Re: [rfc] data race in page table setup/walking?

[Benjamin Herrenschmidt <benh-XVmvHMARGAS8U2dJNN8I7kB+6BGkLq7r@public.gmane.org>]

On Tue, 2008-04-29 at 07:00 +0200, Nick Piggin wrote:
> 
> At this point, the spinlock is not guaranteed to have ordered the previous
> stores to initialize the pte page with the subsequent store to put it in the
> page tables. So another Linux page table walker might be walking down (without
> any locks, because we have split-leaf-ptls), and find that new pte we've
> inserted. It might try to take the spinlock before the store from the other
> CPU initializes it. And subsequently it might read a pte_t out before stores
> from the other CPU have cleared the memory.

Funny, we used to have a similar race where the zeros for clearing a
newly allocated anonymous pages end up reaching the coherency domain
after the new PTE in set_pte, causing memory corruption on threaded
apps. I think back then we fixed that with an explicit smp_wmb() before
a set_pte(). Maybe we need that also when setting the higher levels.

Ben.