linux-kernel - Should we use preempt_disable() in sleep_on

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Message-ID: <563a732e0912022249y1e831c6cv7f2d5d376b8a5f63@mail.gmail.com>
Date:	Thu, 3 Dec 2009 14:49:14 +0800
From:	liu pf <kernelfans@...il.com>
To:	linux-kernel@...r.kernel.org
Subject: Should we use preempt_disable() in sleep_on_common()?

Hi:

I am puzzled with the following scenario. Could anyone enlighten me?

Thanks
pfliu


static long __sched
sleep_on_common(wait_queue_head_t *q, int state, long timeout)
{
    unsigned long flags;
    wait_queue_t wait;

    init_waitqueue_entry(&wait, current);

    __set_current_state(state);

    ==========>suppose that after task A  set state=TASK_INTERRUPTIBLE
, it is preempted by task B.

    spin_lock_irqsave(&q->lock, flags);
...............................................................
}

asmlinkage void __sched schedule(void)
{
.......................................................................................................
    if (prev->state && !(preempt_count() & PREEMPT_ACTIVE)) {
        if (unlikely(signal_pending_state(prev->state, prev)))
            prev->state = TASK_RUNNING;
        else
            deactivate_task(rq, prev, 1);
       =============>This will remove task A from rq, but there are no
wait queue referring to A, so we lose A.
        switch_count = &prev->nvcsw;
    }

..................................................................................................
}
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