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Re: [PATCH] perf: Synchronously cleanup child events

From Alexander Shishkin <alexander.shishkin@linux.intel.com>
Newsgroups linux.kernel
Subject Re: [PATCH] perf: Synchronously cleanup child events
Date 2016-01-18 13:20 +0100
Message-ID <qSgWR-uP-11@gated-at.bofh.it> (permalink)
References <qRciC-6vV-27@gated-at.bofh.it> <qRdeG-7bt-31@gated-at.bofh.it> <qRgPh-ZC-21@gated-at.bofh.it>
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Peter Zijlstra <peterz@infradead.org> 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

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Thread

[PATCH] perf: Cleanup user's child events Alexander Shishkin <alexander.shishkin@linux.intel.com> - 2016-01-15 12:30 +0100
  Re: [PATCH] perf: Cleanup user's child events Peter Zijlstra <peterz@infradead.org> - 2016-01-15 14:00 +0100
    Re: [PATCH] perf: Cleanup user's child events Alexander Shishkin <alexander.shishkin@linux.intel.com> - 2016-01-15 14:10 +0100
      Re: [PATCH] perf: Cleanup user's child events Peter Zijlstra <peterz@infradead.org> - 2016-01-15 14:10 +0100
        [PATCH] perf: Synchronously cleanup child events Alexander Shishkin <alexander.shishkin@linux.intel.com> - 2016-01-15 15:10 +0100
          Re: [PATCH] perf: Synchronously cleanup child events Peter Zijlstra <peterz@infradead.org> - 2016-01-15 19:00 +0100
            Re: [PATCH] perf: Synchronously cleanup child events Alexander Shishkin <alexander.shishkin@linux.intel.com> - 2016-01-18 13:20 +0100
            Re: [PATCH] perf: Synchronously cleanup child events Alexander Shishkin <alexander.shishkin@linux.intel.com> - 2016-01-18 13:40 +0100
              Re: [PATCH] perf: Synchronously cleanup child events Peter Zijlstra <peterz@infradead.org> - 2016-01-18 15:50 +0100
                [PATCH v2] perf: Synchronously cleanup child events Alexander Shishkin <alexander.shishkin@linux.intel.com> - 2016-01-19 16:20 +0100
                Re: [PATCH v2] perf: Synchronously cleanup child events Peter Zijlstra <peterz@infradead.org> - 2016-01-19 21:10 +0100
                Re: [PATCH v2] perf: Synchronously cleanup child events Peter Zijlstra <peterz@infradead.org> - 2016-01-19 21:10 +0100
                Re: [PATCH v2] perf: Synchronously cleanup child events Alexei Starovoitov <alexei.starovoitov@gmail.com> - 2016-01-19 23:00 +0100
                Re: [PATCH v2] perf: Synchronously cleanup child events Peter Zijlstra <peterz@infradead.org> - 2016-01-20 09:40 +0100
                Re: [PATCH v2] perf: Synchronously cleanup child events Alexei Starovoitov <alexei.starovoitov@gmail.com> - 2016-01-21 06:00 +0100
                Re: [PATCH v2] perf: Synchronously cleanup child events Alexander Shishkin <alexander.shishkin@linux.intel.com> - 2016-01-20 08:10 +0100
                Re: [PATCH v2] perf: Synchronously cleanup child events Peter Zijlstra <peterz@infradead.org> - 2016-01-20 09:10 +0100
                Re: [PATCH v2] perf: Synchronously cleanup child events Alexander Shishkin <alexander.shishkin@linux.intel.com> - 2016-01-22 12:40 +0100
                Re: [PATCH v2] perf: Synchronously cleanup child events Peter Zijlstra <peterz@infradead.org> - 2016-01-22 13:20 +0100
                Re: [PATCH v2] perf: Synchronously cleanup child events Peter Zijlstra <peterz@infradead.org> - 2016-01-22 13:40 +0100
                Re: [PATCH v2] perf: Synchronously cleanup child events Alexei Starovoitov <alexei.starovoitov@gmail.com> - 2016-01-22 20:50 +0100
                Re: [PATCH v2] perf: Synchronously cleanup child events Peter Zijlstra <peterz@infradead.org> - 2016-01-25 12:50 +0100
                Re: [PATCH v2] perf: Synchronously cleanup child events Peter Zijlstra <peterz@infradead.org> - 2016-01-25 16:00 +0100
                Re: [PATCH v2] perf: Synchronously cleanup child events Peter Zijlstra <peterz@infradead.org> - 2016-01-25 22:10 +0100
                Re: [PATCH v2] perf: Synchronously cleanup child events Alexei Starovoitov <alexei.starovoitov@gmail.com> - 2016-01-26 06:10 +0100
                Re: [PATCH v2] perf: Synchronously cleanup child events Peter Zijlstra <peterz@infradead.org> - 2016-01-26 17:20 +0100
                Re: [PATCH v2] perf: Synchronously cleanup child events Peter Zijlstra <peterz@infradead.org> - 2016-01-26 18:30 +0100
                Re: [PATCH v2] perf: Synchronously cleanup child events Alexei Starovoitov <alexei.starovoitov@gmail.com> - 2016-01-27 00:40 +0100
                Re: [PATCH v2] perf: Synchronously cleanup child events Peter Zijlstra <peterz@infradead.org> - 2016-01-27 11:00 +0100
                Re: [PATCH v2] perf: Synchronously cleanup child events Alexei Starovoitov <alexei.starovoitov@gmail.com> - 2016-01-27 19:00 +0100
          Re: [PATCH] perf: Synchronously cleanup child events Ingo Molnar <mingo@kernel.org> - 2016-01-19 08:50 +0100

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