linux-kernel - Re: [PATCH] It may not be assumed that wake_up(), finish

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Date:	Tue, 28 Apr 2009 11:18:51 +0100
From:	David Howells <dhowells@...hat.com>
To:	paulmck@...ux.vnet.ibm.com
Cc:	dhowells@...hat.com, Oleg Nesterov <oleg@...hat.com>,
	Ingo Molnar <mingo@...e.hu>, torvalds@...l.org,
	Andrew Morton <akpm@...ux-foundation.org>, serue@...ibm.com,
	viro@...iv.linux.org.uk, Nick Piggin <nickpiggin@...oo.com.au>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] It may not be assumed that wake_up(), finish_wait() and co. imply a memory barrier

Paul E. McKenney <paulmck@...ux.vnet.ibm.com> wrote:

> But I would strongly suggest at least a note calling this out, preferably a
> "don't do this" example.

How about I add this to the bottom of the new section:

[!] Note that the memory barriers implied by the sleeper and the waker do _not_
order multiple stores before the wake-up with respect to loads of those stored
values after the sleeper has called set_current_state().  For instance, if the
sleeper does:

	set_current_state(TASK_INTERRUPTIBLE);
	if (event_indicated)
		break;
	__set_current_state(TASK_RUNNING);
	do_something(my_data);

and the waker does:

	my_data = value;
	event_indicated = 1;
	wake_up(&event_wait_queue);

there's no guarantee that the change to event_indicated will be perceived by
the sleeper as coming after the change to my_data.  In such a circumstance, the
code on both sides must interpolate its own memory barriers between the
separate data accesses.  Thus the above sleeper ought to do:

	set_current_state(TASK_INTERRUPTIBLE);
	if (event_indicated) {
		smp_rmb();
		do_something(my_data);
	}

and the waker should do:

	my_data = value;
	smp_wmb();
	event_indicated = 1;
	wake_up(&event_wait_queue);

David
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