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

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Date:	Wed, 10 Nov 2010 16:17:13 +0100
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Oleg Nesterov <oleg@...hat.com>
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 Tue, 2010-11-09 at 19:57 +0100, Oleg Nesterov wrote:
> 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... 

I think that's still racy, suppose we do:

void perf_event_exit_task(struct task_struct *child)
{
	struct perf_event *event, *tmp;
	int ctxn;

	mutex_lock(&child->perf_event_mutex);
	list_for_each_entry_safe(event, tmp, &child->perf_event_list,
				 owner_entry) {
		event->owner = NULL;
		list_del_init(&event->owner_entry);
	}
	mutex_unlock(&child->perf_event_mutex);

	for_each_task_context_nr(ctxn)
		perf_event_exit_task_context(child, ctxn);
}


and the close() races with an exit, then couldn't we observe
event->owner after the last put_task_struct()? In which case a
get_task_struct() will result in a double-free.


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