[Xen-devel] Re: Question about x86/mm/gup.c's use of disabled interrupts

[Avi Kivity <avi@xxxxxxxxxx>]

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.
> > > Is this the only reason to disable interrupts?  
> > Another comment says it also prevents pagetable teardown.
> We could take a reference to the mm to get the same effect, no?
Won't stop munmap().

--
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