On Tue, Feb 19, 2008 at 12:03:37AM +0100, Dmitry Adamushko wrote:
>
> Hi,
>
>
> [ description ]
>
> Subject: kthread: add a memory barrier to kthread_stop()
>
> 'kthread' threads do a check in the following order:
> - set_current_state(TASK_INTERRUPTIBLE);
> - kthread_should_stop();
>
> and set_current_state() implies an smp_mb().
>
> on another side (kthread_stop), wake_up_process() does not seem to
> guarantee a full mb.
>
> And 'kthread_stop_info.k' must be visible before wake_up_process()
> checks for/modifies a state of the 'kthread' task.
>
> (the patch is at the end of the message)
>
>
> [ more detailed description ]
>
> the current code might well be safe in case a to-be-stopped 'kthread'
> task is _not_ running on another CPU at the moment when kthread_stop()
> is called (in this case, 'rq->lock' will act as a kind of synch.
> point/barrier).
>
> Another case is as follows:
>
> CPU#0:
>
> ...
> while (kthread_should_stop()) {
>
> if (condition)
> schedule();
>
> /* ... do something useful ... */ <--- EIP
>
> set_current_state(TASK_INTERRUPTIBLE);
> }
>
> so a 'kthread' task is about to call
> set_current_state(TASK_INTERRUPTIBLE) ...
>
>
> (in the mean time)
>
> CPU#1:
>
> kthread_stop()
>
> -> kthread_stop_info.k = k (*)
> -> wake_up_process()
>
> wake_up_process() looks like:
>
> (try_to_wake_up)
>
> IRQ_OFF
> LOCK
>
> old_state = p->state;
> if (!(old_state & state)) (**)
> goto out;
>
> ...
>
> UNLOCK
> IRQ_ON
>
>
> let's suppose (*) and (**) are reordered
> (according to Documentation/memory-barriers.txt, neither IRQ_OFF nor
> LOCK may prevent it from happening).
>
> - the state is TASK_RUNNING, so we are about to return.
>
> - CPU#1 is about to execute (*) (it's guaranteed to be done before
> spin_unlock(&rq->lock) at the end of try_to_wake_up())
>
>
> (in the mean time)
>
> CPU#0:
>
> - set_current_state(TASK_INTERRUPTIBLE);
> - kthread_should_stop();
>
> here, kthread_stop_info.k is not yet visible
>
> - schedule()
>
> ...
>
> we missed a 'kthread_stop' event.
>
> hum?
>
>
> TIA,
>
> ---
>
> From: Dmitry Adamushko <[EMAIL PROTECTED]>
> Subject: kthread: add a memory barrier to kthread_stop()
>
> 'kthread' threads do a check in the following order:
> - set_current_state(TASK_INTERRUPTIBLE);
> - kthread_should_stop();
>
> and set_current_state() implies an smp_mb().
>
> on another side (kthread_stop), wake_up_process() is not guaranteed to
> act as a full mb.
>
> 'kthread_stop_info.k' must be visible before wake_up_process() checks
> for/modifies a state of the 'kthread' task.
>
>
> Signed-off-by: Dmitry Adamushko <[EMAIL PROTECTED]>
>
>
> diff --git a/kernel/kthread.c b/kernel/kthread.c
> index 0ac8878..5167110 100644
> --- a/kernel/kthread.c
> +++ b/kernel/kthread.c
> @@ -211,6 +211,10 @@ int kthread_stop(struct task_struct *k)
>
> /* Now set kthread_should_stop() to true, and wake it up. */
> kthread_stop_info.k = k;
> +
> + /* The previous store operation must not get ahead of the wakeup. */
> + smp_mb();
> +
> wake_up_process(k);
> put_task_struct(k);
The rules as written do seem to support your theory. The CPU has every
right to delay the .k = k as late as the UNLOCK operation.
On the read-side there is a full barrier implied by the
set_current_state(TASK_INTERRUPTIBLE), however this synchronises us with
the current global state, which may well not have the updated version
of .k.
That seems to imply that a write memory barrier would be sufficient to
cover this.
So three comments. First, should this not be an smp_wmb. Second, this
memory barrier is paired with the barrier in
set_current_state(TASK_INTERRUPTIBLE) and that probabally should be
documented as part of this patch. Finally, I think the comment as is is
hard to understand I got the sense of it backwards on first reading;
perhaps something like this:
/*
* Ensure kthread_stop_info.k is visible before wakeup, paired
* with barrier in set_current_state().
*/
-apw
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