Re: [PATCH 0/3] 64-bit futexes: Intro

[Ingo Molnar <mingo@elte.hu>]

* Linus Torvalds <torvalds@linux-foundation.org> wrote:

> > That bit can be used as a lock and if all access to the state of 
> > that atomic variable uses it, arbitrary higher-order atomic state 
> > transitions can be derived from it. The cost would be a bit more 
> > instructions in the fastpath, but there would still only be a single 
> > atomic op (the acquire op), as the unlock would be a natural barrier 
> > (on x86 at least).
> 
> No, "unlocks as a natural barrier" only works for exclusive kernel 
> locks (spin_unlock and write_unlock). There we can just do a write to 
> unlock. But for anything that wants to handle contention differently 
> than just spinning, the unlock path needs to be able to do an atomic 
> "unlock and test if I need to do something else", because it may need 
> to wake things up.

yeah, indeed. Compared to all the other costs that have to be dealt with 
here, having a second atomic op isnt all that much of an issue either, 
especially on latest hw. An atomic op will probably never be as cheap as 
a non-atomic op, but ~20 cycles is still plenty fast for most practical 
purposes.

	Ingo