[PATCH RESEND v7 4/6] arm64: futex: refactor futex atomic operation

Yeoreum Yun
2025-08-16T15:19:44Z

Refactor futex atomic operations using ll/sc method with
clearing PSTATE.PAN to prepare to apply FEAT_LSUI on them.

Signed-off-by: Yeoreum Yun <yeoreum.yun@arm.com>
---
arch/arm64/include/asm/futex.h | 132 +++++++++++++++++++++------------
1 file changed, 84 insertions(+), 48 deletions(-)

diff --git a/arch/arm64/include/asm/futex.h b/arch/arm64/include/asm/futex.h
index bc06691d2062..ab7003cb4724 100644
--- a/arch/arm64/include/asm/futex.h
+++ b/arch/arm64/include/asm/futex.h
@@ -7,17 +7,21 @@

#include <linux/futex.h>
#include <linux/uaccess.h>
+#include <linux/stringify.h>

#include <asm/errno.h>

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

-#define __futex_atomic_op(insn, ret, oldval, uaddr, tmp, oparg) \
-do { \
- unsigned int loops = FUTEX_MAX_LOOPS; \
+#define LLSC_FUTEX_ATOMIC_OP(op, insn) \
+static __always_inline int \
+__llsc_futex_atomic_##op(int oparg, u32 __user *uaddr, int *oval) \
+{ \
+ unsigned int loops = LLSC_MAX_LOOPS; \
+ int ret, oldval, tmp; \
\
uaccess_enable_privileged(); \
- asm volatile( \
+ asm volatile("// __llsc_futex_atomic_" #op "\n" \
" prfm pstl1strm, %2\n" \
"1: ldxr %w1, %2\n" \
insn "\n" \
@@ -35,45 +39,103 @@ do { \
: "r" (oparg), "Ir" (-EAGAIN) \
: "memory"); \
uaccess_disable_privileged(); \
-} while (0)
+ \
+ if (!ret) \
+ *oval = oldval; \
+ \
+ 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")
+
+static __always_inline int
+__llsc_futex_cmpxchg(u32 __user *uaddr, u32 oldval, u32 newval, u32 *oval)
+{
+ int ret = 0;
+ unsigned int loops = LLSC_MAX_LOOPS;
+ u32 val, tmp;
+
+ uaccess_enable_privileged();
+ asm volatile("//__llsc_futex_cmpxchg\n"
+" prfm pstl1strm, %2\n"
+"1: ldxr %w1, %2\n"
+" eor %w3, %w1, %w5\n"
+" cbnz %w3, 4f\n"
+"2: stlxr %w3, %w6, %2\n"
+" cbz %w3, 3f\n"
+" sub %w4, %w4, %w3\n"
+" cbnz %w4, 1b\n"
+" mov %w0, %w7\n"
+"3:\n"
+" dmb ish\n"
+"4:\n"
+ _ASM_EXTABLE_UACCESS_ERR(1b, 4b, %w0)
+ _ASM_EXTABLE_UACCESS_ERR(2b, 4b, %w0)
+ : "+r" (ret), "=&r" (val), "+Q" (*uaddr), "=&r" (tmp), "+r" (loops)
+ : "r" (oldval), "r" (newval), "Ir" (-EAGAIN)
+ : "memory");
+ uaccess_disable_privileged();
+
+ if (!ret)
+ *oval = val;
+
+ return ret;
+}
+
+#define FUTEX_ATOMIC_OP(op) \
+static __always_inline int \
+__futex_atomic_##op(int oparg, u32 __user *uaddr, int *oval) \
+{ \
+ return __llsc_futex_atomic_##op(oparg, uaddr, oval); \
+}
+
+FUTEX_ATOMIC_OP(add)
+FUTEX_ATOMIC_OP(or)
+FUTEX_ATOMIC_OP(and)
+FUTEX_ATOMIC_OP(eor)
+FUTEX_ATOMIC_OP(set)
+
+static __always_inline int
+__futex_cmpxchg(u32 __user *uaddr, u32 oldval, u32 newval, u32 *oval)
+{
+ return __llsc_futex_cmpxchg(uaddr, oldval, newval, oval);
+}

static inline int
arch_futex_atomic_op_inuser(int op, int oparg, int *oval, u32 __user *_uaddr)
{
- int oldval = 0, ret, tmp;
- u32 __user *uaddr = __uaccess_mask_ptr(_uaddr);
+ int ret;
+ u32 __user *uaddr;

if (!access_ok(_uaddr, sizeof(u32)))
return -EFAULT;

+ uaddr = __uaccess_mask_ptr(_uaddr);
+
switch (op) {
case FUTEX_OP_SET:
- __futex_atomic_op("mov %w3, %w5",
- ret, oldval, uaddr, tmp, oparg);
+ ret = __futex_atomic_set(oparg, uaddr, oval);
break;
case FUTEX_OP_ADD:
- __futex_atomic_op("add %w3, %w1, %w5",
- ret, oldval, uaddr, tmp, oparg);
+ ret = __futex_atomic_add(oparg, uaddr, oval);
break;
case FUTEX_OP_OR:
- __futex_atomic_op("orr %w3, %w1, %w5",
- ret, oldval, uaddr, tmp, oparg);
+ ret = __futex_atomic_or(oparg, uaddr, oval);
break;
case FUTEX_OP_ANDN:
- __futex_atomic_op("and %w3, %w1, %w5",
- ret, oldval, uaddr, tmp, ~oparg);
+ ret = __futex_atomic_and(~oparg, uaddr, oval);
break;
case FUTEX_OP_XOR:
- __futex_atomic_op("eor %w3, %w1, %w5",
- ret, oldval, uaddr, tmp, oparg);
+ ret = __futex_atomic_eor(oparg, uaddr, oval);
break;
default:
ret = -ENOSYS;
}

- if (!ret)
- *oval = oldval;
-
return ret;
}

@@ -81,40 +143,14 @@ static inline int
futex_atomic_cmpxchg_inatomic(u32 *uval, u32 __user *_uaddr,
u32 oldval, u32 newval)
{
- int ret = 0;
- unsigned int loops = FUTEX_MAX_LOOPS;
- u32 val, tmp;
u32 __user *uaddr;

if (!access_ok(_uaddr, sizeof(u32)))
return -EFAULT;

uaddr = __uaccess_mask_ptr(_uaddr);
- uaccess_enable_privileged();
- asm volatile("// futex_atomic_cmpxchg_inatomic\n"
-" prfm pstl1strm, %2\n"
-"1: ldxr %w1, %2\n"
-" sub %w3, %w1, %w5\n"
-" cbnz %w3, 4f\n"
-"2: stlxr %w3, %w6, %2\n"
-" cbz %w3, 3f\n"
-" sub %w4, %w4, %w3\n"
-" cbnz %w4, 1b\n"
-" mov %w0, %w7\n"
-"3:\n"
-" dmb ish\n"
-"4:\n"
- _ASM_EXTABLE_UACCESS_ERR(1b, 4b, %w0)
- _ASM_EXTABLE_UACCESS_ERR(2b, 4b, %w0)
- : "+r" (ret), "=&r" (val), "+Q" (*uaddr), "=&r" (tmp), "+r" (loops)
- : "r" (oldval), "r" (newval), "Ir" (-EAGAIN)
- : "memory");
- uaccess_disable_privileged();
-
- if (!ret)
- *uval = val;

- return ret;
+ return __futex_cmpxchg(uaddr, oldval, newval, uval);
}

#endif /* __ASM_FUTEX_H */
--
LEVI:{C3F47F37-75D8-414A-A8BA-3980EC8A46D7}