51B0A5A1.1040805@redhat.com> In-Reply-To: <51B0A5A1.1040805@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 Content-Length: 3136 Lines: 98 On Thu 06 Jun 2013 11:07:13 PM CST, Jerome Marchand wrote: > On 06/06/2013 04:36 PM, Jiang Liu wrote: >> 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()". > > Indeed, failed_reads is updated there. > >> Read side is protected by down_read(&zram->lock). > > which does not prevent concurrent read access. The counter isn't > protected by zram_lock here. Hi Jerome, Yeah, it's true. The down_read(&zram->lock) can't protect modification to stat counters because there may be multiple readers. So zram uses zram_stat_inc() here. Regards! Gerry > > Jerome > >> 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 >>>> >>> >> >> > -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/