linux-kernel - Re: Q: perf_event && event->owner

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Message-ID: <20101109174219.GA8279@redhat.com>
Date:	Tue, 9 Nov 2010 18:42:20 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc:	Frederic Weisbecker <fweisbec@...il.com>,
	Alan Stern <stern@...land.harvard.edu>,
	Arnaldo Carvalho de Melo <acme@...hat.com>,
	Ingo Molnar <mingo@...e.hu>, Paul Mackerras <paulus@...ba.org>,
	Prasad <prasad@...ux.vnet.ibm.com>,
	Roland McGrath <roland@...hat.com>,
	linux-kernel@...r.kernel.org
Subject: Re: Q: perf_event && event->owner

On 11/09, Peter Zijlstra wrote:
>
> Ah,.. quite so. So how about we explicitly destroy the list when the
> task dies?

Yes, I think it makes sense to destroy the list and set ->owner = NULL.
If we reset the owner, we can also avoid get_task_struct().

The only problem is perf_event_release_kernel(), it can race with the
exiting event->owner. It can do get_task_struct() under rcu lock temporary,
just to take the mutex and remove the entry.

> > And ptrace(), it doesn't use sys_perf_event_open() to create the event.
>
> Right, I guess it uses kernel based things, I guess we could not add
> kernel based counters to the list.

Agreed, another case when event->owner should be NULL.

Hmm. With or without these changes. Shouldn't perf_event_release_kernel()
remove the event from list before anything else? Otherwise, afaics a thread
which does close(event_fd) can race with creator doing prctl(EVENTS_ENABLE),
no?

Oleg.

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