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    SubjectRe: [PATCH] arch/powerpc/include/asm/barrier.h: redefine rmb and wmb to lwsync
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    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@linux.vnet.ibm.com>
    > ---
    > 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

    > +#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

    By the way, why put it inside a ({ }) ?
    And I think nowdays we use "asm volatile" not "__asm__ __volatile__"

    Shouldn't you also consider the same for mb() ?

    Note that your series will conflict with b6e259297a6b ("powerpc/kcsan:
    Memory barriers semantics") in powerpc/next tree.

    > +/* 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.

    > +#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

    This line shouldn't be changed by your patch

    > +#define SMPWMB eieio
    > #endif
    >
    > /* clang defines this macro for a builtin, which will not work with runtime patching */
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