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

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Date:	Fri, 2 Apr 2010 12:46:14 -0700 (PDT)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Jason Wessel <jason.wessel@...driver.com>
cc:	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

On Fri, 2 Apr 2010, Linus Torvalds wrote:
> 
> 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.

Put another way: from a kernel standpoint, cpu_relax() in _no_ way implies 
a memory barrier. That has always been true, and that continues to be 
true.

But Linux does expect that if some other CPU modifies a memory location, 
then we _will_ see that modification eventually. If the CPU needs help to 
do so, then cpu_relax() needs to do that. Again - this has nothing to do 
with memory barriers. It's just a basic requirement.

				Linus
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