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

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Message-ID: <20160122121222.GR6357@twins.programming.kicks-ass.net>
Date:	Fri, 22 Jan 2016 13:12:22 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Alexander Shishkin <alexander.shishkin@...ux.intel.com>
Cc:	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>, alexei.starovoitov@...il.com
Subject: Re: [PATCH v2] perf: Synchronously cleanup child events

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 :-(
> 
> Also, it does indeed race with
> __perf_event_exit_task()/sync_child_event(), but that one I'd fix by
> simply wrapping the sync_child_event()/free_event() in
> 
> mutex_lock(&parent_event->child_mutex);
> if (!is_orphan_event(parent_event)) {
>    sync_child_event(child_event);
>    free_event(child_event);
> }
> mutex_unlock(&parent_event->child_event);

So I've been staring at exactly that code for a while because Ingo
managed to trigger that race (and I could reproduce with his
'workload').

But I'm not seeing how; both sites hold ctx->mutex and remove the event
from the ctx->event_list.

So the way I'm seeing it, either the orphan_work find and frees it, or
the __perf_event_exit_task() one does, but I'm a bit stumped on how they
can both do.

Sure, the sync stuff is pointless if we're orphan, but I don't see how
it can harm.

> At some later point in time the code there could use a bit of
> reshuffling, I guess.

Yes.