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Subject[PATCH 6/5] x86/percpu: Optimize raw_cpu_xchg()

And because it's one of _those_ days, I forgot to include one patch...

---
Subject: x86/percpu: Optimize raw_cpu_xchg()
From: Peter Zijlstra <peterz@infradead.org>
Date: Wed Feb 27 11:09:56 CET 2019

Since raw_cpu_xchg() doesn't need to be IRQ-safe, like
this_cpu_xchg(), we can use a simple load-store instead of the cmpxchg
loop.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
---
arch/x86/include/asm/percpu.h | 18 +++++++++++++++---
1 file changed, 15 insertions(+), 3 deletions(-)

--- a/arch/x86/include/asm/percpu.h
+++ b/arch/x86/include/asm/percpu.h
@@ -407,9 +407,21 @@ do { \
#define raw_cpu_or_1(pcp, val) percpu_to_op(, "or", (pcp), val)
#define raw_cpu_or_2(pcp, val) percpu_to_op(, "or", (pcp), val)
#define raw_cpu_or_4(pcp, val) percpu_to_op(, "or", (pcp), val)
-#define raw_cpu_xchg_1(pcp, val) percpu_xchg_op(, pcp, val)
-#define raw_cpu_xchg_2(pcp, val) percpu_xchg_op(, pcp, val)
-#define raw_cpu_xchg_4(pcp, val) percpu_xchg_op(, pcp, val)
+
+/*
+ * raw_cpu_xchg() can use a load-store since it is not required to be
+ * IRQ-safe.
+ */
+#define raw_percpu_xchg_op(var, nval) \
+({ \
+ typeof(var) pxo_ret__ = raw_cpu_read(var); \
+ raw_cpu_write(var, (nval)); \
+ pxo_ret__; \
+})
+
+#define raw_cpu_xchg_1(pcp, val) raw_percpu_xchg_op(pcp, val)
+#define raw_cpu_xchg_2(pcp, val) raw_percpu_xchg_op(pcp, val)
+#define raw_cpu_xchg_4(pcp, val) raw_percpu_xchg_op(pcp, val)

#define this_cpu_read_1(pcp) percpu_from_op(volatile, "mov", pcp)
#define this_cpu_read_2(pcp) percpu_from_op(volatile, "mov", pcp)
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