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

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Message-ID: <20101109185748.GA12138@redhat.com>
Date:	Tue, 9 Nov 2010 19:57:48 +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:
>
> I think you're right, how about something like this?

I need to read it with a fresh head ;)

At first glance,

> @@ -2254,6 +2249,12 @@ static int perf_release(struct inode *in
>
>  	file->private_data = NULL;
>
> +	if (event->owner) {
> +		mutex_lock(&event->owner->perf_event_mutex);
> +		list_del_init(&event->owner_entry);
> +		mutex_unlock(&event->owner->perf_event_mutex);
> +	}

Agreed, it is better to do this in perf_release().

But, this can use the already freed task_struct, event->owner.

Either sys_perf_open() should do get_task_struct() like we currently
do, or perf_event_exit_task() should clear event->owner and then
perf_release() should do something like

	rcu_read_lock();
	owner = event->owner;
	if (owner)
		get_task_struct(owner);
	rcu_read_unlock();

	if (owner) {
		mutex_lock(&event->owner->perf_event_mutex);
		list_del_init(&event->owner_entry);
		mutex_unlock(&event->owner->perf_event_mutex);
		put_task_struct(owner);
	}

Probably this can be simplified...

Oleg.

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