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

    * Linus Torvalds <> 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


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