Re: [PATCH v3 01/11] MIPS: Add nudges to writes for bit unlocks.

[Ralf Baechle <ralf@linux-mips.org>]

On Mon, Nov 13, 2017 at 10:30:17PM -0600, Steven J. Hill wrote:

> From: Chad Reese <kreese@caviumnetworks.com>
> 
> Flushing the writes lets other CPUs waiting for the lock to get it
> sooner.
> 
> Signed-off-by: Chad Reese <kreese@caviumnetworks.com>
> Signed-off-by: David Daney <david.daney@cavium.com>
> Signed-off-by: Steven J. Hill <steven.hill@cavium.com>
> ---
>  arch/mips/include/asm/bitops.h | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/arch/mips/include/asm/bitops.h b/arch/mips/include/asm/bitops.h
> index fa57cef..da1b871 100644
> --- a/arch/mips/include/asm/bitops.h
> +++ b/arch/mips/include/asm/bitops.h
> @@ -456,6 +456,7 @@ static inline void __clear_bit_unlock(unsigned long nr, volatile unsigned long *
>  {
>  	smp_mb__before_llsc();
>  	__clear_bit(nr, addr);
> +	nudge_writes();
>  }

Hmm...

This patch looks ok, it's more the definition of nudge_writes() which I'm
concerned about.  __clear_bit_unlock() does not need a memory barrier
after __clear_bit() but for non-Octeon processors nudge_writes() expands
into one anyway.

The good news is nudge_writes() is currently unused anyway so we can
change the definition to something like nudge_writes do { } while (0)
for the Octeon.  Suggested patch below.

The Octeon definition of nudge_writes() has a memory clobber and I'm
wondering if that one is actually required or if we could make things a
little easier on the compiler by removing it.

  Ralf

Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

 arch/mips/include/asm/barrier.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/mips/include/asm/barrier.h b/arch/mips/include/asm/barrier.h
index a5eb1bb199a7..0e8e6afbf80a 100644
--- a/arch/mips/include/asm/barrier.h
+++ b/arch/mips/include/asm/barrier.h
@@ -216,7 +216,7 @@
 #else
 #define smp_mb__before_llsc() smp_llsc_mb()
 #define __smp_mb__before_llsc() smp_llsc_mb()
-#define nudge_writes() mb()
+#define nudge_writes() do { } while (0)
 #endif
 
 #define __smp_mb__before_atomic()	__smp_mb__before_llsc()