On Mon, Jun 16, 2008 at 05:57:26PM -0400, Kyle McMartin wrote:
> On Mon, Jun 16, 2008 at 05:54:24PM -0400, Carlos O'Donell wrote:
> > On Mon, Jun 16, 2008 at 5:06 PM, Helge Deller <deller@xxxxxx> wrote:
> > > So, your proposal is (copy-and-pasted in here) the following ?
> > >
> > > diff --git a/include/asm-parisc/system.h b/include/asm-parisc/system.h
> > > index ee80c92..daeae39 100644
> > > --- a/include/asm-parisc/system.h
> > > +++ b/include/asm-parisc/system.h
> > > @@ -169,7 +169,7 @@ static inline void set_eiem(unsigned long val)
> > > #define __ldcw(a) ({ \
> > > unsigned __ret; \
> > > __asm__ __volatile__(__LDCW " 0(%1),%0" \
> > > - : "=r" (__ret) : "r" (a)); \
> > > + : "=r" (__ret) : "r" (a) : "memory" ); \
> > > __ret; \
> > > })
> >
> > Yes. The asm should clobber memory thus forcing the compiler to avoid
> > memory temporaries.
>
> It shouldn't need to, since we're only ever accessing one word (the one
> specified in the operand.)
>
> Otherwise basically every inline asm everywhere ever is going to need a
> memory clobber, and that's just BROKEN.
Carlos' and Helge's point (I think) is that the __ldcw() doesn't clobber
memory, so gcc can cache other things in registers across a call to
__ldcw(). While this is true, our definition of __raw_spin_lock() has
two calls to mb() in it, which is defined to clobber memory.
The only users of __ldcw() are in spinlock.h which has the mb()s in
place. I don't think there's a problem here.
--
Intel are signing my paycheques ... these opinions are still mine
"Bill, look, we understand that you're interested in selling us this
operating system, but compare it to ours. We can't possibly take such
a retrograde step."
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