[PATCH][RT][PPC64] Fix preempt unsafe paths accessing per_cpu variables

[Benjamin Herrenschmidt]

> All these operations are done assuming that tlb_gather_mmu disables
> preemption and tlb_finish_mmu enables preemption again.
> This is not true for -rt.
> For x86, none of the code paths between tlb_gather_mmu and
> tlb_finish_mmu access any per_cpu variables.
> But this is not true for powerpc64 as we can see.
> 
> One way could be to make tlb_gather_mmu disable preemption as it does
> in mainline but only for powerpc.
> Although i am not sure, if this is the right step ahead.
> 
> I am attaching a patch below for the same.
> I have left out the tce bits, as they are fine.
> 
> Note: I haven't extensively tested the patch

A better option is to make sure that a context switch does the right
thing, flushing the pending batch. I think that's already the case,
which means that your original patch may work, but that needs to
be double-checked and commented properly.

Cheers,
Ben.