From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755237AbbFCLVw (ORCPT ); Wed, 3 Jun 2015 07:21:52 -0400 Received: from mail-wi0-f169.google.com ([209.85.212.169]:36622 "EHLO mail-wi0-f169.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753395AbbFCLVr (ORCPT ); Wed, 3 Jun 2015 07:21:47 -0400 Date: Wed, 3 Jun 2015 13:21:41 +0200 From: Ingo Molnar To: Milos Vyletel Cc: Jiri Olsa , Peter Zijlstra , Paul Mackerras , Ingo Molnar , Arnaldo Carvalho de Melo , Masami Hiramatsu , Namhyung Kim , Jiri Olsa , He Kuang , Adrian Hunter , "open list:PERFORMANCE EVENT..." Subject: Re: [PATCH 2/2] perf/tools: put new buildid locks to use Message-ID: <20150603112141.GA25670@gmail.com> References: <1431588493-1474-1-git-send-email-milos@redhat.com> <1431588493-1474-3-git-send-email-milos@redhat.com> <20150514104059.GA27771@gmail.com> <20150514113821.GB1313@krava.redhat.com> <20150514173807.GA19338@gmail.com> <20150603111310.GA3217@lenivo.usersys.redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20150603111310.GA3217@lenivo.usersys.redhat.com> User-Agent: Mutt/1.5.23 (2014-03-12) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org * Milos Vyletel wrote: > On Thu, May 14, 2015 at 07:38:08PM +0200, Ingo Molnar wrote: > > > > > * Milos Vyletel wrote: > > > > > On Thu, May 14, 2015 at 01:38:21PM +0200, Jiri Olsa wrote: > > > > > > > On Thu, May 14, 2015 at 12:40:59PM +0200, Ingo Molnar wrote: > > > > > > > > > > * Milos Vyletel wrote: > > > > > > > > > > > Use new read/write locks when accesing buildid directory on places where > > > > > > we may race if multiple instances are run simultaneously. > > > > > > > > > > Dunno, this will create locking interaction between multiple instances > > > > > of perf - hanging each other, etc. > > > > > > > > > > And it seems unnecessary: the buildid hierarchy is already spread out. > > > > > What kind of races might there be? > > > > > > > > there was just recently one fixed by commit: > > > > 0635b0f71424 perf tools: Fix race in build_id_cache__add_s() > > > > > > > > havent checked the final patch yet, but the idea is to > > > > protect us from similar bugs > > > > > > right. on top of race with EEXIST couple more are possible (EMLINK, > > > ENOSPC, EDQUOT, ENOMEM... the only way to prevent them all is to > > > lock this kind of operations and make sure we run one at a time. > > > > Yeah, so the race pointed out in 0635b0f71424 can be (and should be) > > fixed without locking: > > > > - first create the file under a process-private name under > > ~/.debug/tmp/ if the target does not exist yet > > > > - then fully fill it in with content > > > > - then link(2) it to the public target name, which VFS operation is > > atomic and may fail safely: at which point it got already created > > by someone else. > > > > - finally unlink() the private instance name and the target will now > > be the only instance left: either created by us, or by some other > > perf instance in the rare racy case. > > > > Since all of ~/.debug is on the same filesystem this should work fine. > > > > Beyond avoiding locking this approach has another advantage: it's > > transaction safe, so a crashed/interrupted perf instance won't corrupt > > the debug database, it will only put fully constructed files into the > > public build-id namespace. It at most leaves a stale private file > > around in ~/.debug/tmp/. > > > > Ingo, > > I finally found some time to make this change. While going over the code I've > noticed one thing that would make concurrent creation even easier to solve. > Instead of copying the file to temp file what about simply opening file with > O_CREAT|O_EXCL? creat itself > > "creat() is equivalent to open() with flags equal to O_CREAT|O_WRONLY|O_TRUNC." > > addition of O_EXCL would > > "Ensure that this call creates the file: if this flag is specified in > conjunction with O_CREAT, and pathname already exists, then open() will fail." > > This we would prevent truncation of already linked file in case link() races as > in 0635b0f71424. What do you think? But it would not prevent the problem of creating a not yet fully constructed file - which some other tool invocation could attempt to parse in an incomplete fashion. Using create+link+unlink avoids that race, the files in the publicly visible namespace will always be fully constructed by the time they are made visible (atomically). Thanks, Ingo