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Date: Mon, 28 Aug 2017 21:23:07 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: Jiri Olsa <jolsa@kernel.org>
Cc: Arnaldo Carvalho de Melo <acme@kernel.org>,
        lkml <linux-kernel@vger.kernel.org>, Ingo Molnar <mingo@kernel.org>,
        Alexander Shishkin <alexander.shishkin@linux.intel.com>,
        Namhyung Kim <namhyung@kernel.org>, David Ahern <dsahern@gmail.com>,
        Andi Kleen <andi@firstfloor.org>, Mark Rutland <mark.rutland@arm.com>
Subject: Re: [PATCH 02/10] perf: Fix leader for removed sibling event in
 perf_group_detach
Message-ID: <20170828192307.5mxis44vg2m5ph3a@hirez.programming.kicks-ass.net>
References: <20170824162737.7813-1-jolsa@kernel.org>
 <20170824162737.7813-3-jolsa@kernel.org>
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On Thu, Aug 24, 2017 at 06:27:29PM +0200, Jiri Olsa wrote:
> Mark reported warning in x86_perf_event_update caused
> by reading unscheduled leader of an event that was
> already removed from the group.
> 
> As he pointed out we don't properly reset event's leader
> once it's been detached from the group and he posted the
> attached fix.
> 
> [1] https://marc.info/?l=linux-kernel&m=150238662928203&w=2
> 
> Originally-From: Mark Rutland <mark.rutland@arm.com>
> Signed-off-by: Jiri Olsa <jolsa@kernel.org>
> ---
>  kernel/events/core.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/kernel/events/core.c b/kernel/events/core.c
> index d704e23914bf..30e30e94ea32 100644
> --- a/kernel/events/core.c
> +++ b/kernel/events/core.c
> @@ -1727,6 +1727,7 @@ static void perf_group_detach(struct perf_event *event)
>  	if (event->group_leader != event) {
>  		list_del_init(&event->group_entry);
>  		event->group_leader->nr_siblings--;
> +		event->group_leader = event;
>  		goto out;
>  	}

No, I think this breaks other things... but I again forgot what. The
original issue he reported is fixed by patch:

  2bcb88eb04d3 ("perf/core: fix group {cpu,task} validation")

Also by Mark.