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

[Jeremy Fitzhardinge <jeremy@xxxxxxxx>]

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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