Wikipedia, Spinlock - Wikipedia
One good reason to avoid spin locks is performance: as described in the main text, if a thread is interrupted while holding a lock, other threads that use spin locks will spend a large amount of CPU time just waiting for the lock to become available.
However, it turns out there is another interesting reason to avoid spin locks on some systems: correctness. The problem to be wary of is known as priority inversion, which unfortunately is an intergalactic scourge. Just avoiding the use of spin locks, unfortunately, does not avoid the problem of inversion (alas). You can address the priority inversion problem in a number of ways:
- In the specific case where spin locks cause the problem, you can avoid using spin locks.
- More generally, a higher-priority thread waiting for a lower-priority thread can temporarily boost the lower thread’s priority, thus enabling it to run and overcoming the inversion, a technique known as priority inheritance.
- A last solution is simplest: ensure all threads have the same priority.
Energy efficient spin-locking in multi-core machines
Example implementation:
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:
xor eax, eax ; Set the EAX register to 0.
xchg eax, [locked] ; Atomically swap the EAX register with
; the lock variable.
ret ; The lock has been released.
Spin Locks Considered Harmful, and How to Write Them When We Must
除了 PAUSE，还有 TPAUSE、UMONITOR 和 UMWAIT：
- TPAUSE 可以等待到指定 TSC 时钟。
- UMONITOR 可以监视地址写入，然后用 UMWAIT 等待。
- 不过似乎只有 Clang 支持，MSVC 不支持？