Re: [Xen-devel] [PATCH 2 of 3] Make p2m lookups fully synchronized wrt modifications

[Tim Deegan <tim@xxxxxxx>]

At 10:03 -0800 on 14 Nov (1321264988), Andres Lagar-Cavilla wrote:
> Hi there,
> > At 16:42 -0500 on 08 Nov (1320770545), Andres Lagar-Cavilla wrote:
> >>  xen/arch/x86/mm/mm-locks.h |  13 +++++++------
> >>  xen/arch/x86/mm/p2m.c      |  18 +++++++++++++++++-
> >>  xen/include/asm-x86/p2m.h  |  39
> >> ++++++++++++++++++++++++---------------
> >>  3 files changed, 48 insertions(+), 22 deletions(-)
> >>
> >>
> >> We achieve this by locking/unlocking the global p2m_lock in get/put_gfn.
> >> This brings about a few consequences for the p2m_lock:
> >>
> >>  - not ordered anymore in mm-locks.h: unshare does get_gfn -> shr_lock,
> >>    there are code paths that do paging_lock -> get_gfn. All of these
> >>    would cause mm-locks.h to panic.
> >
> > In that case there's a potential deadlock in the sharing code, and
> > turning off the safety catches is not an acceptable response to that. :)
> >
> > ISTR you had a plan to get rid of the shr_lock entirely, or am
> > I misremembering?
> Sharing is actually fine, I can reorder those safely until I get rid of
> the shr_lock. Except for sharing audits, which basically lock the whole
> hypervisor, and _no one is using at all_.
> 
> I have a more fundamental problem with the paging lock. sh_update_cr3 can
> be called from a variety of situations. It will walk the four top level
> PAE mappings, acquiring the p2m entry for each, with the paging lock held.
> This is an inversion & deadlock, if I try to synchronize p2m lookups with
> the p2m lock.

Is sh_update_cr3() really called with p2m locks/refs held?  Since it
doesn't take a frame number as an argument it might be possible to
shuffle things around at the callers.

> Any suggestions here? Other than disabling ordering of the p2m lock?

Disabling the ordering won't do, so we need to find something else.

Tim.


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