Syntax C11 and C ++ 11: semantics of the semantics of getting and freeing memory

Question

I use C11 * atomics to control the state enumeration between multiple threads. The code resembles the following:

static _Atomic State state;

void setToFoo(void)
{
    atomic_store_explicit(&state, STATE_FOO, memory_order_release);
}

bool stateIsBar(void)
{
    return atomic_load_explicit(&state, memory_order_acquire) == STATE_BAR;
}

This compiles (for ARM Cortex-M4):

<setToFoo>:
   ldr  r3, [pc, #8]
   dmb  sy ; Memory barrier
   movs r2, #0
   strb r2, [r3, #0] ; store STATE_FOO
   bx   lr
   .word    0x00000000

<stateIsBar>:
  ldr   r3, [pc, #16]
  ldrb  r0, [r3, #0] ; load state
  dmb   sy ; Memory barrier
  sub.w r0, r0, #2 ; Comparison and return follows
  clz   r0, r0
  lsrs  r0, r0, #5
  bx    lr
  .word 0x00000000

Why does the fence face release and after acquisition? My mental model assumed that after release, a barrier will be placed (to “distribute” the stored variable and all other stores to other flows) and until received (to receive all previous stores from other flows).

---

* C11, ++ 11, ( ), . gcc g++ . . http://en.cppreference.com/w/c/atomic/memory_order http://en.cppreference.com/w/cpp/atomic/memory_order

Answer 1

, . , , . , .

T1: on-deps(A) -> fence -> write(A)

T2: read(A) -> fence -> deps-on(A)

read (A) deps-on (A)

write (A) on-deps (A)

, , , , (, ).

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