To implement a spinlock in assembly. Here I post a solution I came up with. Is it correct? Do you know a shorter one?
lock:
mov ecx, 0
.loop:
xchg [eax], ecx
cmp ecx, 0
je .loop
release:
lock dec dword [eax]
eax is initialized to -1 (which means lock is free). This should work for many threads (not necessarily 2).
Shortest would probably be:
acquire:
lock bts [eax],0
jc acquire
release:
mov [eax],0
For performance, it's best to use a "test, test and set" approach, and use pause, like this:
acquire:
lock bts [eax],0 ;Optimistic first attempt
jnc l2 ;Success if acquired
l1:
pause
test [eax],1
jne l1 ;Don't attempt again unless there's a chance
lock bts [eax],0 ;Attempt to acquire
jc l1 ;Wait again if failed
l2:
release:
mov [eax],0
For debugging, you can add extra data to make it easier to detect problems, like this:
acquire:
lock bts [eax],31 ;Optimistic first attempt
jnc l2 ;Success if acquired
mov ebx,[CPUnumber]
lea ebx,[ebx+0x80000000]
cmp [eax],ebx ;Is the lock acquired by this CPU?
je .bad ; yes, deadlock
lock inc dword [eax+4] ;Increase "lock contention counter"
l1:
pause
test [eax],0x80000000
jne l1 ;Don't attempt again unless there's a chance
lock bts [eax],31 ;Attempt to acquire
jc l1 ;Wait again if failed
l2: mov [eax],ebx ;Store CPU number
release:
mov ebx,[CPUnumber]
lea ebx,[ebx+0x80000000]
cmp [eax],ebx ;Is lock acquired, and is CPU same?
jne .bad ; no, either not acquired or wrong CPU
mov [eax],0