MIPS: cmpxchg: Omit redundant barriers for Loongson3

When building a kernel configured to support Loongson3 LL/SC workarounds
(ie. CONFIG_CPU_LOONGSON3_WORKAROUNDS=y) the inline assembly in
__xchg_asm() & __cmpxchg_asm() already emits completion barriers, and as
such we don't need to emit extra barriers from the xchg() or cmpxchg()
macros. Add compile-time constant checks causing us to omit the
redundant memory barriers.

Signed-off-by: Paul Burton <paul.burton@mips.com>
Cc: linux-mips@vger.kernel.org
Cc: Huacai Chen <chenhc@lemote.com>
Cc: Jiaxun Yang <jiaxun.yang@flygoat.com>
Cc: linux-kernel@vger.kernel.org

diff --git a/arch/mips/include/asm/cmpxchg.h b/arch/mips/include/asm/cmpxchg.h
index fc121d20a980891bf5d33339588ff20bc81f59d4..820df68e32e1cac7bb7e3345a764d7b14554cbcd 100644 (file)
--- a/arch/mips/include/asm/cmpxchg.h
+++ b/arch/mips/include/asm/cmpxchg.h
@@ -94,7 +94,13 @@ static inline unsigned long __xchg(volatile void *ptr, unsigned long x,
 ({                                                                     \
        __typeof__(*(ptr)) __res;                                       \
                                                                        \
-       smp_mb__before_llsc();                                          \
+       /*                                                              \
+        * In the Loongson3 workaround case __xchg_asm() already        \
+        * contains a completion barrier prior to the LL, so we don't   \
+        * need to emit an extra one here.                              \
+        */                                                             \
+       if (!__SYNC_loongson3_war)                                      \
+               smp_mb__before_llsc();                                  \
                                                                        \
        __res = (__typeof__(*(ptr)))                                    \
                __xchg((ptr), (unsigned long)(x), sizeof(*(ptr)));      \
@@ -179,9 +185,23 @@ static inline unsigned long __cmpxchg(volatile void *ptr, unsigned long old,
 ({                                                                     \
        __typeof__(*(ptr)) __res;                                       \
                                                                        \
-       smp_mb__before_llsc();                                          \
+       /*                                                              \
+        * In the Loongson3 workaround case __cmpxchg_asm() already     \
+        * contains a completion barrier prior to the LL, so we don't   \
+        * need to emit an extra one here.                              \
+        */                                                             \
+       if (!__SYNC_loongson3_war)                                      \
+               smp_mb__before_llsc();                                  \
+                                                                       \
        __res = cmpxchg_local((ptr), (old), (new));                     \
-       smp_llsc_mb();                                                  \
+                                                                       \
+       /*                                                              \
+        * In the Loongson3 workaround case __cmpxchg_asm() already     \
+        * contains a completion barrier after the SC, so we don't      \
+        * need to emit an extra one here.                              \
+        */                                                             \
+       if (!__SYNC_loongson3_war)                                      \
+               smp_llsc_mb();                                          \
                                                                        \
        __res;                                                          \
 })