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    Subject[ 36/91] workqueue: skip nr_running sanity check in worker_enter_idle() if trustee is active
    3.4-stable review patch.  If anyone has any objections, please let me know.

    ------------------

    From: Tejun Heo <tj@kernel.org>

    commit 544ecf310f0e7f51fa057ac2a295fc1b3b35a9d3 upstream.

    worker_enter_idle() has WARN_ON_ONCE() which triggers if nr_running
    isn't zero when every worker is idle. This can trigger spuriously
    while a cpu is going down due to the way trustee sets %WORKER_ROGUE
    and zaps nr_running.

    It first sets %WORKER_ROGUE on all workers without updating
    nr_running, releases gcwq->lock, schedules, regrabs gcwq->lock and
    then zaps nr_running. If the last running worker enters idle
    inbetween, it would see stale nr_running which hasn't been zapped yet
    and trigger the WARN_ON_ONCE().

    Fix it by performing the sanity check iff the trustee is idle.

    Signed-off-by: Tejun Heo <tj@kernel.org>
    Reported-by: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

    ---
    kernel/workqueue.c | 9 +++++++--
    1 file changed, 7 insertions(+), 2 deletions(-)

    --- a/kernel/workqueue.c
    +++ b/kernel/workqueue.c
    @@ -1210,8 +1210,13 @@ static void worker_enter_idle(struct wor
    } else
    wake_up_all(&gcwq->trustee_wait);

    - /* sanity check nr_running */
    - WARN_ON_ONCE(gcwq->nr_workers == gcwq->nr_idle &&
    + /*
    + * Sanity check nr_running. Because trustee releases gcwq->lock
    + * between setting %WORKER_ROGUE and zapping nr_running, the
    + * warning may trigger spuriously. Check iff trustee is idle.
    + */
    + WARN_ON_ONCE(gcwq->trustee_state == TRUSTEE_DONE &&
    + gcwq->nr_workers == gcwq->nr_idle &&
    atomic_read(get_gcwq_nr_running(gcwq->cpu)));
    }




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