On 07/06/2013 09:37 PM, Marcus Gelderie wrote:
> This patch fixes a race condition whereby the process can be caused to
> sleep indefinitely. The problem occurs when the process is preempted
> after having set its state to TASK_INTERRUPTIBLE but before starting the
> alarm.
>
I don't think there is any such problem. The __schedule() function does
this check before dequeuing a task:
if (prev->state && !(preempt_count() & PREEMPT_ACTIVE))
The latter part helps prevent the kind of indefinite sleeping you described
above. Also, take a look at preempt_schedule(), preempt_schedule_irq()
and friends to see how in-kernel preemption is dealt with, by using
PREEMPT_ACTIVE. That would clarify why there is no bug here.
Regards,
Srivatsa S. Bhat
> ---
> kernel/time/alarmtimer.c | 6 +++++-
> 1 file changed, 5 insertions(+), 1 deletion(-)
>
> diff --git a/kernel/time/alarmtimer.c b/kernel/time/alarmtimer.c
> index f11d83b..81c8b31 100644
> --- a/kernel/time/alarmtimer.c
> +++ b/kernel/time/alarmtimer.c
> @@ -585,15 +585,19 @@ static int alarmtimer_do_nsleep(struct alarm *alarm,
> ktime_t absexp)
> {
> alarm->data = (void *)current;
> do {
> + preempt_disable();
> set_current_state(TASK_INTERRUPTIBLE);
> alarm_start(alarm, absexp);
> + preempt_enable_no_resched();
> if (likely(alarm->data))
> schedule();
>
> + preempt_disable();
> alarm_cancel(alarm);
> + __set_current_state(TASK_RUNNING);
> + preempt_enable_no_resched();
> } while (alarm->data && !signal_pending(current));
>
> - __set_current_state(TASK_RUNNING);
>
> return (alarm->data == NULL);
> }
>
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