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    Subject[PATCH 3.8 29/86] genirq: Remove racy waitqueue_active check
    Date
    3.8.13.21 -stable review patch.  If anyone has any objections, please let me know.

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

    From: Chuansheng Liu <chuansheng.liu@intel.com>

    commit c685689fd24d310343ac33942e9a54a974ae9c43 upstream.

    We hit one rare case below:

    T1 calling disable_irq(), but hanging at synchronize_irq()
    always;
    The corresponding irq thread is in sleeping state;
    And all CPUs are in idle state;

    After analysis, we found there is one possible scenerio which
    causes T1 is waiting there forever:
    CPU0 CPU1
    synchronize_irq()
    wait_event()
    spin_lock()
    atomic_dec_and_test(&threads_active)
    insert the __wait into queue
    spin_unlock()
    if(waitqueue_active)
    atomic_read(&threads_active)
    wake_up()

    Here after inserted the __wait into queue on CPU0, and before
    test if queue is empty on CPU1, there is no barrier, it maybe
    cause it is not visible for CPU1 immediately, although CPU0 has
    updated the queue list.
    It is similar for CPU0 atomic_read() threads_active also.

    So we'd need one smp_mb() before waitqueue_active.that, but removing
    the waitqueue_active() check solves it as wel l and it makes
    things simple and clear.

    Signed-off-by: Chuansheng Liu <chuansheng.liu@intel.com>
    Cc: Xiaoming Wang <xiaoming.wang@intel.com>
    Link: http://lkml.kernel.org/r/1393212590-32543-1-git-send-email-chuansheng.liu@intel.com
    Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
    Signed-off-by: Kamal Mostafa <kamal@canonical.com>
    ---
    kernel/irq/manage.c | 3 +--
    1 file changed, 1 insertion(+), 2 deletions(-)

    diff --git a/kernel/irq/manage.c b/kernel/irq/manage.c
    index a9302d0..f3feefc 100644
    --- a/kernel/irq/manage.c
    +++ b/kernel/irq/manage.c
    @@ -801,8 +801,7 @@ static irqreturn_t irq_thread_fn(struct irq_desc *desc,

    static void wake_threads_waitq(struct irq_desc *desc)
    {
    - if (atomic_dec_and_test(&desc->threads_active) &&
    - waitqueue_active(&desc->wait_for_threads))
    + if (atomic_dec_and_test(&desc->threads_active))
    wake_up(&desc->wait_for_threads);
    }

    --
    1.8.3.2


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