Re: [PATCH 4/5] kgdb: Use atomic operators which use barriers
From: Russell King - ARM Linux
Date: Sat Apr 03 2010 - 12:08:59 EST
On Fri, Apr 02, 2010 at 04:24:57PM -0700, Linus Torvalds wrote:
> 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
Yet again you read something into what I say that wasn't there.
Wait for me to return from holiday, as I said, and I'll respond further.
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