linux-kernel - Re: [PATCH 04/16] perf, mmap: Factor out perf_get

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Message-ID: <20140425145205.GV13658@twins.programming.kicks-ass.net>
Date:	Fri, 25 Apr 2014 16:52:05 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Robert Richter <rric@...nel.org>
Cc:	Jean Pihet <jean.pihet@...aro.org>, Borislav Petkov <bp@...en8.de>,
	Ingo Molnar <mingo@...nel.org>,
	Arnaldo Carvalho de Melo <acme@...radead.org>,
	Jiri Olsa <jolsa@...hat.com>, linux-kernel@...r.kernel.org,
	Al Viro <viro@...IV.linux.org.uk>
Subject: Re: [PATCH 04/16] perf, mmap: Factor out perf_get_fd()

On Fri, Apr 25, 2014 at 04:43:01PM +0200, Peter Zijlstra wrote:
> On Fri, Apr 25, 2014 at 03:54:13PM +0200, Robert Richter wrote:
> > I guess its this patch from Al you referring to:
> > 
> >  ea635c6 Fix racy use of anon_inode_getfd() in perf_event.c
> > 
> > I think we do not introduce it back due to:
> > 
> >  a6fa941 perf_event: Switch to internal refcount, fix race with close()
> > 
> > which removes the user of event_file.
> 
> linux-2.6# git show a6fa941
> error: short SHA1 a6fa941 is ambiguous.
> error: short SHA1 a6fa941 is ambiguous.
> fatal: ambiguous argument 'a6fa941': unknown revision or path not in the working tree.
> 
> Which is I think why Linus makes us use 12 chars instead of the git
> default of 8.
> 
> Of course its also entirely useless of git to not list the full IDs for
> those it did find to be ambiguous.

ok, so its a6fa941d94b4.

But no, I don't think that helps, its still true that the moment you get
a fd another thread can immediately close(). That would drop the last
ref and free it, meanwhile perf_event_open() is happily poking at it.

Now I think you could cure this by adding an extra ref before calling
your perf_get_fd() and dropping that extra ref at the end, where we used
to have fd_install().
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