csky: atomic: Optimize cmpxchg with acquire & release

Optimize cmpxchg with ASM acquire/release fence ASM instructions
instead of previous generic based. Prevent a fence when cmxchg's
first load != old.

Comments by Rutland:

8e86f0b ("arm64: atomics: fix use of acquire + release for
full barrier semantics")

Comments by Boqun:

FWIW, you probably need to make sure that a barrier instruction inside
an lr/sc loop is a good thing. IIUC, the execution time of a barrier
instruction is determined by the status of store buffers and invalidate
queues (and probably other stuffs), so it may increase the execution
time of the lr/sc loop, and make it unlikely to succeed. But this really
depends on how the arch executes these instructions.

Link: https://lore.kernel.org/linux-riscv/CAJF2gTSAxpAi=LbAdu7jntZRUa=-dJwL0VfmDfBV5MHB=rcZ-w@mail.gmail.com/T/#m27a0f1342995deae49ce1d0e1f2683f8a181d6c3
Signed-off-by: Guo Ren <[email protected]>
Signed-off-by: Guo Ren <[email protected]>
Cc: Mark Rutland <[email protected]>

Author: guoren83

arch/csky/include/asm/barrier.h

@@ -37,17 +37,21 @@
  * bar.brar
  * bar.bwaw
  */
+#define FULL_FENCE		".long 0x842fc000\n"
+#define ACQUIRE_FENCE		".long 0x8427c000\n"
+#define RELEASE_FENCE		".long 0x842ec000\n"
+
 #define __bar_brw()	asm volatile (".long 0x842cc000\n":::"memory")
 #define __bar_br()	asm volatile (".long 0x8424c000\n":::"memory")
 #define __bar_bw()	asm volatile (".long 0x8428c000\n":::"memory")
 #define __bar_arw()	asm volatile (".long 0x8423c000\n":::"memory")
 #define __bar_ar()	asm volatile (".long 0x8421c000\n":::"memory")
 #define __bar_aw()	asm volatile (".long 0x8422c000\n":::"memory")
-#define __bar_brwarw()	asm volatile (".long 0x842fc000\n":::"memory")
-#define __bar_brarw()	asm volatile (".long 0x8427c000\n":::"memory")
+#define __bar_brwarw()	asm volatile (FULL_FENCE:::"memory")
+#define __bar_brarw()	asm volatile (ACQUIRE_FENCE:::"memory")
 #define __bar_bwarw()	asm volatile (".long 0x842bc000\n":::"memory")
 #define __bar_brwar()	asm volatile (".long 0x842dc000\n":::"memory")
-#define __bar_brwaw()	asm volatile (".long 0x842ec000\n":::"memory")
+#define __bar_brwaw()	asm volatile (RELEASE_FENCE:::"memory")
 #define __bar_brar()	asm volatile (".long 0x8425c000\n":::"memory")
 #define __bar_brar()	asm volatile (".long 0x8425c000\n":::"memory")
 #define __bar_bwaw()	asm volatile (".long 0x842ac000\n":::"memory")
@@ -56,7 +60,6 @@
 #define __smp_rmb()	__bar_brar()
 #define __smp_wmb()	__bar_bwaw()
 
-#define ACQUIRE_FENCE		".long 0x8427c000\n"
 #define __smp_acquire_fence()	__bar_brarw()
 #define __smp_release_fence()	__bar_brwaw()
 

arch/csky/include/asm/cmpxchg.h

@@ -64,15 +64,71 @@ extern void __bad_xchg(void);
 #define arch_cmpxchg_relaxed(ptr, o, n) \
 	(__cmpxchg_relaxed((ptr), (o), (n), sizeof(*(ptr))))
 
-#define arch_cmpxchg(ptr, o, n) 				\
+#define __cmpxchg_acquire(ptr, old, new, size)			\
 ({								\
+	__typeof__(ptr) __ptr = (ptr);				\
+	__typeof__(new) __new = (new);				\
+	__typeof__(new) __tmp;					\
+	__typeof__(old) __old = (old);				\
+	__typeof__(*(ptr)) __ret;				\
+	switch (size) {						\
+	case 4:							\
+		asm volatile (					\
+		"1:	ldex.w		%0, (%3) \n"		\
+		"	cmpne		%0, %4   \n"		\
+		"	bt		2f       \n"		\
+		"	mov		%1, %2   \n"		\
+		"	stex.w		%1, (%3) \n"		\
+		"	bez		%1, 1b   \n"		\
+		ACQUIRE_FENCE					\
+		"2:				 \n"		\
+			: "=&r" (__ret), "=&r" (__tmp)		\
+			: "r" (__new), "r"(__ptr), "r"(__old)	\
+			:);					\
+		break;						\
+	default:						\
+		__bad_xchg();					\
+	}							\
+	__ret;							\
+})
+
+#define arch_cmpxchg_acquire(ptr, o, n) \
+	(__cmpxchg_acquire((ptr), (o), (n), sizeof(*(ptr))))
+
+#define __cmpxchg(ptr, old, new, size)				\
+({								\
+	__typeof__(ptr) __ptr = (ptr);				\
+	__typeof__(new) __new = (new);				\
+	__typeof__(new) __tmp;					\
+	__typeof__(old) __old = (old);				\
 	__typeof__(*(ptr)) __ret;				\
-	__smp_release_fence();					\
-	__ret = arch_cmpxchg_relaxed(ptr, o, n);		\
-	__smp_acquire_fence();					\
+	switch (size) {						\
+	case 4:							\
+		asm volatile (					\
+		RELEASE_FENCE					\
+		"1:	ldex.w		%0, (%3) \n"		\
+		"	cmpne		%0, %4   \n"		\
+		"	bt		2f       \n"		\
+		"	mov		%1, %2   \n"		\
+		"	stex.w		%1, (%3) \n"		\
+		"	bez		%1, 1b   \n"		\
+		FULL_FENCE					\
+		"2:				 \n"		\
+			: "=&r" (__ret), "=&r" (__tmp)		\
+			: "r" (__new), "r"(__ptr), "r"(__old)	\
+			:);					\
+		break;						\
+	default:						\
+		__bad_xchg();					\
+	}							\
 	__ret;							\
 })
 
+#define arch_cmpxchg(ptr, o, n)					\
+	(__cmpxchg((ptr), (o), (n), sizeof(*(ptr))))
+
+#define arch_cmpxchg_local(ptr, o, n)				\
+	(__cmpxchg_relaxed((ptr), (o), (n), sizeof(*(ptr))))
 #else
 #include <asm-generic/cmpxchg.h>
 #endif