Re: Race condition in time/alarmtimer.c

[Marcus Gelderie]

Hi,

bouncing this mail because originally my mail address was mangled due to MUA misconfig.

Sorry
Marcus


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.
> 
> 
> Cheers
> Marcus
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