Re: [PATCH] arch/powerpc/include/asm/barrier.h: redefine rmb and wmb to lwsync
From: Kautuk Consul
Date: Wed Feb 22 2023 - 03:17:12 EST
On Wed, Feb 22, 2023 at 07:02:34AM +0000, Christophe Leroy wrote:
>
>
> Le 22/02/2023 à 07:01, Kautuk Consul a écrit :
> > A link from ibm.com states:
> > "Ensures that all instructions preceding the call to __lwsync
> > complete before any subsequent store instructions can be executed
> > on the processor that executed the function. Also, it ensures that
> > all load instructions preceding the call to __lwsync complete before
> > any subsequent load instructions can be executed on the processor
> > that executed the function. This allows you to synchronize between
> > multiple processors with minimal performance impact, as __lwsync
> > does not wait for confirmation from each processor."
> >
> > Thats why smp_rmb() and smp_wmb() are defined to lwsync.
> > But this same understanding applies to parallel pipeline
> > execution on each PowerPC processor.
> > So, use the lwsync instruction for rmb() and wmb() on the PPC
> > architectures that support it.
> >
> > Also removed some useless spaces.
> >
> > Signed-off-by: Kautuk Consul <kconsul@xxxxxxxxxxxxxxxxxx>
> > ---
> > arch/powerpc/include/asm/barrier.h | 12 +++++++++---
> > 1 file changed, 9 insertions(+), 3 deletions(-)
> >
> > diff --git a/arch/powerpc/include/asm/barrier.h b/arch/powerpc/include/asm/barrier.h
> > index e80b2c0e9315..553f5a5d20bd 100644
> > --- a/arch/powerpc/include/asm/barrier.h
> > +++ b/arch/powerpc/include/asm/barrier.h
> > @@ -41,11 +41,17 @@
> >
> > /* The sub-arch has lwsync */
> > #if defined(CONFIG_PPC64) || defined(CONFIG_PPC_E500MC)
> > -# define SMPWMB LWSYNC
>
> This line shouldn't be changed by your patch
I mentioned it in the commit message.
But if you want I'll remove this. Did this because the rest
of the file doesn't have these spaces.
>
> > +#undef rmb
> > +#undef wmb
>
> I see no point with defining something and undefining them a few lines
> later.
>
> Instead, why not do:
>
> #define mb() __asm__ __volatile__ ("sync" : : : "memory")
>
> #if defined(CONFIG_PPC64) || defined(CONFIG_PPC_E500MC)
> #define rmb() ({__asm__ __volatile__ ("lwsync" : : : "memory"); })
> #define wmb() ({__asm__ __volatile__ ("lwsync" : : : "memory"); })
> #else
> #define rmb() __asm__ __volatile__ ("sync" : : : "memory")
> #define wmb() __asm__ __volatile__ ("sync" : : : "memory")
> #endif
>
I thought of doing that earlier, but there exists one more #elif
for CONFIG_BOOKE and then the #else.
That way we would have to put 3 different #defines for rmb and wmb
and I wanted to avoid that.
> By the way, why put it inside a ({ }) ?
checkpatch.pl asks for ({}).
> And I think nowdays we use "asm volatile" not "__asm__ __volatile__"
I was just following what was there in the file already.
Can change this if required.
>
> Shouldn't you also consider the same for mb() ?
My change wasn't meant to address newer usages of as volatile
#defines. I just wanted to redefine the rmb and wmb #defines
to lwsync.
>
> Note that your series will conflict with b6e259297a6b ("powerpc/kcsan:
> Memory barriers semantics") in powerpc/next tree.
I thought of defining the __rmb and __wmb macros but I decided against
it because I didn't understand kcsan completely.
I used the standard Linus' tree, not powerpc/next.
Can you tell me which branch or git repo I should make this patch on ?
>
> > +/* Redefine rmb() to lwsync. */
>
> WHat's the added value of this comment ? Isn't it obvious in the line
> below that rmb() is being defined to lwsync ? Please avoid useless comments.
Sure.
>
> > +#define rmb() ({__asm__ __volatile__ ("lwsync" : : : "memory"); })
> > +/* Redefine wmb() to lwsync. */
> > +#define wmb() ({__asm__ __volatile__ ("lwsync" : : : "memory"); })
> > +#define SMPWMB LWSYNC
> > #elif defined(CONFIG_BOOKE)
> > -# define SMPWMB mbar
>
> This line shouldn't be changed by your patch
>
> > +#define SMPWMB mbar
> > #else
> > -# define SMPWMB eieio
Ok. Can change my patch.
>
> This line shouldn't be changed by your patch
>
> > +#define SMPWMB eieio
> > #endif
Sure. Will retain this too.
> >
> > /* clang defines this macro for a builtin, which will not work with runtime patching */