(s/mm-commits/lkml, cc Steven and Linus).
On 02/22, Andrew Morton wrote:
>
> From: Dmitry Adamushko <[EMAIL PROTECTED]>
>
> We must ensure that kthread_stop_info.k has been updated before kthread's
> wakeup. This is required to properly support the use of kthread_should_stop()
> in the main loop of kthread.
>
> wake_up_process() doesn't imply a full memory barrier,
> so we add an explicit one.
I tried to raise the similar issue twice without success.
> --- a/kernel/kthread.c~kthread-add-a-missing-memory-barrier-to-kthread_stop
> +++ a/kernel/kthread.c
> @@ -53,6 +53,19 @@ static struct kthread_stop_info kthread_
> * When someone calls kthread_stop() on your kthread, it will be woken
> * and this will return true. You should then return, and your return
> * value will be passed through to kthread_stop().
> + *
> + * In order to safely use kthread_stop() for kthread, there is a requirement
> + * on how its main loop has to be orginized. Namely, the sequence of
> + * events that lead to kthread being blocked (schedule()) has to be
> + * ordered as follows:
> + *
> + * - set_current_state(TASK_INTERRUPTIBLE);
> + * - if (kthread_should_stop()) break;
> + * - schedule() or similar.
> + *
> + * set_current_state() implies a full memory barrier. kthread_stop()
> + * has a matching barrier right after an update of kthread_stop_info.k
> + * and before kthread's wakeup.
> */
> int kthread_should_stop(void)
> {
> @@ -211,6 +224,15 @@ int kthread_stop(struct task_struct *k)
>
> /* Now set kthread_should_stop() to true, and wake it up. */
> kthread_stop_info.k = k;
> +
> + /*
> + * We must ensure that kthread_stop_info.k has been updated before
> + * the following wakeup. This is required to properly support the use
> + * of kthread_should_stop() in the main loop of kthread
> + * (see description of kthread_should_stop() for more details).
> + */
> + smp_mb();
> +
> wake_up_process(k);
> put_task_struct(k);
I think we should fix wake_up_process() instead.
Please look at http://marc.info/?l=linux-kernel&m=118503598307267
and at this thread: http://marc.info/?t=116275561700001
In short: wake_up_process() doesn't imply mb(), this means that _in theory_
the commonly used code like
set_current_state(TASK_INTERRUPTIBLE);
if (CONDITION)
return;
schedule();
is racy wrt
CONDITION = 1;
wake_up_process(p);
I'll be happy to be wrong though, please correct me.
Oleg.
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to [EMAIL PROTECTED]
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/
