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From: Nick Piggin <nickpiggin@yahoo.com.au>
To: Jeremy Fitzhardinge <jeremy@goop.org>
Subject: Re: [ANNOUNCE] mdb: Merkey's Linux Kernel Debugger  2.6.27-rc4 released
Date: Fri, 22 Aug 2008 11:37:50 +1000
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Cc: jmerkey@wolfmountaingroup.com, Stefan Richter <stefanr@s5r6.in-berlin.de>,
       paulmck@linux.vnet.ibm.com, Peter Zijlstra <peterz@infradead.org>,
       linux-kernel@vger.kernel.org,
       Linus Torvalds <torvalds@linux-foundation.org>,
       David Howells <dhowells@redhat.com>
References: <200808210250.m7L2obNX028353@wolfmountaingroup.com> <200808212337.38626.nickpiggin@yahoo.com.au> <48ADD8DE.3010209@goop.org>
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On Friday 22 August 2008 07:06, Jeremy Fitzhardinge wrote:
> Nick Piggin wrote:
> > On Thursday 21 August 2008 22:26, jmerkey@wolfmountaingroup.com wrote:
> >> I used the smp_wmb() functions.  I noted a couple of things.  a) some of
> >> these macros just emit __asm__ __volatile__ into the code so why not
> >> just say "volatile" to begin with
> >
> > It is not the same as volatile type. What it does is tell the compiler
> > to clobber all registers or temporaries. This something pretty well
> > defined and hard to get wrong compared to volatile type.
>
> No, that's not what "asm volatile" means.  Its *only* meaning is "emit
> this, even if it doesn't look like it has side-effects and its results
> are not used".  An asm() with no outputs is "volatile" by default, which
> makes most of the uses of "asm volatile" in the kernel redundant.  "asm
> volatile" also has no effect on the ordering of the asm with respect to
> other code; you must use constraints to do that.
>
> An asm with a "memory" clobber is sufficient to make sure that gcc
> doesn't cache memory values in registers; perhaps that's what you mean.

That is what I meant, yes.