On Wed, Jan 27, 2021 at 09:36:27PM +0100, Alexander A Sverdlin wrote:
> From: Alexander Sverdlin <alexander.sverdlin@xxxxxxxxx>
>
> It makes no sense to fold smp_mb__before_llsc()/smp_llsc_mb() again and
> again, leave only one barrier pair in the outer function.
>
> This removes one SYNCW from __xchg_small() and brings around 10%
> performance improvement in a tight spinlock loop with 6 threads on a 6 core
> Octeon.
>
> Signed-off-by: Alexander Sverdlin <alexander.sverdlin@xxxxxxxxx>
> ---
> arch/mips/kernel/cmpxchg.c | 4 ++--
> 1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/arch/mips/kernel/cmpxchg.c b/arch/mips/kernel/cmpxchg.c
> index 89107de..122e85f 100644
> --- a/arch/mips/kernel/cmpxchg.c
> +++ b/arch/mips/kernel/cmpxchg.c
> @@ -41,7 +41,7 @@ unsigned long __xchg_small(volatile void *ptr, unsigned long val, unsigned int s
> do {
> old32 = load32;
> new32 = (load32 & ~mask) | (val << shift);
> - load32 = cmpxchg(ptr32, old32, new32);
> + load32 = cmpxchg_local(ptr32, old32, new32);
> } while (load32 != old32);
>
> return (load32 & mask) >> shift;
> @@ -97,7 +97,7 @@ unsigned long __cmpxchg_small(volatile void *ptr, unsigned long old,
> */
> old32 = (load32 & ~mask) | (old << shift);
> new32 = (load32 & ~mask) | (new << shift);
> - load32 = cmpxchg(ptr32, old32, new32);
> + load32 = cmpxchg_local(ptr32, old32, new32);
> if (load32 == old32)
> return old;
> }
This is wrong, please use cmpxchg_relaxed() which you've just
introduced. cmpxchg_local() need not be cross-cpu atomic at all (it is
on mips by accident of implementation).
