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

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Message-ID: <alpine.LFD.2.00.1004021621300.3634@i5.linux-foundation.org>
Date:	Fri, 2 Apr 2010 16:24:57 -0700 (PDT)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Russell King - ARM Linux <linux@....linux.org.uk>
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
Subject: Re: [PATCH 4/5] kgdb: Use atomic operators which use barriers



On Fri, 2 Apr 2010, Russell King - ARM Linux wrote:
> 
> Actually, in future threads you end up agreeing with my position...

I always agreed that it was not a memory barrier.

In fact, the commit that extended on the "volatile-considered-harmful" 
patch from you has this quote from me in the commit logs:

    Linus sayeth:
    
    : I don't think it was ever the intention that it would be seen as anything
    : but a compiler barrier, although it is obviously implied that it might
    : well perform some per-architecture actions that have "memory barrier-like"
    : semantics.
    :
    : After all, the whole and only point of the "cpu_relax()" thing is to tell
    : the CPU that we're busy-looping on some event.
    :
    : And that "event" might be (and often is) about reading the same memory
    : location over and over until it changes to what we want it to be.  So it's
    : quite possible that on various architectures the "cpu_relax()" could be
    : about making sure that such a tight loop on loads doesn't starve cache
    : transactions, for example - and as such look a bit like a memory barrier
    : from a CPU standpoint.
    :
    : But it's not meant to have any kind of architectural memory ordering
    : semantics as far as the kernel is concerned - those must come from other
    : sources.

which I think is pretty clear. 

But that quote seems to be the one where you then think I "agree" with 
you.

			Linus
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