staticintcas(int*ptr,intcmp,intnew_val){intold_value=*ptr;__asm__volatile("lock; cmpxchg %2, %3":"=a"(old_value),"=m"(*ptr):"r"(new_val),"m"(*ptr),"0"(cmp):"cc","memory");returnold_value;} 直接看汇编十分晦涩,是因为cmpxchg这条指令比较奇怪,有个隐藏的%0参数,假设有三个...
最后比较 temp 的值是否等于 data,如果等于那么就将 data 的值变成 temp ,如果不相等(也就是说有其他线程更改了 data 的值,此时不能赋值给 data)回到第一步,这个操作主要是基于指令cmpxchg。 上面的三个步骤当中第三步是一个原子操作对应上面的汇编指令lock cmpxchg %esi,(%rcx),cmpxchg 指令前面加了 lock 主...
atomic_xchg{,_relaxed,_acquire,_release}()atomic_cmpxchg{,_relaxed,_acquire,_release}()atomic_tr...
编译器针对 atomic 产生对应的 code 是依赖不同的 architecture 下提供的指令集的, x86 下 load 可以用 mov 但 store 需要 xchg(锁内存 bus,这等价于 full fence),CAS 可以用 cmpxchg itanium 平台下有专门的指令:ld.acq 来读、st.rel; mf(memory fence) 来写,cmpxchg.rel;mf 来做 CAS,看起来更适合服务...
= val) {val = lock->val; return false;} //没持锁*/if(atomic_try_cmpxchg_relaxed(&lock->...
while ((old = cmpxchg(&v->counter, c, c c_op i)) != c) \ c = old; \ \ return c; \ } #else #include <linux/irqflags.h> @@ -88,6 +100,20 @@ static inline int atomic_##op##_return(int i, atomic_t *v) \ return ret; \ } #define ATOMIC_FETCH_OP(op, c_op) ...
这种思想预设所有线程在执行过程中都不会发生冲突，每一个线程都会乐观地认为自己能够成功执行，从而大大...
old = atomic_cmpxchg_relaxed(&r->refs, val, new); if (old == val) break;val = old; } } while (!atomic_try_cmpxchg_relaxed(&r->refs, &val, new));WARN_ONCE(new == UINT_MAX, "refcount_t: saturated; leaking memory.\n");@...
For other things, like atomic<struct> or atomic<double>, the only option on x86 is a retry loop with cmpxchg (or TSX). Atomic compare-and-swap (CAS) is usable as a lock-free building-block for any atomic RMW operation, up to the max hardware-supported CAS width. On x86-...
10 @@ #include <cmpxchg_loop.h> +#include "atomic_helpers.h" + template <class T> -void -test() +void test() { { typedef std::atomic<T> A; @@ -63,35 +64,7 @@ } } -struct A -{ - int i; - - explicit A(int d = 0) noexcept {i=d;} - - friend bool operator==...