Re: [RFC][PATCH 05/12] arch: Introduce arch_{,try_}_cmpxchg128{,_local}()

Boqun Feng
2022-12-22T01:25:56Z

On Mon, Dec 19, 2022 at 04:35:30PM +0100, Peter Zijlstra wrote:
> For all architectures that currently support cmpxchg_double()
> implement the cmpxchg128() family of functions that is basically the
> same but with a saner interface.
>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
> ---
> arch/arm64/include/asm/atomic_ll_sc.h | 38 +++++++++++++++++++++++
> arch/arm64/include/asm/atomic_lse.h | 33 +++++++++++++++++++-
> arch/arm64/include/asm/cmpxchg.h | 26 ++++++++++++++++
> arch/s390/include/asm/cmpxchg.h | 33 ++++++++++++++++++++
> arch/x86/include/asm/cmpxchg_32.h | 3 +
> arch/x86/include/asm/cmpxchg_64.h | 55 +++++++++++++++++++++++++++++++++-
> 6 files changed, 185 insertions(+), 3 deletions(-)
>
> --- a/arch/arm64/include/asm/atomic_ll_sc.h
> +++ b/arch/arm64/include/asm/atomic_ll_sc.h
> @@ -326,6 +326,44 @@ __CMPXCHG_DBL( , , , )
> __CMPXCHG_DBL(_mb, dmb ish, l, "memory")
>
> #undef __CMPXCHG_DBL
> +
> +union __u128_halves {
> + u128 full;
> + struct {
> + u64 low, high;
> + };
> +};
> +
> +#define __CMPXCHG128(name, mb, rel, cl) \
> +static __always_inline u128 \
> +__ll_sc__cmpxchg128##name(volatile u128 *ptr, u128 old, u128 new) \
> +{ \
> + union __u128_halves r, o = { .full = (old) }, \
> + n = { .full = (new) }; \
> + \
> + asm volatile("// __cmpxchg128" #name "\n" \
> + " prfm pstl1strm, %2\n" \
> + "1: ldxp %0, %1, %2\n" \
> + " eor %3, %0, %3\n" \
> + " eor %4, %1, %4\n" \
> + " orr %3, %4, %3\n" \
> + " cbnz %3, 2f\n" \
> + " st" #rel "xp %w3, %5, %6, %2\n" \
> + " cbnz %w3, 1b\n" \
> + " " #mb "\n" \
> + "2:" \
> + : "=&r" (r.low), "=&r" (r.high), "+Q" (*(unsigned long *)ptr) \
> + : "r" (o.low), "r" (o.high), "r" (n.low), "r" (n.high) \
> + : cl); \
> + \
> + return r.full; \
> +}
> +
> +__CMPXCHG128( , , , )
> +__CMPXCHG128(_mb, dmb ish, l, "memory")
> +
> +#undef __CMPXCHG128
> +
> #undef K
>
> #endif /* __ASM_ATOMIC_LL_SC_H */
> --- a/arch/arm64/include/asm/atomic_lse.h
> +++ b/arch/arm64/include/asm/atomic_lse.h
> @@ -151,7 +151,7 @@ __lse_atomic64_fetch_##op##name(s64 i, a
> " " #asm_op #mb " %[i], %[old], %[v]" \
> : [v] "+Q" (v->counter), \
> [old] "=r" (old) \
> - : [i] "r" (i) \
> + : [i] "r" (i) \
> : cl); \
> \
> return old; \
> @@ -324,4 +324,35 @@ __CMPXCHG_DBL(_mb, al, "memory")
>
> #undef __CMPXCHG_DBL
>
> +#define __CMPXCHG128(name, mb, cl...) \
> +static __always_inline u128 \
> +__lse__cmpxchg128##name(volatile u128 *ptr, u128 old, u128 new) \
> +{ \
> + union __u128_halves r, o = { .full = (old) }, \
> + n = { .full = (new) }; \
> + register unsigned long x0 asm ("x0") = o.low; \
> + register unsigned long x1 asm ("x1") = o.high; \
> + register unsigned long x2 asm ("x2") = n.low; \
> + register unsigned long x3 asm ("x3") = n.high; \
> + register unsigned long x4 asm ("x4") = (unsigned long)ptr; \
> + \
> + asm volatile( \
> + __LSE_PREAMBLE \
> + " casp" #mb "\t%[old1], %[old2], %[new1], %[new2], %[v]\n"\
> + : [old1] "+&r" (x0), [old2] "+&r" (x1), \
> + [v] "+Q" (*(unsigned long *)ptr) \
> + : [new1] "r" (x2), [new2] "r" (x3), [ptr] "r" (x4), \

Issue #1: the line below can be removed, otherwise..

> + [oldval1] "r" (r.low), [oldval2] "r" (r.high) \

warning:

./arch/arm64/include/asm/atomic_lse.h: In function '__lse__cmpxchg128_mb':
./arch/arm64/include/asm/atomic_lse.h:309:27: warning: 'r.<U97b8>.low' is used uninitialized [-Wuninitialized]
309 | [oldval1] "r" (r.low), [oldval2] "r" (r.high)

> + : cl); \
> + \
> + r.low = x0; r.high = x1; \
> + \
> + return r.full; \
> +}
> +
> +__CMPXCHG128( , )
> +__CMPXCHG128(_mb, al, "memory")
> +
> +#undef __CMPXCHG128
> +
> #endif /* __ASM_ATOMIC_LSE_H */
> --- a/arch/arm64/include/asm/cmpxchg.h
> +++ b/arch/arm64/include/asm/cmpxchg.h
> @@ -147,6 +147,19 @@ __CMPXCHG_DBL(_mb)
>
> #undef __CMPXCHG_DBL
>
> +#define __CMPXCHG128(name) \
> +static inline long __cmpxchg128##name(volatile u128 *ptr, \

Issue #2: this should be

static inline u128 __cmpxchg128##name(..)

because cmpxchg* needs to return the old value.

Regards,
Boqun

> + u128 old, u128 new) \
> +{ \
> + return __lse_ll_sc_body(_cmpxchg128##name, \
> + ptr, old, new); \
> +}
> +
> +__CMPXCHG128( )
> +__CMPXCHG128(_mb)
> +
> +#undef __CMPXCHG128
> +
> #define __CMPXCHG_GEN(sfx) \
> static __always_inline unsigned long __cmpxchg##sfx(volatile void *ptr, \
> unsigned long old, \
> @@ -229,6 +242,19 @@ __CMPXCHG_GEN(_mb)
> __ret; \
> })
>
> +/* cmpxchg128 */
> +#define system_has_cmpxchg128() 1
> +
> +#define arch_cmpxchg128(ptr, o, n) \
> +({ \
> + __cmpxchg128_mb((ptr), (o), (n)); \
> +})
> +
> +#define arch_cmpxchg128_local(ptr, o, n) \
> +({ \
> + __cmpxchg128((ptr), (o), (n)); \
> +})
> +
> #define __CMPWAIT_CASE(w, sfx, sz) \
> static inline void __cmpwait_case_##sz(volatile void *ptr, \
> unsigned long val) \
> --- a/arch/s390/include/asm/cmpxchg.h
> +++ b/arch/s390/include/asm/cmpxchg.h
> @@ -201,4 +201,37 @@ static __always_inline int __cmpxchg_dou
> (unsigned long)(n1), (unsigned long)(n2)); \
> })
>
> +#define system_has_cmpxchg128() 1
> +
> +static __always_inline u128 arch_cmpxchg128(volatile u128 *ptr, u128 old, u128 new)
> +{
> + asm volatile(
> + " cdsg %[old],%[new],%[ptr]\n"
> + : [old] "+&d" (old)
> + : [new] "d" (new),
> + [ptr] "QS" (*(unsigned long *)ptr)
> + : "memory", "cc");
> + return old;
> +}
> +
> +static __always_inline bool arch_try_cmpxchg128(volatile u128 *ptr, u128 *oldp, u128 new)
> +{
> + u128 old = *oldp;
> + int cc;
> +
> + asm volatile(
> + " cdsg %[old],%[new],%[ptr]\n"
> + " ipm %[cc]\n"
> + " srl %[cc],28\n"
> + : [cc] "=&d" (cc), [old] "+&d" (old)
> + : [new] "d" (new),
> + [ptr] "QS" (*(unsigned long *)ptr)
> + : "memory", "cc");
> +
> + if (unlikely(!cc))
> + *oldp = old;
> +
> + return likely(cc);
> +}
> +
> #endif /* __ASM_CMPXCHG_H */
> --- a/arch/x86/include/asm/cmpxchg_32.h
> +++ b/arch/x86/include/asm/cmpxchg_32.h
> @@ -103,6 +103,7 @@ static inline bool __try_cmpxchg64(volat
>
> #endif
>
> -#define system_has_cmpxchg_double() boot_cpu_has(X86_FEATURE_CX8)
> +#define system_has_cmpxchg_double() boot_cpu_has(X86_FEATURE_CX8)
> +#define system_has_cmpxchg64() boot_cpu_has(X86_FEATURE_CX8)
>
> #endif /* _ASM_X86_CMPXCHG_32_H */
> --- a/arch/x86/include/asm/cmpxchg_64.h
> +++ b/arch/x86/include/asm/cmpxchg_64.h
> @@ -20,6 +20,59 @@
> arch_try_cmpxchg((ptr), (po), (n)); \
> })
>
> -#define system_has_cmpxchg_double() boot_cpu_has(X86_FEATURE_CX16)
> +union __u128_halves {
> + u128 full;
> + struct {
> + u64 low, high;
> + };
> +};
> +
> +static __always_inline u128 arch_cmpxchg128(volatile u128 *ptr, u128 old, u128 new)
> +{
> + union __u128_halves o = { .full = old, }, n = { .full = new, };
> +
> + asm volatile(LOCK_PREFIX "cmpxchg16b %[ptr]"
> + : [ptr] "+m" (*ptr),
> + "+a" (o.low), "+d" (o.high)
> + : "b" (n.low), "c" (n.high)
> + : "memory");
> +
> + return o.full;
> +}
> +
> +static __always_inline u128 arch_cmpxchg128_local(volatile u128 *ptr, u128 old, u128 new)
> +{
> + union __u128_halves o = { .full = old, }, n = { .full = new, };
> +
> + asm volatile("cmpxchg16b %[ptr]"
> + : [ptr] "+m" (*ptr),
> + "+a" (o.low), "+d" (o.high)
> + : "b" (n.low), "c" (n.high)
> + : "memory");
> +
> + return o.full;
> +}
> +
> +static __always_inline bool arch_try_cmpxchg128(volatile u128 *ptr, u128 *old, u128 new)
> +{
> + union __u128_halves o = { .full = *old, }, n = { .full = new, };
> + bool ret;
> +
> + asm volatile(LOCK_PREFIX "cmpxchg16b %[ptr]"
> + CC_SET(e)
> + : CC_OUT(e) (ret),
> + [ptr] "+m" (*ptr),
> + "+a" (o.low), "+d" (o.high)
> + : "b" (n.low), "c" (n.high)
> + : "memory");
> +
> + if (unlikely(!ret))
> + *old = o.full;
> +
> + return likely(ret);
> +}
> +
> +#define system_has_cmpxchg_double() boot_cpu_has(X86_FEATURE_CX16)
> +#define system_has_cmpxchg128() boot_cpu_has(X86_FEATURE_CX16)
>
> #endif /* _ASM_X86_CMPXCHG_64_H */
>
>