Add a new cmpxchg interface:
bool try_cmpxchg(u{8,16,32,64} *ptr, u{8,16,32,64} *val, u{8,16,32,64} new);
Where the boolean returns the result of the compare; and thus if the
exchange happened; and in case of failure, the new value of *ptr is
returned in *val.
This allows simplification/improvement of loops like:
for (;;) {
new = val $op $imm;
old = cmpxchg(ptr, val, new);
if (old == val)
break;
val = old;
}
into:
for (;;) {
new = val $op $imm;
if (try_cmpxchg(ptr, &val, new))
break;
}
while also generating better code (GCC6 and onwards).
Signed-off-by: Peter Zijlstra (Intel) <[email protected]>
---
--- a/arch/x86/include/asm/atomic.h
+++ b/arch/x86/include/asm/atomic.h
@@ -186,6 +186,12 @@ static __always_inline int atomic_cmpxch
return cmpxchg(&v->counter, old, new);
}
+#define atomic_try_cmpxchg atomic_try_cmpxchg
+static __always_inline bool atomic_try_cmpxchg(atomic_t *v, int *old, int new)
+{
+ return try_cmpxchg(&v->counter, old, new);
+}
+
static inline int atomic_xchg(atomic_t *v, int new)
{
return xchg(&v->counter, new);
--- a/arch/x86/include/asm/cmpxchg.h
+++ b/arch/x86/include/asm/cmpxchg.h
@@ -153,6 +153,75 @@ extern void __add_wrong_size(void)
#define cmpxchg_local(ptr, old, new) \
__cmpxchg_local(ptr, old, new, sizeof(*(ptr)))
+
+#define __raw_try_cmpxchg(_ptr, _pold, _new, size, lock) \
+({ \
+ bool success; \
+ __typeof__(_ptr) _old = (_pold); \
+ __typeof__(*(_ptr)) __old = *_old; \
+ __typeof__(*(_ptr)) __new = (_new); \
+ switch (size) { \
+ case __X86_CASE_B: \
+ { \
+ volatile u8 *__ptr = (volatile u8 *)(_ptr); \
+ asm volatile(lock "cmpxchgb %[new], %[ptr]" \
+ CC_SET(z) \
+ : CC_OUT(z) (success), \
+ [ptr] "+m" (*__ptr), \
+ [old] "+a" (__old) \
+ : [new] "q" (__new) \
+ : "memory"); \
+ break; \
+ } \
+ case __X86_CASE_W: \
+ { \
+ volatile u16 *__ptr = (volatile u16 *)(_ptr); \
+ asm volatile(lock "cmpxchgw %[new], %[ptr]" \
+ CC_SET(z) \
+ : CC_OUT(z) (success), \
+ [ptr] "+m" (*__ptr), \
+ [old] "+a" (__old) \
+ : [new] "r" (__new) \
+ : "memory"); \
+ break; \
+ } \
+ case __X86_CASE_L: \
+ { \
+ volatile u32 *__ptr = (volatile u32 *)(_ptr); \
+ asm volatile(lock "cmpxchgl %[new], %[ptr]" \
+ CC_SET(z) \
+ : CC_OUT(z) (success), \
+ [ptr] "+m" (*__ptr), \
+ [old] "+a" (__old) \
+ : [new] "r" (__new) \
+ : "memory"); \
+ break; \
+ } \
+ case __X86_CASE_Q: \
+ { \
+ volatile u64 *__ptr = (volatile u64 *)(_ptr); \
+ asm volatile(lock "cmpxchgq %[new], %[ptr]" \
+ CC_SET(z) \
+ : CC_OUT(z) (success), \
+ [ptr] "+m" (*__ptr), \
+ [old] "+a" (__old) \
+ : [new] "r" (__new) \
+ : "memory"); \
+ break; \
+ } \
+ default: \
+ __cmpxchg_wrong_size(); \
+ } \
+ *_old = __old; \
+ success; \
+})
+
+#define __try_cmpxchg(ptr, pold, new, size) \
+ __raw_try_cmpxchg((ptr), (pold), (new), (size), LOCK_PREFIX)
+
+#define try_cmpxchg(ptr, pold, new) \
+ __try_cmpxchg((ptr), (pold), (new), sizeof(*(ptr)))
+
/*
* xadd() adds "inc" to "*ptr" and atomically returns the previous
* value of "*ptr".
--- a/include/linux/atomic.h
+++ b/include/linux/atomic.h
@@ -423,6 +423,28 @@
#endif
#endif /* atomic_cmpxchg_relaxed */
+#ifndef atomic_try_cmpxchg
+
+#define __atomic_try_cmpxchg(type, _p, _po, _n)
\
+({ \
+ typeof(_po) __po = (_po); \
+ typeof(*(_po)) __o = *__po; \
+ bool success = (atomic_cmpxchg##type((_p), __o, (_n)) == __o); \
+ *__po = __o; \
+ success; \
+})
+
+#define atomic_try_cmpxchg(_p, _po, _n) __atomic_try_cmpxchg(,
_p, _po, _n)
+#define atomic_try_cmpxchg_relaxed(_p, _po, _n)
__atomic_try_cmpxchg(_relaxed, _p, _po, _n)
+#define atomic_try_cmpxchg_acquire(_p, _po, _n)
__atomic_try_cmpxchg(_acquire, _p, _po, _n)
+#define atomic_try_cmpxchg_release(_p, _po, _n)
__atomic_try_cmpxchg(_release, _p, _po, _n)
+
+#else /* atomic_try_cmpxchg */
+#define atomic_try_cmpxchg_relaxed atomic_try_cmpxchg
+#define atomic_try_cmpxchg_acquire atomic_try_cmpxchg
+#define atomic_try_cmpxchg_release atomic_try_cmpxchg
+#endif /* atomic_try_cmpxchg */
+
/* cmpxchg_relaxed */
#ifndef cmpxchg_relaxed
#define cmpxchg_relaxed cmpxchg
