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    SubjectRe: [CFT][RFC] HT scheduler
    On Tue, 16 Dec 2003, Linus Torvalds wrote:

    > On Tue, 16 Dec 2003, Davide Libenzi wrote:
    > > > I bet it is. In a big way.
    > > >
    > > > The lock does two independent things:
    > > > - it tells the core that it can't just crack up the load and store.
    > > > - it also tells other memory ops that they can't re-order around it.
    > >
    > > You forgot the one where no HT knowledge can be used for optimizations.
    > >
    > > - Asserting the CPU's #LOCK pin to take control of the system bus. That in
    > > MP system translate into the signaling CPU taking full control of the
    > > system bus until the pin is deasserted.
    > I think this is historical and no longer true.
    > All modern CPU's do atomic accesses on a cache coherency (and write buffer
    > ordering) level, and it has nothing to do with the external bus any more.
    > I suspect locked accesses to memory-mapped IO _may_ still set an external
    > flag, but I seriously doubt anybody cares. I wouldn't be surprised if the
    > pin doesn't even exist any more.

    I just checked the P6 stuff. It does exist but it can is some cases
    (write-back access on data completely on cache, for example) be caught by
    the cache controller that handles the following ops using normal cache
    coherency mechanisms.

    - Davide

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