[PATCH 2/2] riscv: cmpxchg: implement arch_cmpxchg64_{relaxed|acquire|release}

From: Jisheng Zhang
Date: Sat Nov 25 2023 - 03:34:36 EST


After selecting ARCH_USE_CMPXCHG_LOCKREF, one straight futher
optimization is implementing the arch_cmpxchg64_relaxed() because the
lockref code does not need the cmpxchg to have barrier semantics. At
the same time, implement arch_cmpxchg64_acquire and
arch_cmpxchg64_release as well.

However, on both TH1520 and JH7110 platforms, I didn't see obvious
performance improvement with Linus' test case [1]. IMHO, this may
be related with the fence and lr.d/sc.d hw implementations. In theory,
lr/sc without fence could give performance improvement over lr/sc plus
fence, so add the code here to leave performance improvement room on
newer HW platforms.

Link: http://marc.info/?l=linux-fsdevel&m=137782380714721&w=4 [1]
Signed-off-by: Jisheng Zhang <jszhang@xxxxxxxxxx>
---
arch/riscv/include/asm/cmpxchg.h | 18 ++++++++++++++++++
1 file changed, 18 insertions(+)

diff --git a/arch/riscv/include/asm/cmpxchg.h b/arch/riscv/include/asm/cmpxchg.h
index 2f4726d3cfcc..6318187f426f 100644
--- a/arch/riscv/include/asm/cmpxchg.h
+++ b/arch/riscv/include/asm/cmpxchg.h
@@ -360,4 +360,22 @@
arch_cmpxchg_relaxed((ptr), (o), (n)); \
})

+#define arch_cmpxchg64_relaxed(ptr, o, n) \
+({ \
+ BUILD_BUG_ON(sizeof(*(ptr)) != 8); \
+ arch_cmpxchg_relaxed((ptr), (o), (n)); \
+})
+
+#define arch_cmpxchg64_acquire(ptr, o, n) \
+({ \
+ BUILD_BUG_ON(sizeof(*(ptr)) != 8); \
+ arch_cmpxchg_acquire((ptr), (o), (n)); \
+})
+
+#define arch_cmpxchg64_release(ptr, o, n) \
+({ \
+ BUILD_BUG_ON(sizeof(*(ptr)) != 8); \
+ arch_cmpxchg_release((ptr), (o), (n)); \
+})
+
#endif /* _ASM_RISCV_CMPXCHG_H */
--
2.42.0