Re: About spinlock implementation

[qznc via Digitalmars-d-learn]

On Thursday, 1 September 2016 at 06:44:13 UTC, mogu wrote:
I found an implementation of spinlock in concurrency.d.
```
static shared struct SpinLock
{
    void lock() { while (!cas(&locked, false, true)) { 
Thread.yield(); } }
    void unlock() { atomicStore!(MemoryOrder.rel)(locked, 
false); }
    bool locked;
}
```
Why atomicStore use MemoryOrder.rel instead of MemoryOrder.raw?

I'm not sure I understand rel [0], but raw is too weak. Raw means 
no sequencing barrier, so

  local_var = protected_value;
  spinlock.unlock();

could be transformed (by compiler or CPU) to

  spinlock.unlock();
  local_var = protected_value;

This effectively makes the access to the protected value 
unprotected and nullifies the effect of the spinlock.

I find the documentation on MemoryOrder lacking about the 
semantics of rel. :(

[0] https://dlang.org/library/core/atomic/memory_order.html