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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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