Re: Race condition in time/alarmtimer.c

[John Stultz]

On 06/29/2013 06:47 AM, Marcus Gelderie wrote:
> On Mo, Jun 24, 2013 at 09:12:03PM +0200, Marcus Gelderie wrote:
> > Hi,
> >
> > there seems to be a race condition in kernel/time/alarmtimer.c
> >
> > More specifically, the following function (line numbers correspond to actual file):
> >
> > 584 static int alarmtimer_do_nsleep(struct alarm *alarm, ktime_t absexp)
> > 585 {
> > 586         alarm->data = (void *)current;
> > 587         do {
> > 588                 set_current_state(TASK_INTERRUPTIBLE);
> > 589                 alarm_start(alarm, absexp);
> > 590                 if (likely(alarm->data))
> > 591                         schedule();
> > 592
> > 593                 alarm_cancel(alarm);
> > 594         } while (alarm->data && !signal_pending(current));
> > 595
> > 596         __set_current_state(TASK_RUNNING);
> > 597
> > 598         return (alarm->data == NULL);
> > 599 }
> >
> > has a race: If the task is preempted after set_current_state(TASK_INTERRUPTIBLE)
> > but before the alarm is started in the next line, the task never wakes up.
> >
> > Swapping both lines is not an option either, because then the alarm might trigger before
> > the thread sets itself to TASK_INTERRUPTIBLE, thereby loosing the wakeup.
> >
> > A spinlock would disable preemption and protect alarm->data against the race from another CPU.
> > We could wrap lines 588 and 589 with a spin lock. Then the wakeup code would also aquire the
> > lock, of course. The lock could be attached to struct alarm.
> >
> > An alternative would be a waitqueue, of course.
> >
> > If folks agree with me, I will provide a patch.

So does this race also affect the hrtimer do_nanosleep?

thanks
-john


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