Mutex Unlock using add

While checking Mutex unlock codes around, I found some that add 1 into the mutex variable instead of setting it to "1" directly. Is there any pros/con开发者_Python百科s of this?

Thanks

---

I wonder if it is possible that you are refereeing to the difference between mutex and semaphore resource access control.

---

Edit

That's all about CPU cycles needed for those two OPs. To my knowledge add uses less CPU cycles than mov. But then again it is very arch-dependent and questionable. Also, bear in mind that assemblers choice of how to encode a higher-level language instruction is very dependent on the surrounding instructions

---

It's just important that whatever operation is used, that it is atomic. It makes most sense to me to doing a set, rather than an add, particularly if there's a test-and-set instruction or implementation.

I found this implementation of a TestAndSet function for the x86 architecture. Here it uses a set (mov) instruction but it could have also used add or inc to do this but it would require eax to be 0 with the xchg instruction being used for atomicity. I suppose that requiring eax to be zero could be a con.