[PATCH v2 33/36] MIPS: barrier: Make __smp_mb__before_atomic() a no-op for Loongson3

From: Paul Burton
Date: Tue Oct 01 2019 - 17:54:25 EST


Loongson3 systems with CONFIG_CPU_LOONGSON3_WORKAROUNDS enabled already
emit a full completion barrier as part of the inline assembly containing
LL/SC loops for atomic operations. As such the barrier emitted by
__smp_mb__before_atomic() is redundant, and we can remove it.

Signed-off-by: Paul Burton <paul.burton@xxxxxxxx>
---

Changes in v2: None

arch/mips/include/asm/barrier.h | 12 +++++++++++-
1 file changed, 11 insertions(+), 1 deletion(-)

diff --git a/arch/mips/include/asm/barrier.h b/arch/mips/include/asm/barrier.h
index 6d92d5ccdafa..49ff172a72b9 100644
--- a/arch/mips/include/asm/barrier.h
+++ b/arch/mips/include/asm/barrier.h
@@ -119,7 +119,17 @@ static inline void wmb(void)
#define nudge_writes() mb()
#endif

-#define __smp_mb__before_atomic() __smp_mb__before_llsc()
+/*
+ * In the Loongson3 LL/SC workaround case, all of our LL/SC loops already have
+ * a completion barrier immediately preceding the LL instruction. Therefore we
+ * can skip emitting a barrier from __smp_mb__before_atomic().
+ */
+#ifdef CONFIG_CPU_LOONGSON3_WORKAROUNDS
+# define __smp_mb__before_atomic()
+#else
+# define __smp_mb__before_atomic() __smp_mb__before_llsc()
+#endif
+
#define __smp_mb__after_atomic() smp_llsc_mb()

static inline void sync_ginv(void)
--
2.23.0