Re: [PATCH 1/6] wake_q: Cleanup + Documentation update.

[Davidlohr Bueso]

On Sat, 12 Oct 2019, Manfred Spraul wrote:

> 1) wake_q_add() contains a memory barrier, and callers such as
> ipc/mqueue.c rely on this barrier.
> Unfortunately, this is documented in ipc/mqueue.c, and not in the
> description of wake_q_add().
> Therefore: Update the documentation.
> Removing/updating ipc/mqueue.c will happen with the next patch in the
> series.
>
> 2) wake_q_add() ends with get_task_struct(), which is an
> unordered refcount increase. Add a clear comment that the callers
> are responsible for a barrier: most likely spin_unlock() or
> smp_store_release().
>
> 3) wake_up_q() relies on the memory barrier in try_to_wake_up().
> Add a comment, to simplify searching.
>
> 4) wake_q.next is accessed without synchroniyation by wake_q_add(),
> using cmpxchg_relaxed(), and by wake_up_q().
> Therefore: Use WRITE_ONCE in wake_up_q(), to ensure that the
> compiler doesn't perform any tricks.
>
> Signed-off-by: Manfred Spraul <manfred@xxxxxxxxxxxxxxxx>
> Cc: Davidlohr Bueso <dave@xxxxxxxxxxxx>
> ---
> kernel/sched/core.c | 17 ++++++++++++++---
> 1 file changed, 14 insertions(+), 3 deletions(-)
>
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index dd05a378631a..60ae574317fd 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -440,8 +440,16 @@ static bool __wake_q_add(struct wake_q_head *head, struct task_struct *task)
>  * @task: the task to queue for 'later' wakeup
>  *
>  * Queue a task for later wakeup, most likely by the wake_up_q() call in the
> - * same context, _HOWEVER_ this is not guaranteed, the wakeup can come
> - * instantly.
> + * same context, _HOWEVER_ this is not guaranteed. Especially, the wakeup
> + * may happen before the function returns.
> + *
> + * What is guaranteed is that there is a memory barrier before the wakeup,
> + * callers may rely on this barrier.
> + *
> + * On the other hand, the caller must guarantee that @task does not disappear
> + * before wake_q_add() completed. wake_q_add() does not contain any memory
> + * barrier to ensure ordering, thus the caller may need to use
> + * smp_store_release().

This is why we have wake_q_add_safe(). I think this last paragraph is unnecessary
and confusing.

Thanks,
Davidlohr

>  *
>  * This function must be used as-if it were wake_up_process(); IOW the task
>  * must be ready to be woken at this location.
> @@ -486,11 +494,14 @@ void wake_up_q(struct wake_q_head *head)
> 		BUG_ON(!task);
> 		/* Task can safely be re-inserted now: */
> 		node = node->next;
> -		task->wake_q.next = NULL;
> +
> +		WRITE_ONCE(task->wake_q.next, NULL);
>
> 		/*
> 		 * wake_up_process() executes a full barrier, which pairs with
> 		 * the queueing in wake_q_add() so as not to miss wakeups.
> +		 * The barrier is the smp_mb__after_spinlock() in
> +		 * try_to_wake_up().
> 		 */
> 		wake_up_process(task);
> 		put_task_struct(task);
> --
> 2.21.0