Messages in this thread | | | Date | Wed, 24 Nov 2010 18:47:12 +0200 | Subject | Re: [thiscpuops upgrade 10/10] Lockless (and preemptless) fastpaths for slub | From | Pekka Enberg <> |
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On Wed, 24 Nov 2010, Pekka Enberg wrote: >> > 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.
On Wed, Nov 24, 2010 at 6:45 PM, Christoph Lameter <cl@linux.com> wrote: > The compiler is still free to rearrange the tid fetch. A possible > optimization that the compiler may do is to move the tid fetch into the > next if statement since that is the only block in which the tid variable > is actually used.
Yes, which is why we need a *compiler barrier* but not a *memory barrier*.
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