linux-kernel - Re: + kthread-add-a-missing-memory-barrier-to-kthread

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Date:	Sat, 23 Feb 2008 22:07:53 +0100
From:	"Dmitry Adamushko" <dmitry.adamushko@...il.com>
To:	"Linus Torvalds" <torvalds@...ux-foundation.org>
Cc:	"Oleg Nesterov" <oleg@...sign.ru>,
	"Andrew Morton" <akpm@...ux-foundation.org>,
	"Linux Kernel Mailing List" <linux-kernel@...r.kernel.org>,
	a.p.zijlstra@...llo.nl, apw@...dowen.org,
	"Ingo Molnar" <mingo@...e.hu>, nickpiggin@...oo.com.au,
	paulmck@...ux.vnet.ibm.com, rusty@...tcorp.com.au,
	"Steven Rostedt" <rostedt@...dmis.org>, linux-arch@...r.kernel.org
Subject: Re: + kthread-add-a-missing-memory-barrier-to-kthread_stop.patch added to -mm tree

On 23/02/2008, Linus Torvalds <torvalds@...ux-foundation.org> wrote:
>
>  On Sat, 23 Feb 2008, Dmitry Adamushko wrote:
>  >
>  > it's not a LOAD that escapes *out* of the region. It's a MODIFY that gets *in*:
>
>
> Not with the smp_wmb(). That's the whole point.
>
>  Ie the patch I'm suggesting is sufficient is appended, and the point is
>  that any write before the critical region will be ordered wrt the critical
>  region because of the write barrier before the spinlock (which itself is a
>  write).

Yeah, good point!

(heh... I wouldn't dare to say this 'obvious thing' only to Anton
Blanchard who is "the only person who always 'have a point' by
definition" :-))

> This is also why I mentioned that if you have a really odd architecure
> that considers spinlocks to be "outside" the normal cache coherency
> domain, that would be broken, but I cannot think of a single valid case of
> that, and I consider it insane.

Yeah, some potential implementations come into my mind but, I guess,
they are as far away from real hardware as science-fiction from
science :-/

So how should we proceed with this issue?

let's use your patch and declare try_to_wake_up() a 'full' mb for the case:

MODIFY
try_to_wake_up
LOAD or MODIFY (that take place either after or inside try_to_wake_up())

so we'll fix (lots of) potentially problematic cases with a single shot.

and

LOAD
try_to_wake_up()
LOAD or MODIFY

is probably not that common so we don't care.


-- 
Best regards,
Dmitry Adamushko
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