linux-kernel - Re: [PATCH] kernel:time fix race condition in alaramtimer.c

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Date:	Mon, 08 Jul 2013 11:01:49 +0530
From:	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>
To:	Marcus Gelderie <redmnic@...il.com>
CC:	artem.bityutskiy@...ux.intel.com, herbert@...dor.apana.org.au,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] kernel:time fix race condition in alaramtimer.c

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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