Re: [Xen-devel] [PATCH] xen: avoid race in p2m handling

[Juergen Gross <jgross@xxxxxxxx>]

On 10/16/2014 05:50 PM, David Vrabel wrote:
> On 16/10/14 07:13, Juergen Gross wrote:
> > When a new p2m leaf is allocated this leaf is linked into the p2m tree
> > via cmpxchg. Unfortunately the compare value for checking the success
> > of the update is read after checking for the need of a new leaf. It is
> > possible that a new leaf has been linked into the tree concurrently
> > in between. This could lead to a leaked memory page and to the loss of
> > some p2m entries.
> >
> > Avoid the race by using the read compare value for checking the need
> > of a new p2m leaf.
> [...]
> > @@ -579,11 +580,10 @@ static bool alloc_p2m(unsigned long pfn)
> >                 }
> >         }
> >
> > -       if (p2m_top[topidx][mididx] == p2m_identity ||
> > -           p2m_top[topidx][mididx] == p2m_missing) {
> > +       p2m_orig = p2m_top[topidx][mididx];
> Do you need to use ACCESS_ONCE() here?
Yes, you are probably right. Should I send a new patch or do you want
to modify it?

Juergen


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