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Date: Fri, 20 Mar 2015 10:26:15 -0300
From: Arnaldo Carvalho de Melo <acme@kernel.org>
To: Jiri Olsa <jolsa@redhat.com>
Cc: Milos Vyletel <milos@redhat.com>, Peter Zijlstra <a.p.zijlstra@chello.nl>,
        Paul Mackerras <paulus@samba.org>, Ingo Molnar <mingo@redhat.com>,
        Namhyung Kim <namhyung@kernel.org>, Jiri Olsa <jolsa@kernel.org>,
        Stephane Eranian <eranian@google.com>,
        "open list:PERFORMANCE EVENT..." <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH] perf: fix race in build_id_cache__add_s()
Message-ID: <20150320132615.GE16485@kernel.org>
References: <1426847846-11112-1-git-send-email-milos@redhat.com>
 <20150320121807.GB6304@krava.brq.redhat.com>
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Em Fri, Mar 20, 2015 at 01:18:07PM +0100, Jiri Olsa escreveu:
> On Fri, Mar 20, 2015 at 11:37:25AM +0100, Milos Vyletel wrote:
> > int build_id_cache__add_s(const char *sbuild_id, const char *debugdir,
> >                           const char *name, bool is_kallsyms, bool is_vdso)
> > {
> > ...
> >         if (access(filename, F_OK)) {
> >                ^--------------------------------------------------------- [1]
> >                 if (is_kallsyms) {
> >                          if (copyfile("/proc/kallsyms", filename))
> >                                 goto out_free;
> >                 } else if (link(realname, filename) && copyfile(name, filename))
> >                              ^-----------------------------^------------- [2]
> >                                                             \------------ [3]
> >                         goto out_free;
> >         }
> > ...
> > 
> > when multiple instances of perf record get to [1] at more or less same time and
> > run access() one or more may get failure because the file does not exist yet
> > (since the first instance did not have chance to link it yet). at this point the
> > race moves to link() at [2] where first thread to get there links file and goes
> > on but second one gets -EEXIST so it runs copyfile [3] which truncates the file.
> 
> nice.. :-\ 
> 
> Acked-by: Jiri Olsa <jolsa@kernel.org>
> 
> in addition we should use some inter-perf lock
> on all .debug dir operations

Yeah, would be nice to have that improved.

Thanks, applied.

- Arnaldo