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    SubjectRe: [PATCH 3/3] ipc/mqueue: remove STATE_PENDING
    On 04/07/2015 05:03 PM, Sebastian Andrzej Siewior wrote:
    > This patch moves the wakeup_process() invocation so it is not done under
    > the info->lock. With this change, the waiter is woken up once it is
    > "ready" which means its state is STATE_READY and it does not need to loop
    > on SMP if it is still in STATE_PENDING.
    > In the timeout case we still need to grab the info->lock to verify the state.
    >
    > This change should also avoid the introduction of preempt_disable() in
    > -RT which avoids a busy-loop which pools for the STATE_PENDING -> STATE_READY
    > change if the waiter has a higher priority compared to the waker.

    > @@ -909,9 +905,8 @@ SYSCALL_DEFINE1(mq_unlink, const char __user *, u_name)
    > * bypasses the message array and directly hands the message over to the
    > * receiver.
    > * The receiver accepts the message and returns without grabbing the queue
    > - * spinlock. Therefore an intermediate STATE_PENDING state and memory barriers
    > - * are necessary. The same algorithm is used for sysv semaphores, see
    > - * ipc/sem.c for more details.
    > + * spinlock. The same algorithm is used for sysv semaphores, see ipc/sem.c
    > + * for more details.
    No. With your change, ipc/sem.c and ipc/msg.c use different algorithms.
    Please update the comment and describe the new approach:

    Current approach:
    - set pointer to message
    - STATE_PENDING
    - wake_up_process()
    - STATE_READY
    (now the receiver can continue)

    New approach:
    - set pointer to message
    - get_task_struct
    - STATE_READY
    (now the receiver can continue, e.g. woken up due to an unrelated
    SIGKILL)
    - wake_up_process()
    - put_task_struct()


    > + if (r_sender) {
    > + wake_up_process(r_sender);
    > + put_task_struct(r_sender);
    > + }
    > ret = 0;
    Could you double-check that it is safe to call wake_up_process on a
    killed and reaped thread, only with a get_task_struct reference?

    And: please test it, too. (patch the kernel so that you can trigger this
    case).

    --
    Manfred



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