Re: [PATCH 02/11] wait: Provide infrastructure to deal with nested blocking

Oleg Nesterov
2014-09-29T21:05:41Z

On 09/24, Peter Zijlstra wrote:
>
> There are a few places that call blocking primitives from wait loops,
> provide infrastructure to support this without the typical
> task_struct::state collision.
>
> We record the wakeup in wait_queue_t::flags which leaves
> task_struct::state free to be used by others.

Sorry for delay. FWIW,

Reviewed-by: Oleg Nesterov <oleg@redhat.com>

> +/*
> + * DEFINE_WAIT_FUNC(wait, woken_wait_func);
^^^^^^^^^^^^^^^^
woken_wake_function ;)

> +int woken_wake_function(wait_queue_t *wait, unsigned mode, int sync, void *key)
> +{
> + /*
> + * Although this function is called under waitqueue lock, LOCK
> + * doesn't imply write barrier and the users expects write
> + * barrier semantics on wakeup functions. The following
> + * smp_wmb() is equivalent to smp_wmb() in try_to_wake_up()
> + * and is paired with set_mb() in wait_woken().
> + */
> + smp_wmb(); /* C */
> + wait->flags |= WQ_FLAG_WOKEN;

Perhaps it is just me, but I was a bit confused by the comment above wmb().
Afaics, it is not that "users expects write barrier semantics", just we
need to ensure that

CONDITION = true;
wait->flags |= WQ_FLAG_WOKEN;

can't be reordered (and this differs from smp_wmb() in try_to_wake_up()).
Otherwise we can obviously race with

// wait_woken() -> set_mb()
wait->flags &= ~WQ_FLAG_WOKEN;
mb();

if (CONDITION)
break;

Oleg.