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


* Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> 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
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