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From: Peter Zijlstra <peterz@infradead.org>
To: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Ingo Molnar <mingo@redhat.com>,
	linux-kernel@vger.kernel.org, vince@deater.net,
	eranian@google.com, Arnaldo Carvalho de Melo <acme@infradead.org>,
	Jiri Olsa <jolsa@kernel.org>,
	alexei.starovoitov@gmail.com
Subject: Re: [PATCH v2] perf: Synchronously cleanup child events
Date: Tue, 19 Jan 2016 21:05:58 +0100	[thread overview]
Message-ID: <20160119200558.GC6357@twins.programming.kicks-ass.net> (raw)
In-Reply-To: <1453216354-9282-1-git-send-email-alexander.shishkin@linux.intel.com>

On Tue, Jan 19, 2016 at 05:12:34PM +0200, Alexander Shishkin wrote:

> +static void __put_event(struct perf_event *event)
>  {
>  	struct perf_event_context *ctx;
>  
>  	if (!is_kernel_event(event))
>  		perf_remove_from_owner(event);
>  

> +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;
>  
> +retry:

	<snip>

> +	/*
> +	 * If this is the last reference, we're done here, otherwise
> +	 * we must have raced with inherit_event(), in which case, repeat
> +	 */
> +	if (!put_event_last(event))
> +		goto retry;
>  
> +	return 0;
> +}

So I think there's a number of problems still :-(

I all starts with having two perf_remove_from_owner() calls (as I
mentioned on IRC), this doesn't make sense.

I think the moment you close the file and userspace looses control over
it, we should drop the owner bit, which is exactly the one
remove_from_owner in perf_release().

If, for some magical reason, the event lives on after that (and we'll
get to that), it should live on owner-less.

Now, assume someone has such a magical reference, then our
put_event_last() goto again loop will never terminate, this seems like a
bad thing.

The most obvious place that generates such magical references would be
the bpf arraymap doing perf_event_get() on things. There are a few other
places that take temp references (perf_mmap_close), but those are
'short' lived and while ugly will not cause massive grief. The BPF one
OTOH is a real problem here.

And looking at the BPF stuff, that code seems to assume
perf_event_kernel_release() := put_event(), so this patch breaks that
too.


Alexei, is there a reason the arraymap stuff needs a perf event ref as
opposed to a file ref? I'm forever a little confused on how perf<->bpf
works.

  reply	other threads:[~2016-01-19 20:06 UTC|newest]

Thread overview: 33+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2016-01-15 11:22 [PATCH] perf: Cleanup user's child events Alexander Shishkin
2016-01-15 12:54 ` Peter Zijlstra
2016-01-15 13:05   ` Alexander Shishkin
2016-01-15 13:09     ` Peter Zijlstra
2016-01-15 14:07       ` [PATCH] perf: Synchronously cleanup " Alexander Shishkin
2016-01-15 17:57         ` Peter Zijlstra
2016-01-18 12:07           ` Alexander Shishkin
2016-01-18 12:37           ` Alexander Shishkin
2016-01-18 14:44             ` Peter Zijlstra
2016-01-19 15:12               ` [PATCH v2] " Alexander Shishkin
2016-01-19 20:05                 ` Peter Zijlstra [this message]
2016-01-19 21:58                   ` Alexei Starovoitov
2016-01-20  8:32                     ` Peter Zijlstra
2016-01-21  4:55                       ` Alexei Starovoitov
2016-01-20  7:04                   ` Alexander Shishkin
2016-01-20  8:03                     ` Peter Zijlstra
2016-01-22 11:35                   ` Alexander Shishkin
2016-01-22 12:12                     ` Peter Zijlstra
2016-01-22 12:38                     ` Peter Zijlstra
2016-01-22 19:44                       ` Alexei Starovoitov
2016-01-25 11:48                         ` Peter Zijlstra
2016-01-25 14:54                           ` Peter Zijlstra
2016-01-25 21:04                             ` Peter Zijlstra
2016-01-26  4:59                               ` Alexei Starovoitov
2016-01-26 16:16                                 ` Peter Zijlstra
2016-01-26 17:24                                   ` Peter Zijlstra
2016-01-26 23:31                                     ` Alexei Starovoitov
2016-01-27  9:58                                       ` Peter Zijlstra
2016-01-27 17:52                                         ` Alexei Starovoitov
2016-01-29 11:28                                 ` [tip:perf/urgent] perf/bpf: Convert perf_event_array to use struct file tip-bot for Alexei Starovoitov
2016-01-29 20:01                                   ` Alexei Starovoitov
2016-01-19 20:07                 ` [PATCH v2] perf: Synchronously cleanup child events Peter Zijlstra
2016-01-19  7:45         ` [PATCH] " Ingo Molnar

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