linux-kernel - Re: [PATCH v2] perf: Synchronously cleanup child events

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Message-ID: <20160125114846.GW6357@twins.programming.kicks-ass.net>
Date:	Mon, 25 Jan 2016 12:48:46 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc:	Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
	Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
	vince@...ter.net, eranian@...gle.com,
	Arnaldo Carvalho de Melo <acme@...radead.org>,
	Jiri Olsa <jolsa@...nel.org>
Subject: Re: [PATCH v2] perf: Synchronously cleanup child events

On Fri, Jan 22, 2016 at 11:44:03AM -0800, Alexei Starovoitov wrote:
> On Fri, Jan 22, 2016 at 01:38:47PM +0100, Peter Zijlstra wrote:
> > On Fri, Jan 22, 2016 at 01:35:40PM +0200, Alexander Shishkin wrote:
> > > Peter Zijlstra <peterz@...radead.org> writes:
> > > 
> > > > So I think there's a number of problems still :-(
> 
> I've been looking at how perf_event->owner is handled and couldn't
> figure out how you deal with the case of passing perf_event_fd via scm_rights.
> It seems one process can open an event, pass it to another process,
> but when current process exists owner will still point to dead task,
> since refcount > 0.
> Which part am I missing?

Nothing, you raised a good point. I think this shows we cannot link
!event->owner to an event being 'dead'.

If we keep these two states separate, the scm_rights thing should work
again.