Re: [PATCH] It may not be assumed that wake_up(), finish_wait() and co. imply a memory barrier

Oleg Nesterov
2009-04-23T17:01:28Z

On 04/23, David Howells wrote:
>
> + complete();
> + try_to_wake_up();
> + wake_up();
> + wake_up_all();
> + wake_up_bit();
> + wake_up_interruptible();
> + wake_up_interruptible_all();
> + wake_up_interruptible_nr();
> + wake_up_interruptible_poll();
> + wake_up_interruptible_sync();
> + wake_up_interruptible_sync_poll();
> + wake_up_locked();
> + wake_up_locked_poll();
> + wake_up_nr();
> + wake_up_poll();
> +
> +After waking, and assuming it doesn't take a matching lock, the sleeper may
> +need to interpolate a read or full memory barrier before accessing that state
> +as finish_wait() does not imply a barrier either, and schedule() only implies a
> +barrier on entry.

Well. I am starting to suspect I missed something, but I disagree. Or I just
(this is very possible) misunderstand the above.

finish_wait() doesn't imply a barrier, but why this matters?

And if we don't use prepare_to_wait() and just do

for (;;) {
set_current_state(WHATEVER);
if (!CONDITION)
schedule();
break;
}

we do have mb(), but

> + *
> + * It should not be assumed that this function implies any sort of memory
> + * barrier.
> */
> static int try_to_wake_up(struct task_struct *p, unsigned int state, int sync)

unless try_to_wake_up() has a barrier semantics too, this code

CONDITION = 1;
wake_up_process(waiter);

is not right, and that mb above can't help.

Could you please give the code example which shows we need a barrier
after finish_wait() ?

I am just trying to understand what I missed.

Oleg.