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    SubjectRe: [thiscpuops upgrade 10/10] Lockless (and preemptless) fastpaths for slub
    On Wed, Nov 24, 2010 at 6:17 PM, Christoph Lameter <> wrote:
    > On Wed, 24 Nov 2010, Pekka Enberg wrote:
    >> > +       /*
    >> > +        * The transaction ids are globally unique per cpu and per operation on
    >> > +        * a per cpu queue. Thus they can be guarantee that the cmpxchg_double
    >> > +        * occurs on the right processor and that there was no operation on the
    >> > +        * linked list in between.
    >> > +        */
    >> > +       tid = c->tid;
    >> > +       barrier();
    >> You're using a compiler barrier after every load from c->tid. Why?
    > To make sure that the compiler does not do something like loading the tid
    > later. The tid must be obtained before the rest of the information from
    > the per cpu slab data is retrieved in order to ensure that we have a
    > consistent set of data to operate on.
    > The critical section begins with the retrieval of the tid and it ends with
    > the replacement of the tid with the newly generated one. This means that
    > all state data for the alloc and free operation needs to be retrieved in
    > that critical section. The change must be saved with the final
    > cmpxchg_double of the critical section.

    Right and we don't need a *memory barrier* here because we're
    accessing a per-CPU variable which means operations appear in-order.

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