linux-kernel - Re: [PATCH 4/5] kgdb: Use atomic operators which use barriers

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Message-ID: <20100405092126.GA1419@ucw.cz>
Date:	Mon, 5 Apr 2010 11:21:27 +0200
From:	Pavel Machek <pavel@....cz>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Jason Wessel <jason.wessel@...driver.com>,
	Will Deacon <will.deacon@....com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	kgdb-bugreport@...ts.sourceforge.net, linux-arm@...r.kernel.org,
	Russell King - ARM Linux <linux@....linux.org.uk>
Subject: Re: [PATCH 4/5] kgdb: Use atomic operators which use barriers

Hi!

> > Russell had this thread: 
> > http://permalink.gmane.org/gmane.linux.ports.arm.kernel/75717
> 
> Russell is wrong.
> 
> Yes, originally it was about P4's overheating. But let me repeat: the fact 
> is, this _is_ valid kernel code:
> 
> 	kernel/sched.c-		while (task_is_waking(p))
> 	kernel/sched.c:			cpu_relax();


And this is valid (but ugly and not optimal) kernel code:

 	kernel/sched.c-		while (task_is_waking(p))
 	kernel/sched.c:			asm volatile("" :: "memory");


> (where that "task_is_waking()" is simply doing two regular reads, and 
> expects another CPU to be changing them).
> 
> This has _nothing_ to do with memory barriers, or with overheating. 
...
> All that matters is that the above kind of while loop must work. The 
> architecture needs to do whatever it needs to do to make it work. End of 
> discussion. If on ARM6 that means "smp_mb()", then that's an ARM6 
> implementation issue.

...so I don't think inserting smp_mb() into cpu_relax() and udelay()
and similar can ever fix the problem fully.

Run smp_mb() from periodic interrupt?
									Pavel
-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html
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