atomic_add_unless

是說：add unless valus is the given number

arm 版 在 arch/arm/include/asm : atomic.h: static inline int atomic_add_unless(atomic_t *v, int a, int u) { int c, old; c = atomic_read(v); while (c != u && (old = atomic_cmpxchg((v), c, c + a)) != c) c = old; return c != u; } 然後 cmpxchg 是 static inline int atomic_cmpxchg(atomic_t *ptr, int old, int new) { unsigned long oldval, res; smp_mb(); do { __asm__ __volatile__("@ atomic_cmpxchg\n" "ldrex %1, [%3]\n" "mov %0, #0\n" "teq %1, %4\n" "strexeq %0, %5, [%3]\n" : "=&r" (res), "=&r" (oldval), "+Qo" (ptr->counter) : "r" (&ptr->counter), "Ir" (old), "r" (new) : "cc"); } while (res); smp_mb(); return oldval; } 就是 swp. armv6 版.

所以重點在 atomic_cmpxchg((v),c , c+a))

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關於 atomic 的操作，可以看 Documentation/atomic_ops.txt

		Semantics and Behavior of Atomic and
		         Bitmask Operations

這個 documsnt 的說明好像跟實做一饃一樣。

atomic_t 宣告在 linux/types.h typedef struct { int counter; } atomic_t; 宣告後，要初始化： (atomic.h) #define ATOMIC_INIT(i) { (i) }

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這一篇說得很清楚喔： http://blog.roodo.com/use_the_force/archives/3420371.html