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

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Date:	Thu, 23 Apr 2009 14:12:28 -0700 (PDT)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	David Howells <dhowells@...hat.com>
cc:	Ingo Molnar <mingo@...e.hu>, Oleg Nesterov <oleg@...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] It may not be assumed that wake_up(), finish_wait() and
 co. imply a memory barrier

On Thu, 23 Apr 2009, David Howells wrote:
> 
> Well, Ingo's point is it could be left up to the caller of wake_up() to supply
> the barrier:
> 
> 	*my_variable = 1234;
> 	smp_wmb();
> 	wake_up(&my_queue);

That's bogus.

EVERY SINGLE wake_up() would need it.

There is _always_ a reason for the wakeup. And yes, you can re-order 
things, but we normally don't. Just make the rule be that there's an 
implied smp_wmb() instead.

It's not like it's going to cost anything on any sane architecture anyway. 
So asking people to add "smp_wmb()" calls before their wakups just makes 
the source code unreadable and fragile, for no actual advantage.

		Linus
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