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    SubjectRe: [patch] cpufreq: mark cpufreq_tsc() as core_initcall_sync
    On Tue, 21 Nov 2006, Oleg Nesterov wrote:

    > On 11/20, Alan Stern wrote:
    > >
    > > Both CPUs execute their "mb" instructions. The mb forces each
    > > cache to wait until it receives an Acknowdgement for the
    > > Invalidate it sent.
    > >
    > > Both caches send an Acknowledgement message to the other. The
    > > mb instructions complete.
    > >
    > > "b = B" and "a = A" execute. The caches return A==0 and B==0
    > > because they haven't yet invalidated their cache lines.
    > >
    > > The reason the code failed is because the mb instructions didn't force
    > > the caches to wait until the Invalidate messages in their queues had been
    > > fully carried out (i.e., the lines had actually been invalidated).
    >
    > However, from
    > http://marc.theaimsgroup.com/?l=linux-kernel&m=113435711112941
    >
    > Paul E. McKenney wrote:
    > >
    > > 2. rmb() guarantees that any changes seen by the interconnect
    > > preceding the rmb() will be seen by any reads following the
    > > rmb().
    > >
    > > 3. mb() combines the guarantees made by rmb() and wmb().
    >
    > Confused :(

    I'm not certain the odd behavior can occur on systems that use an
    interconnect like Paul described. In the context I was describing, rmb()
    guarantees only that any changes seen _and acknowledged_ by the cache
    preceding the rmb() will be seen by any reads following the rmb(). It's a
    weaker guarantee, but it still suffices to show that

    A = 1 b = B
    wmb rmb
    B = 2 a = A

    will work as expected.

    Alan

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