[PATCH v7 RESEND 5/6] arm64: futex: small optimisation for __llsc_futex_atomic_set()

Yeoreum Yun
2025-08-16T15:19:47Z

__llsc_futex_atomic_set() is implmented using
LLSC_FUTEX_ATOMIC_OP() macro with "mov %w3, %w5".
But this instruction isn't required to implement fux_atomic_set()
so make a small optimisation by implementing __llsc_futex_atomic_set()
as separate function.

This will make usage of LLSC_FUTEX_ATOMIC_OP() macro more simple.

Signed-off-by: Yeoreum Yun <yeoreum.yun@arm.com>
---
arch/arm64/include/asm/futex.h | 43 ++++++++++++++++++++++++++++------
1 file changed, 36 insertions(+), 7 deletions(-)

diff --git a/arch/arm64/include/asm/futex.h b/arch/arm64/include/asm/futex.h
index ab7003cb4724..22a6301a9f3d 100644
--- a/arch/arm64/include/asm/futex.h
+++ b/arch/arm64/include/asm/futex.h
@@ -13,7 +13,7 @@

#define LLSC_MAX_LOOPS 128 /* What's the largest number you can think of? */

-#define LLSC_FUTEX_ATOMIC_OP(op, insn) \
+#define LLSC_FUTEX_ATOMIC_OP(op, asm_op) \
static __always_inline int \
__llsc_futex_atomic_##op(int oparg, u32 __user *uaddr, int *oval) \
{ \
@@ -24,7 +24,7 @@ __llsc_futex_atomic_##op(int oparg, u32 __user *uaddr, int *oval) \
asm volatile("// __llsc_futex_atomic_" #op "\n" \
" prfm pstl1strm, %2\n" \
"1: ldxr %w1, %2\n" \
- insn "\n" \
+" " #asm_op " %w3, %w1, %w5\n" \
"2: stlxr %w0, %w3, %2\n" \
" cbz %w0, 3f\n" \
" sub %w4, %w4, %w0\n" \
@@ -46,11 +46,40 @@ __llsc_futex_atomic_##op(int oparg, u32 __user *uaddr, int *oval) \
return ret; \
}

-LLSC_FUTEX_ATOMIC_OP(add, "add %w3, %w1, %w5")
-LLSC_FUTEX_ATOMIC_OP(or, "orr %w3, %w1, %w5")
-LLSC_FUTEX_ATOMIC_OP(and, "and %w3, %w1, %w5")
-LLSC_FUTEX_ATOMIC_OP(eor, "eor %w3, %w1, %w5")
-LLSC_FUTEX_ATOMIC_OP(set, "mov %w3, %w5")
+LLSC_FUTEX_ATOMIC_OP(add, add)
+LLSC_FUTEX_ATOMIC_OP(or, orr)
+LLSC_FUTEX_ATOMIC_OP(and, and)
+LLSC_FUTEX_ATOMIC_OP(eor, eor)
+
+static __always_inline int
+__llsc_futex_atomic_set(int oparg, u32 __user *uaddr, int *oval)
+{
+ unsigned int loops = LLSC_MAX_LOOPS;
+ int ret, oldval;
+
+ uaccess_enable_privileged();
+ asm volatile("//__llsc_futex_xchg\n"
+" prfm pstl1strm, %2\n"
+"1: ldxr %w1, %2\n"
+"2: stlxr %w0, %w4, %2\n"
+" cbz %w3, 3f\n"
+" sub %w3, %w3, %w0\n"
+" cbnz %w3, 1b\n"
+" mov %w0, %w5\n"
+"3:\n"
+" dmb ish\n"
+ _ASM_EXTABLE_UACCESS_ERR(1b, 3b, %w0)
+ _ASM_EXTABLE_UACCESS_ERR(2b, 3b, %w0)
+ : "=&r" (ret), "=&r" (oldval), "+Q" (*uaddr), "+r" (loops)
+ : "r" (oparg), "Ir" (-EAGAIN)
+ : "memory");
+ uaccess_disable_privileged();
+
+ if (!ret)
+ *oval = oldval;
+
+ return ret;
+}

static __always_inline int
__llsc_futex_cmpxchg(u32 __user *uaddr, u32 oldval, u32 newval, u32 *oval)
--
LEVI:{C3F47F37-75D8-414A-A8BA-3980EC8A46D7}