linux-kernel - Re: [PATCH] Document that wake_up(), complete() and co. imply a full memory barrier

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Date:	Thu, 23 Apr 2009 18:36:24 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	David Howells <dhowells@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>, serue@...ibm.com,
	viro@...iv.linux.org.uk,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Nick Piggin <nickpiggin@...oo.com.au>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] Document that wake_up(), complete() and co. imply a
	full memory barrier

On 04/22, Ingo Molnar wrote:
>
> * David Howells <dhowells@...hat.com> wrote:
>
> The dangerous pattern is lockless code doing wakeups. But lockless
> code always has to use proper barriers or atomics anyway, and has to
> be aware of the fact that kernel primitives they call are not
> necessarily full memory barriers.
>
> In fact i'd encourage to _not_ document try_to_lock() as a write
> barrier either - but rather have explicit barriers where they are
> needed. Then we could remove that barrier from try_to_wake_up() too
> ;-)

Well. I think in that case almost every try_to_wake_up/wake_up_process
needs mb().

For example:

	do_nanosleep:

		for (;;) {
			set_current_state(TASK_INTERRUPTIBLE);
			if (likely(t->task))
				schedule();
			break;
		}

	hrtimer_wakeup:

		task = t->task
		t->task = NULL;
		wake_up_process(task);

If try_to_wake_up() has no the barrier semantics, we can miss the event.
"t->task = NULL" and the reading of task->state must not be reordered.

Oleg.

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