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    SubjectRe: [patch] sys_epoll ...
    Davide Libenzi wrote:
    >
    > On Sun, 20 Oct 2002, Andrew Morton wrote:
    >
    > > + if (ep->eventcnt || !timeout)
    > > + break;
    > > + if (signal_pending(current)) {
    > > + res = -EINTR;
    > > + break;
    > > + }
    > > +
    > > + set_current_state(TASK_INTERRUPTIBLE);
    > > +
    > > + write_unlock_irqrestore(&ep->lock, flags);
    > > + timeout = schedule_timeout(timeout);
    > >
    > > You should set current->state before performing the checks.
    >
    > Why this Andrew ?
    >

    Well I'm assuming that you don't want to sleep if, say,
    ep->eventcnt is non-zero. The code is currently (simplified):

    add_wait_queue(...);
    if (ep->eventcnt)
    break;
    /* window here */
    set_current_state(TASK_INTERRUPTIBLE);
    schedule();

    If another CPU increments eventcnt and sends this task a wakeup in that
    window, it is missed and we still sleep. The conventional fix for that
    is:

    add_wait_queue(...);
    set_current_state(TASK_INTERRUPTIBLE);
    if (ep->eventcnt)
    break;
    /* harmless window here */
    schedule();

    So if someone delivers a wakeup in the "harmless window" then this task
    still calls schedule(), but the wakeup has turned the state from
    TASK_INTERRUPTIBLE into TASK_RUNNING, so the schedule() doesn't actually
    take this task off the runqueue. This task will zoom straight through the
    schedule() and will then loop back and notice the incremented ep->eventcnt.

    So it is important that the waker increment eventcnt _before_ delivering
    the wake_up, too.
    -
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