Re: [PATCH 2/2] perf/tools: put new buildid locks to use

From: Ingo Molnar <mingo@kernel.org>
To: Milos Vyletel <milos@redhat.com>
Cc: Jiri Olsa <jolsa@redhat.com>,
	Peter Zijlstra <a.p.zijlstra@chello.nl>,
	Paul Mackerras <paulus@samba.org>, Ingo Molnar <mingo@redhat.com>,
	Arnaldo Carvalho de Melo <acme@kernel.org>,
	Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>,
	Namhyung Kim <namhyung@kernel.org>, Jiri Olsa <jolsa@kernel.org>,
	He Kuang <hekuang@huawei.com>,
	Adrian Hunter <adrian.hunter@intel.com>,
	"open list:PERFORMANCE EVENT..." <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH 2/2] perf/tools: put new buildid locks to use
Date: Thu, 14 May 2015 19:38:49 +0200	[thread overview]
Message-ID: <20150514173849.GB19338@gmail.com> (raw)
In-Reply-To: <20150514154420.GD2247@lenivo.usersys.redhat.com>

* Milos Vyletel <milos@redhat.com> 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 <milos@redhat.com> 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.

No, that's not the only way to avoid the race - see my previous mail.

Thanks,

	Ingo