Messages in this thread | | | Date | Wed, 24 Nov 2010 10:55:40 -0600 (CST) | From | Christoph Lameter <> | Subject | Re: [thiscpuops upgrade 10/10] Lockless (and preemptless) fastpaths for slub |
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On Wed, 24 Nov 2010, Pekka Enberg wrote:
> 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*.
Exactly. That is the reason there is a compiler barrier there. A memory barrier would be smp_mb() or so.
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