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    Subject[tip:perf/core] perf: Fix group moving
    Commit-ID:  74c3337c2fc6389d3a57a622a936036b6db6b2e8
    Gitweb: http://git.kernel.org/tip/74c3337c2fc6389d3a57a622a936036b6db6b2e8
    Author: Peter Zijlstra <a.p.zijlstra@chello.nl>
    AuthorDate: Fri, 15 Oct 2010 11:40:29 +0200
    Committer: Ingo Molnar <mingo@elte.hu>
    CommitDate: Mon, 18 Oct 2010 19:58:51 +0200

    perf: Fix group moving

    Matt found we trigger the WARN_ON_ONCE() in perf_group_attach() when we take
    the move_group path in perf_event_open().

    Since we cannot de-construct the group (we rely on it to move the events), we
    have to simply ignore the double attach. The group state is context invariant
    and doesn't need changing.

    Reported-by: Matt Fleming <matt@console-pimps.org>
    Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
    LKML-Reference: <1287135757.29097.1368.camel@twins>
    Signed-off-by: Ingo Molnar <mingo@elte.hu>
    ---
    kernel/perf_event.c | 7 ++++++-
    1 files changed, 6 insertions(+), 1 deletions(-)

    diff --git a/kernel/perf_event.c b/kernel/perf_event.c
    index 99b9700..346dc0e 100644
    --- a/kernel/perf_event.c
    +++ b/kernel/perf_event.c
    @@ -315,7 +315,12 @@ static void perf_group_attach(struct perf_event *event)
    {
    struct perf_event *group_leader = event->group_leader;

    - WARN_ON_ONCE(event->attach_state & PERF_ATTACH_GROUP);
    + /*
    + * We can have double attach due to group movement in perf_event_open.
    + */
    + if (event->attach_state & PERF_ATTACH_GROUP)
    + return;
    +
    event->attach_state |= PERF_ATTACH_GROUP;

    if (group_leader == event)

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