Re: [ANNOUNCE] mdb: Merkey's Linux Kernel Debugger 2.6.27-rc4 released

[Nick Piggin <nickpiggin@yahoo.com.au>]

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.