Messages in this thread | | | From | Andi Kleen <> | Subject | Re: broken local_t on i386 | Date | Mon, 12 Jun 2006 19:06:28 +0200 |
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On Monday 12 June 2006 18:54, Christoph Lameter wrote: > On Mon, 12 Jun 2006, Andi Kleen wrote: > > > > #define __get_cpu_var(var) (*RELOC_HIDE(&per_cpu__##var, __my_cpu_offset())) > > > > It is also affected by your race. The inc would only be atomic if the counter > > was in the PDA, but standard per cpu data isn't. So it has to follow > > a pointer and then it could already have switched. > > I thought the above would refer to a PDA memory area that is specially > mapped by each processor? That is the only thing that would get this > working right because we would map to a different PDA if the process > would be mapped to a different processor.
It does, but the per cpu data that everybody uses doesn't reside in the PDA because it wasn't possible to make this work with binutils
It would require a relocation relative to another symbol which isn't really supported.
At some point I considered using runtime patching to work around this limitation, but it would be some work and relatively complex.
So the PDA just contains a pointer to the real per CPU area and it's added. Unfortunately it's three instructions or so and not atomic (mov, add, reference)
> > > Fix would be to disable preemption. I don't think it needs cli/sti > > on non preemptible kernels. > > Yuck. The advantage of local.t was that it does not need any of these > tricks. What is the point of local.t if one needs to disable preemption?
No atomic operations. Preemption just requires to increase a counter in thread info.
Also on non preemptive kernels - which are the majority - it's a single instruction on x86. I guess preempt users can live with a bit more overhead ...
-Andi
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