Re: [PATCH] perf: Synchronously cleanup child events

[Alexander Shishkin]

Peter Zijlstra <peterz@xxxxxxxxxxxxx> writes:

> On Fri, Jan 15, 2016 at 04:07:41PM +0200, Alexander Shishkin wrote:
>>  int perf_event_release_kernel(struct perf_event *event)
>>  {
>> +	struct perf_event *child, *tmp;
>> +	LIST_HEAD(child_list);
>>  
>> +	if (!is_kernel_event(event))
>> +		perf_remove_from_owner(event);
>>  
>> +	event->owner = NULL;
>>  
>> +	/*
>> +	 * event::child_mutex nests inside ctx::lock, so move children
>> +	 * to a safe place first and avoid inversion
>> +	 */
>> +	mutex_lock(&event->child_mutex);
>> +	list_splice_init(&event->child_list, &child_list);
>> +	mutex_unlock(&event->child_mutex);
>
> I suspect this races against inherit_event(), like:
>
> 	inherit_event()			perf_event_release_kernel()
>
> 	if (is_orphaned_event(parent_event) /* false */
>
> 					event->owner = NULL
>
> 	mutex_lock(child_mutex);
> 	list_splice
> 	mutex_unlock(child_mutex);
>
> 					mutex_lock(child_mutex);
> 					list_add_tail
> 					mutex_unlock(child_mutex);

Indeed, this is possible.

>
> Something like this would fix that I think, not sure its the best way
> though...
>
>
> --- a/kernel/events/core.c
> +++ b/kernel/events/core.c
> @@ -979,8 +979,8 @@ static void put_ctx(struct perf_event_co
>   * Lock order:
>   *	task_struct::perf_event_mutex
>   *	  perf_event_context::mutex
> - *	    perf_event_context::lock
>   *	    perf_event::child_mutex;
> + *	      perf_event_context::lock
>   *	    perf_event::mmap_mutex
>   *	    mmap_sem
>   */

This is, actually, the order that we have already:

perf_ioctl():                ctx::mutex
-> perf_event_for_each():    event::child_mutex
   -> _perf_event_enable():  ctx::lock

that is, ctx::lock already nests inside event::child_mutex. So what
you're suggesting is an ok solution.

Regards,
--
Alex