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    SubjectRe: [thiscpuops upgrade 10/10] Lockless (and preemptless) fastpaths for slub
    On Wed, 24 Nov 2010, Jeremy Fitzhardinge wrote:

    > On 11/24/2010 08:17 AM, 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.
    >
    > Isn't that best expressed with ACCESS_ONCE()?

    ACCESS_ONCE does not prevent reordering if used once it seems when one
    reads the comments. ACCESS_ONCE() uses volatile? Uggh.




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