linux-kernel - Re: [patch] sched: fix bad missed wakeups in the i386, x86

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Date:	Thu, 21 Dec 2006 14:00:12 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Linus Torvalds <torvalds@...l.org>
Cc:	Fernando Lopez-Lezcano <nando@...ma.Stanford.EDU>,
	Andrew Morton <akpm@...l.org>, linux-kernel@...r.kernel.org,
	Andi Kleen <ak@...e.de>, Len Brown <len.brown@...el.com>
Subject: Re: [patch] sched: fix bad missed wakeups in the i386, x86_64, ia64, ACPI and APM idle code


* Ingo Molnar <mingo@...e.hu> wrote:

> CPU idle wakeup very much depends on ordered memory ops, the clearing 
> of the TS_POLLING flag must always be done before we test 
> need_resched() and hit the idle instruction(s). [Symmetrically, the 
> wakeup code needs to set NEED_RESCHED before it tests the TS_POLLING 
> flag, so memory ordering is paramount.]

Btw., this means that an smp_rmb() ought to be enough too. (I sent such 
a test-patch to Fernando.) Also, i'd like the change to smp_rmb() be 
done separately from the fix so that we have the most-conservative 
variant in and thus it becomes bisectable. I'd like to avoid another few 
months of latency for fixing yet another bug in this area...

	Ingo
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