Пример реализации Википедии предусматривает спин-блокировку с помощью команды x86 XCHG:

; Intel syntax

locked:                      ; The lock variable. 1 = locked, 0 = unlocked.
     dd      0

spin_lock:
     mov     eax, 1          ; Set the EAX register to 1.

     xchg    eax, [locked]   ; Atomically swap the EAX register with
                             ;  the lock variable.
                             ; This will always store 1 to the lock, leaving
                             ;  the previous value in the EAX register.

     test    eax, eax        ; Test EAX with itself. Among other things, this will
                             ;  set the processor's Zero Flag if EAX is 0.
                             ; If EAX is 0, then the lock was unlocked and
                             ;  we just locked it.
                             ; Otherwise, EAX is 1 and we didn't acquire the lock.

     jnz     spin_lock       ; Jump back to the MOV instruction if the Zero Flag is
                             ;  not set; the lock was previously locked, and so
                             ; we need to spin until it becomes unlocked.

     ret                     ; The lock has been acquired, return to the calling
                             ;  function.

spin_unlock:
     mov     eax, 0          ; Set the EAX register to 0.

     xchg    eax, [locked]   ; Atomically swap the EAX register with
                             ;  the lock variable.

     ret                     ; The lock has been released.

отсюдаhttps://en.wikipedia.org/wiki/Spinlock#Example_implementation

Я не понимаю, почему разблокировка должна быть атомарной. Что случилось с

spin_unlock:
     mov     [locked], 0