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

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Date:	Thu, 3 Dec 2009 08:57:32 +0100
From:	Dmitry Adamushko <dmitry.adamushko@...il.com>
To:	liu pf <kernelfans@...il.com>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: Should we use preempt_disable() in sleep_on_common()?

2009/12/3 liu pf <kernelfans@...il.com>:
> 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;
>    }

In this case, (preempt_count() & PREEMPT_ACTIVE) == 1(see
preempt_schedule_irq() and other use-cases of PREEMPT_ACTIVE) so we
don't  enter this block.

i.e. a preempted task stays on its queue (with state != TASK_RUNNING
but that doesn't really matter).


-- Dmitry
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