Re: [Xen-devel] [PATCH v2 4/4] The locking order is: first rank lock, then vgic lock. The order is respected everywhere, except for gic_update_one_lr.

[Julien Grall <julien.grall@xxxxxxx>]

Hi Stefano,

On 03/01/17 22:51, Stefano Stabellini wrote:
> On Wed, 28 Dec 2016, Julien Grall wrote:
> > Hi Stefano,
> >
> > On 22/12/16 02:15, Stefano Stabellini wrote:
> > > gic_update_one_lr is called with the vgic lock held, but it calls
> > > vgic_get_target_vcpu, which tries to obtain the rank lock. This can
> > > cause deadlocks.
> > >
> > > We already have a version of vgic_get_target_vcpu that doesn't take the
> > > rank lock: __vgic_get_target_vcpu.
> > >
> > > Solve the lock inversion problem, by not taking the rank lock in
> > > gic_update_one_lr (calling __vgic_get_target_vcpu instead of
> > > vgic_get_target_vcpu).  This is safe, because vcpu target modifications
> > > are protected by the same vgic vcpu lock.
> > I maintain what I said on the first version of this patch. All this patch
> > could be simplified by moving the call to irq_set_affinity in
> > vgic_irq_migrate.
> >
> > There are no restriction to do that and no need to have any lock taken but the
> > rank lock.
> All right. I'll submit a patch that does exactly that. It is not
> perfect, because it drops interrupts in the problematic scenario, but it
> should be a good place to start for a technical discussion.
I think I have missed something. What is the problematic scenario you 
are describing? Looking at the v3, I don't see any mention of this 
problem. Does it mean it has been solved?
Cheers,

--
Julien Grall

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