Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932406AbdC1NVH (ORCPT ); Tue, 28 Mar 2017 09:21:07 -0400 Received: from mail-ot0-f177.google.com ([74.125.82.177]:34962 "EHLO mail-ot0-f177.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932102AbdC1NUy (ORCPT ); Tue, 28 Mar 2017 09:20:54 -0400 MIME-Version: 1.0 In-Reply-To: References: <20170328095029.3500369-1-arnd@arndb.de> From: Arnd Bergmann Date: Tue, 28 Mar 2017 15:20:47 +0200 X-Google-Sender-Auth: Fnm80wJSufKrf2SGB0b4Pq_hQt0 Message-ID: Subject: Re: [PATCH] Revert "md: raid1: use bio helper in process_checks()" To: Ming Lei Cc: Shaohua Li , NeilBrown , Jens Axboe , "colyli@suse.de" , Guoqing Jiang , Mike Christie , "open list:SOFTWARE RAID (Multiple Disks) SUPPORT" , Linux Kernel Mailing List Content-Type: text/plain; charset=UTF-8 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2205 Lines: 52 On Tue, Mar 28, 2017 at 1:42 PM, Ming Lei wrote: > On Tue, Mar 28, 2017 at 7:35 PM, Arnd Bergmann wrote: >> On Tue, Mar 28, 2017 at 12:44 PM, Ming Lei wrote: >>> On Tue, Mar 28, 2017 at 5:49 PM, Arnd Bergmann wrote: >>>> Commit 60928a91b0b3 ("md: raid1: use bio helper in process_checks()") >>>> is probably correct, but I get a new compile-time warning after >>>> it, and have trouble understanding what it fixes: >>>> >>>> drivers/md/raid1.c: In function 'sync_request_write': >>>> drivers/md/raid1.c:2172:9: error: 'page_len$' may be used uninitialized in this function [-Werror=maybe-uninitialized] >>>> if (memcmp(page_address(ppages[j]), >>>> ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ >>>> page_address(spages[j]), >>>> ~~~~~~~~~~~~~~~~~~~~~~~~ >>>> page_len[j])) >>>> ~~~~~~~~~~~~ >>>> drivers/md/raid1.c:2160:7: note: 'page_len$' was declared here >>>> int page_len[RESYNC_PAGES]; >>>> ^~~~~~~~ >>>> >>>> This reverts it to resolve the warning. >>> >>> Please try the following patch: >>> >>> https://lkml.org/lkml/2017/3/28/126 >> >> >> That patch will certainly shut up the warning, but will also prevent >> the compiler from warning when the function gets changed in some >> way that actually leads to an uninitialized use of the page_len array, > > Why do you think that it leads to an uninitialized use of the page_len array? What I meant is that a future change to the function might cause another bug to go unnoticed later. > The following code does initialize the array well enough for future use: > > bio_for_each_segment_all(bi, sbio, j) > page_len[j] = bi->bv_len; > > That is why we don't need to initialize the array explicitly, but just > for killing the warning. It's also a little less clear why that is safe than the original code: We rely on sbio->bi_vcnt to be the same as vcnt, but it requires careful reading of the function to see that this is always true. gcc warns because it cannot prove this to be the case, so if something changed here, it's likely that this would also not get noticed. Arnd