Re: [PATCH] cpu/hotplug: Allow the CPU in CPU_UP_PREPARE state to be brought up again.

[Boris Ostrovsky]

On 12/6/21 6:25 AM, Sebastian Andrzej Siewior wrote:
> On 2021-11-24 21:17:34 [-0500], Boris Ostrovsky wrote:
> > On 11/24/21 5:54 PM, Thomas Gleixner wrote:
> > > Any comment from XEN folks?
> >
> > If memory allocation in cpu_initialize_context() fails we will not be
> > able to bring up the VCPU because xen_cpu_initialized_map bit at the
> > top of that routine will already have been set. We will BUG in
> > xen_pv_cpu_up() on second (presumably successful) attempt because
> > nothing for that VCPU would be initialized. This can in principle be
> > fixed by moving allocation to the top of the routine and freeing
> > context if the bit in the bitmap is already set.
> >
> >
> > Having said that, allocation really should not fail: for PV guests we
> > first bring max number of VCPUs up and then offline them down to
> > however many need to run. I think if we fail allocation during boot we
> > are going to have a really bad day anyway.
> So can we keep the patch as-is or are some changes needed?


I think for the sake of completeness we could add


diff --git a/arch/x86/xen/smp_pv.c b/arch/x86/xen/smp_pv.c
index 6a8f3b53ab83..86368fcef466 100644
--- a/arch/x86/xen/smp_pv.c
+++ b/arch/x86/xen/smp_pv.c
@@ -277,8 +277,11 @@ cpu_initialize_context(unsigned int cpu, struct task_struct *idle)
                return 0;

        ctxt = kzalloc(sizeof(*ctxt), GFP_KERNEL);
-       if (ctxt == NULL)
+       if (ctxt == NULL) {
+               cpumask_clear_cpu(cpu, xen_cpu_initialized_map);
+               cpumask_clear_cpu(cpu, cpu_callout_mask);
                return -ENOMEM;
+       }

        gdt = get_cpu_gdt_rw(cpu);


-boris