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    SubjectRe: [PATCH 0/3] 64-bit futexes: Intro
    On Wed, Jun 04, 2008 at 12:57:13PM -0700, Linus Torvalds wrote:
    >
    >
    > On Tue, 3 Jun 2008, Nick Piggin wrote:
    > >
    > > I think optimised our unlock_page in a way that it can do a
    > > non-atomic unlock in the fastpath (no waiters) using 2 bits. In
    > > practice it was still atomic but only because other page flags
    > > operations could operate on ->flags at the same time.
    >
    > I'd be *very* nervous about this.

    Heh ;) Well I'm not actually trying to do it in Linux (yet).


    > > We don't require any load/store barrier in the unlock_page fastpath
    > > because the bits are in the same word, so cache coherency gives us a
    > > sequential ordering anyway.
    >
    > Yes and no.
    >
    > Yes, the bits are int he same word, so cache coherency guarantees a lot.
    >
    > HOWEVER. If you do the sub-word write using a regular store, you are now
    > invoking the _one_ non-coherent part of the x86 memory pipeline: the store
    > buffer. Normal stores can (and will) be forwarded to subsequent loads from
    > the store buffer, and they are not strongly ordered wrt cache coherency
    > while they are buffered.
    >
    > IOW, on x86, loads are ordered wrt loads, and stores are ordered wrt other
    > stores, but loads are *not* ordered wrt other stores in the absense of a
    > serializing instruction, and it's exactly because of the write buffer.
    >
    > So:
    >
    > > But actually if we're careful, we can put them in seperate parts of the
    > > word and use the sub-word operations on x86 to avoid the atomic
    > > requirement. I'm not aware of any architecture in which operations to
    > > the same word could be out of order.
    >
    > See above. The above is unsafe, because if you do a regular store to a
    > partial word, with no serializing instructions between that and a
    > subsequent load of the whole word, the value of the store can be bypassed
    > from the store buffer, and the load from the other part of the word can be
    > carried out _before_ the store has actually gotten that cacheline
    > exclusively!
    >
    > So when you do
    >
    > movb reg,(byteptr)
    > movl (byteptr),reg
    >
    > you may actually get old data in the upper 24 bits, along with new data in
    > the lower 8.
    >
    > I think.

    I'd be very surprised if that was the case. But the unlock code needn't
    do that anyway. It could do

    movb reg,(byteptr) # clear PG_locked
    movb (byteptr+1),reg # load PG_waiters



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