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

Yeoreum Yun
2025-09-16T12:51:45Z

Hi,

[...]
> > +#define LSUI_CMPXCHG_HELPER(suffix, start_bit) \
> > +static __always_inline int \
> > +__lsui_cmpxchg_helper_##suffix(u64 __user *uaddr, u32 oldval, u32 newval) \
> > +{ \
> > + int ret = 0; \
> > + u64 oval, nval, tmp; \
> > + \
> > + asm volatile("//__lsui_cmpxchg_helper_" #suffix "\n" \
> > + __LSUI_PREAMBLE \
> > +" prfm pstl1strm, %2\n" \
> > +"1: ldtr %x1, %2\n" \
> > +" mov %x3, %x1\n" \
> > +" bfi %x1, %x5, #" #start_bit ", #32\n" \
> > +" bfi %x3, %x6, #" #start_bit ", #32\n" \
> > +" mov %x4, %x1\n" \
> > +"2: caslt %x1, %x3, %2\n" \
> > +" sub %x1, %x1, %x4\n" \
> > +" cbz %x1, 3f\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" (oval), "+Q" (*uaddr), "=&r" (nval), "=&r" (tmp) \
> > + : "r" (oldval), "r" (newval), "Ir" (-EAGAIN) \
> > + : "memory"); \
>
> The vast majority of this can be written in C.

Here is the version with C base on patch 6:

diff --git a/arch/arm64/include/asm/futex.h b/arch/arm64/include/asm/futex.h
diff --git a/arch/arm64/include/asm/futex.h b/arch/arm64/include/asm/futex.h
index 1d6d9f856ac5..68af15ba545a 100644
--- a/arch/arm64/include/asm/futex.h
+++ b/arch/arm64/include/asm/futex.h
@@ -127,81 +127,77 @@ LSUI_FUTEX_ATOMIC_OP(andnot, ldtclr, al)
LSUI_FUTEX_ATOMIC_OP(set, swpt, al)

static __always_inline int
-__lsui_futex_atomic_and(int oparg, u32 __user *uaddr, int *oval)
+__lsui_cmpxchg_helper(u32 __user *uaddr, u32 oldval, u32 newval, u32 *oval)
{
- return __lsui_futex_atomic_andnot(~oparg, uaddr, oval);
-}
+ int ret = -EAGAIN;
+ u64 __user *uaddr_al;
+ u64 oval64, nval64, tmp;
+ static const u64 lo32_mask = GENMASK_U64(31, 0);
+
+ uaddr_al = (u64 __user *) ALIGN_DOWN((unsigned long)uaddr, sizeof(u64));
+ unsafe_get_user(oval64, uaddr_al, err_fault);
+
+ if ((u32 __user *)uaddr_al != uaddr) {
+ nval64 = ((oval64 & lo32_mask) | ((u64)newval << 32));
+ oval64 = ((oval64 & lo32_mask) | ((u64)oldval << 32));
+ } else {
+ nval64 = ((oval64 & ~lo32_mask) | newval);
+ oval64 = ((oval64 & ~lo32_mask) | oldval);
+ }

-static __always_inline int
-__lsui_futex_atomic_eor(int oparg, u32 __user *uaddr, int *oval)
-{
- unsigned int loops = FUTEX_MAX_LOOPS;
- int ret, oldval, tmp;
+ tmp = oval64;

- /*
- * there are no ldteor/stteor instructions...
- */
- asm volatile("// __lsui_futex_atomic_eor\n"
+ asm volatile("//__lsui_cmpxchg_helper\n"
__LSUI_PREAMBLE
-" prfm pstl1strm, %2\n"
-"1: ldtxr %w1, %2\n"
-" eor %w3, %w1, %w5\n"
-"2: stltxr %w0, %w3, %2\n"
-" cbz %w0, 3f\n"
-" sub %w4, %w4, %w0\n"
-" cbnz %w4, 1b\n"
-" mov %w0, %w6\n"
-"3:\n"
+"1: caslt %x1, %x3, %2\n"
+" sub %x1, %x1, %x4\n"
+" cbnz %x1, 2f\n"
+" mov %w0, %w5\n"
+"2:\n"
" dmb ish\n"
+"3:\n"
_ASM_EXTABLE_UACCESS_ERR(1b, 3b, %w0)
- _ASM_EXTABLE_UACCESS_ERR(2b, 3b, %w0)
- : "=&r" (ret), "=&r" (oldval), "+Q" (*uaddr), "=&r" (tmp),
- "+r" (loops)
- : "r" (oparg), "Ir" (-EAGAIN)
+ : "+r" (ret), "=&r" (oval64), "+Q" (*uaddr_al)
+ : "r" (nval64), "r" (tmp), "Ir" (0)
: "memory");

if (!ret)
*oval = oldval;

+err_fault:
return ret;
}

static __always_inline int
-__lsui_futex_cmpxchg(u32 __user *uaddr, u32 oldval, u32 newval, u32 *oval)
+__lsui_futex_atomic_and(int oparg, u32 __user *uaddr, int *oval)
{
- int ret = 0;
- unsigned int loops = FUTEX_MAX_LOOPS;
- u32 val, tmp;
+ return __lsui_futex_atomic_andnot(~oparg, uaddr, oval);
+}
+
+static __always_inline int
+__lsui_futex_atomic_eor(int oparg, u32 __user *uaddr, int *oval)
+{
+{
+ int ret = -EAGAIN;
+ u32 oldval, newval;

/*
- * cas{al}t doesn't support word size...
+ * there are no ldteor/stteor instructions...
*/
- asm volatile("//__lsui_futex_cmpxchg\n"
- __LSUI_PREAMBLE
-" prfm pstl1strm, %2\n"
-"1: ldtxr %w1, %2\n"
-" eor %w3, %w1, %w5\n"
-" cbnz %w3, 4f\n"
-"2: stltxr %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");
+ unsafe_get_user(oldval, uaddr, err_fault);
+ newval = oldval ^ oparg;

- if (!ret)
- *oval = oldval;
+ ret = __lsui_cmpxchg_helper(uaddr, oldval, newval, oval);

+err_fault:
return ret;
}

+static __always_inline int
+__lsui_futex_cmpxchg(u32 __user *uaddr, u32 oldval, u32 newval, u32 *oval)
+{
+ return __lsui_cmpxchg_helper(uaddr, oldval, newval, oval);
+}
+
#define __lsui_llsc_body(op, ...) \
({ \
alternative_has_cap_likely(ARM64_HAS_LSUI) ? \
(END)

I'm not sure this is good for you.
But If you share your thought, That's would be greatful.
(Note:
When I test with 256 threads for futex_atomic_eor op, there is not much
difference with former assembly version)

Thanks!

--
Sincerely,
Yeoreum Yun