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.
Disabling preemption until the alarm has been set solves this problem.
A second problem may occur if the process reaches the alarm_cancel call
while still being in TASK_INTERRUPTIBLE. I do not see how this could
possibly happen (if schedule returns, the state is TASK_RUNNING; if
schedule is never called, the wakeup callback must have woken the
process (data==NULL) thereby setting its state to TASK_RUNNING) but
the code is clearly written under the assumption that it can happen.
In this case, the state must be set to TASK_RUNNING after the alarm
has been canceled for the same reasons as above.
Signed-off-by: Marcus Gelderie <redm...@gmail.com>
---
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);
}
--
1.8.1.5
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