Messages in this thread | | | Date | Wed, 18 Mar 2009 15:14:08 -0700 | From | Jeremy Fitzhardinge <> | Subject | Re: Question about x86/mm/gup.c's use of disabled interrupts |
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Avi Kivity wrote: > Jeremy Fitzhardinge wrote: >>>> Disabling the interrupt will prevent the tlb flush IPI from coming >>>> in and flushing this cpu's tlb, but I don't see how it will prevent >>>> some other cpu from actually updating the pte in the pagetable, >>>> which is what we're concerned about here. >>> >>> The thread that cleared the pte holds the pte lock and is now >>> waiting for the IPI. The thread that wants to update the pte will >>> wait for the pte lock, thus also waits on the IPI and gup_fast()'s >>> local_irq_enable(). I think. >> >> But hasn't it already done the pte update at that point? >> >> (I think this conversation really is moot because the kernel never >> does P->P pte updates any more; its always P->N->P.) > > I thought you were concerned about cpu 0 doing a gup_fast(), cpu 1 > doing P->N, and cpu 2 doing N->P. In this case cpu 2 is waiting on > the pte lock.
The issue is that if cpu 0 is doing a gup_fast() and other cpus are doing P->P updates, then gup_fast() can potentially get a mix of old and new pte values - where P->P is any aggregate set of unsynchronized P->N and N->P operations on any number of other cpus. Ah, but if every P->N is followed by a tlb flush, then disabling interrupts will hold off any following N->P, allowing gup_fast to get a consistent pte snapshot.
Hm, awkward if flush_tlb_others doesn't IPI...
> Won't stop munmap().
And I guess it does the tlb flush before freeing the pages, so disabling the interrupt helps here too.
Simplest fix is to make gup_get_pte() a pvop, but that does seem like putting a red flag in front of an inner-loop hotspot, or something...
The per-cpu tlb-flush exclusion flag might really be the way to go.
J
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