CODEWITHSUNDEEP
c++multithreadingc++11
L.Koh
L.Koh

Reputation: 109

Do spinlocks guarantee order of acquisition?

I have a simple spinlock implementation similar to this: 

class Spinlock{
  std::atomic_flag flag;
public:
  Spinlock(): flag(ATOMIC_FLAG_INIT) {}
  ~Spinlock() {}

  void lock(){
    while(flag.test_and_set(std::memory_order_acquire));
  }

  void unlock(){
    flag.clear(std::memory_order_release);
  }
};

My question is similar to this one on mutexes but for spinlocks:

Is it guaranteed that Thread 2 will acquire the spinlock before Thread 3?

If not, are there any lock implementations which guarantee the order of acquisition?

Upvotes: 2

Views: 347

Answers (1)

ShadowRanger
ShadowRanger

Reputation: 155363

No, there is no queuing or ordering of any kind, because plain spinlocks are really just repeated races; each time an attempt to acquire fails, there is no memory carried over to the next attempt, it's just racing and hoping to win. Thread 2 or 3 could acquire it, with roughly equal likelihood, even if Thread 2 had been spinning on it for a minute, and Thread 3 for a microsecond.

Upvotes: 7

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