Re: [Xen-devel] [patch 1/4] hotplug: Prevent alloc/free of irq descriptors during cpu up/down

[Boris Ostrovsky]

On 03/12/2016 04:19 AM, Thomas Gleixner wrote:
> Boris,
>
> On Tue, 14 Jul 2015, Boris Ostrovsky wrote:
> > On 07/14/2015 04:15 PM, Thomas Gleixner wrote:
> > > > > The issue here is that all architectures need that protection and just
> > > > > Xen does irq allocations in cpu_up.
> > > > >
> > > > > So moving that protection into architecture code is not really an
> > > > > option.
> > > > >
> > > > > > > > Otherwise we will need to have something like arch_post_cpu_up()
> > > > > > > > after the lock is released.
> > > > > I'm not sure, that this will work. You probably want to do this in the
> > > > > cpu prepare stage, i.e. before calling __cpu_up().
> > > > For PV guests (the ones that use xen_cpu_up()) it will work either before
> > > > or
> > > > after __cpu_up(). At least my (somewhat limited) testing didn't show any
> > > > problems so far.
> > > >
> > > > However, HVM CPUs use xen_hvm_cpu_up() and if you read comments there you
> > > > will
> > > > see that xen_smp_intr_init() needs to be called before native_cpu_up() but
> > > > xen_init_lock_cpu() (which eventually calls irq_alloc_descs()) needs to be
> > > > called after.
> > > >
> > > > I think I can split xen_init_lock_cpu() so that the part that needs to be
> > > > called after will avoid going into irq core code. And then the rest will
> > > > go
> > > > into arch_cpu_prepare().
> > > I think we should revisit this for 4.3. For 4.2 we can do the trivial
> > > variant and move the locking in native_cpu_up() and x86 only. x86 was
> > > the only arch on which such wreckage has been seen in the wild, but we
> > > should have that protection for all archs in the long run.
> > >
> > > Patch below should fix the issue.
> > Thanks! Most of my tests passed, I had a couple of failures but I will need to
> > see whether they are related to this patch.
> Did you ever come around to address that irq allocation from within cpu_up()?
>
> I really want to generalize the protection instead of carrying that x86 only
> hack forever.

Sorry, I completely forgot about this. Let me see how I can take 
allocations from under the lock. I might just be able to put them in CPU 
notifiers --- most into CPU_UP_PREPARE but spinlock interrupt may need 
to go into CPU_ONLINE.

-boris