[PATCH v2 07/10] riscv: Improve amoswap.X use in xchg()

Alexandre Ghiti
2024-06-26T13:11:01Z

xchg() uses amoswap.X instructions from Zabha but still uses
the LR/SC acquire/release semantics which require barriers.

Let's improve that by using proper amoswap acquire/release semantics in
order to avoid any of those barriers.

Suggested-by: Andrea Parri <andrea@rivosinc.com>
Signed-off-by: Alexandre Ghiti <alexghiti@rivosinc.com>
---
arch/riscv/include/asm/cmpxchg.h | 35 +++++++++++++-------------------
1 file changed, 14 insertions(+), 21 deletions(-)

diff --git a/arch/riscv/include/asm/cmpxchg.h b/arch/riscv/include/asm/cmpxchg.h
index eb35e2d30a97..0e57d5fbf227 100644
--- a/arch/riscv/include/asm/cmpxchg.h
+++ b/arch/riscv/include/asm/cmpxchg.h
@@ -11,8 +11,8 @@
#include <asm/fence.h>
#include <asm/alternative.h>

-#define __arch_xchg_masked(sc_sfx, swap_sfx, prepend, sc_append, \
- swap_append, r, p, n) \
+#define __arch_xchg_masked(sc_sfx, swap_sfx, sc_prepend, sc_append, \
+ r, p, n) \
({ \
__label__ zabha, end; \
\
@@ -31,7 +31,7 @@
ulong __rc; \
\
__asm__ __volatile__ ( \
- prepend \
+ sc_prepend \
"0: lr.w %0, %2\n" \
" and %1, %0, %z4\n" \
" or %1, %1, %z3\n" \
@@ -48,9 +48,7 @@
zabha: \
if (IS_ENABLED(CONFIG_RISCV_ISA_ZABHA)) { \
__asm__ __volatile__ ( \
- prepend \
" amoswap" swap_sfx " %0, %z2, %1\n" \
- swap_append \
: "=&r" (r), "+A" (*(p)) \
: "rJ" (n) \
: "memory"); \
@@ -58,19 +56,17 @@ zabha: \
end:; \
})

-#define __arch_xchg(sfx, prepend, append, r, p, n) \
+#define __arch_xchg(sfx, r, p, n) \
({ \
__asm__ __volatile__ ( \
- prepend \
" amoswap" sfx " %0, %2, %1\n" \
- append \
: "=r" (r), "+A" (*(p)) \
: "r" (n) \
: "memory"); \
})

-#define _arch_xchg(ptr, new, sc_sfx, swap_sfx, prepend, \
- sc_append, swap_append) \
+#define _arch_xchg(ptr, new, sc_sfx, swap_sfx, \
+ sc_prepend, sc_append) \
({ \
__typeof__(ptr) __ptr = (ptr); \
__typeof__(*(__ptr)) __new = (new); \
@@ -79,21 +75,19 @@ end:; \
switch (sizeof(*__ptr)) { \
case 1: \
__arch_xchg_masked(sc_sfx, ".b" swap_sfx, \
- prepend, sc_append, swap_append, \
+ sc_prepend, sc_append, \
__ret, __ptr, __new); \
break; \
case 2: \
__arch_xchg_masked(sc_sfx, ".h" swap_sfx, \
- prepend, sc_append, swap_append, \
+ sc_prepend, sc_append, \
__ret, __ptr, __new); \
break; \
case 4: \
- __arch_xchg(".w" swap_sfx, prepend, swap_append, \
- __ret, __ptr, __new); \
+ __arch_xchg(".w" swap_sfx, __ret, __ptr, __new); \
break; \
case 8: \
- __arch_xchg(".d" swap_sfx, prepend, swap_append, \
- __ret, __ptr, __new); \
+ __arch_xchg(".d" swap_sfx, __ret, __ptr, __new); \
break; \
default: \
BUILD_BUG(); \
@@ -102,17 +96,16 @@ end:; \
})

#define arch_xchg_relaxed(ptr, x) \
- _arch_xchg(ptr, x, "", "", "", "", "")
+ _arch_xchg(ptr, x, "", "", "", "")

#define arch_xchg_acquire(ptr, x) \
- _arch_xchg(ptr, x, "", "", "", \
- RISCV_ACQUIRE_BARRIER, RISCV_ACQUIRE_BARRIER)
+ _arch_xchg(ptr, x, "", ".aq", "", RISCV_ACQUIRE_BARRIER)

#define arch_xchg_release(ptr, x) \
- _arch_xchg(ptr, x, "", "", RISCV_RELEASE_BARRIER, "", "")
+ _arch_xchg(ptr, x, "", ".rl", RISCV_RELEASE_BARRIER, "")

#define arch_xchg(ptr, x) \
- _arch_xchg(ptr, x, ".rl", ".aqrl", "", RISCV_FULL_BARRIER, "")
+ _arch_xchg(ptr, x, ".rl", ".aqrl", "", RISCV_FULL_BARRIER)

#define xchg32(ptr, x) \
({ \
--
2.39.2