Thin lock vs. Futex

[Jb]

"Bartosz Milewski" <bartosz at relisoft.com> wrote in message 
news:g9jvui$nbs$1 at digitalmars.com...
> As promised, I posted a blog comparing the thin lock to futex. Please vote 
> it on reddit: http://www.reddit.com/comments/6z4sv/ .

Early on you say this...

"Unlock tests if the futex variable is equal to one (we are the owner, and 
nobody is waiting). If true, it sets it to zero. This is the fast-track 
common-case execution that doesn't make any futex calls whatsoever."

But the futex code you list later in the blog does this....

// we own the lock, so it's either 1 or 2
      if (atomic_decrement(&_word) != 1)

So does the futex actualy need/use an atomic for unlocking?

If it does then that makes it a great deal slower than a thin lock as that 
typicaly doesnt require an atomic for the unlock. As im sure you're aware, 
CAS and such like cost around 120..160 cycles on most cpus, so having an 
algorythm that cuts their usage in half is a big avantage.

So I'd be touting that as good reason to use thin locks. :-)