Re: [Xen-devel] [PATCH v3] x86: correct socket_cpumask allocation

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 10.07.15 at 16:57, <dario.faggioli@xxxxxxxxxx> wrote:
> I added this printk:
> 
> printk("  cpu=%u cpu_to_socket=%u\n", cpu, socket);
> 
> Right before this chunk of code, in cpu_smpboot_free():
> 
>     if ( cpumask_empty(socket_cpumask[socket]) )
>     {
>         xfree(socket_cpumask[socket]);
>         socket_cpumask[socket] = NULL;
>     }
> 
> And it says this:
> 
> ...
> (XEN) Preparing system for ACPI S5 state.
> (XEN) Disabling non-boot CPUs ...
> (XEN) Broke affinity for irq 9
> (XEN)   cpu=1 cpu_to_socket=4294967295
> (XEN) ----[ Xen-4.6-unstable  x86_64  debug=y  Tainted:    C ]----
> (XEN) CPU:    0
> (XEN) RIP:    e008:[<ffff82d0801886c2>] cpu_smpboot_free+0x43/0x28b
> ...
> 
> I.e., it looks like phys_proc_id has already been reset to
> XEN_INVALID_SOCKET_ID, as we're kind-of racing with
> remove_siblinginfo().

Right. We have

cpu_down()
  stop_machine_run(take_cpu_down, ...)
    notifier_call_chain(&cpu_chain, CPU_DYING, ...)
    __cpu_disable()
      remove_siblinginfo()
  __cpu_die()
notifier_call_chain(&cpu_chain, CPU_DEAD, ...)
  cpu_smpboot_free()

I.e. a clear use-after-invalidate.

Jan


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