Messages in this thread | | | Date | Wed, 24 Nov 2010 18:37:19 +0200 | Subject | Re: [thiscpuops upgrade 10/10] Lockless (and preemptless) fastpaths for slub | From | Pekka Enberg <> |
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On Wed, Nov 24, 2010 at 6:17 PM, Christoph Lameter <cl@linux.com> 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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