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

[Peter Zijlstra <peterz@infradead.org>]

On Sat, Oct 12, 2019 at 07:49:53AM +0200, 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@colorfullife.com>
> Cc: Davidlohr Bueso <dave@stgolabs.net>
> ---
>  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.

I would like to stress that this (wake_q_add) provides the same ordering
guarantees as a 'normal' wakeup.

> + *
> + * 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().

Like Davidlohr, I'm slightly puzzled by this last paragraph.

>   *
>   * 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().

We already have wake_up_process() documented as implying this barrier;
why do we want to mention the specifics here? That is, have a look at
the comments before try_to_wake_up().

>  		 */
>  		wake_up_process(task);
>  		put_task_struct(task);
> -- 
> 2.21.0
>