Re: [Xen-devel] [PATCH v3 6/6] VMX: Fixup PI descritpor when cpu is offline

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 02.09.16 at 05:25, <feng.wu@xxxxxxxxx> wrote:
>> From: Jan Beulich [mailto:JBeulich@xxxxxxxx]
>> Sent: Thursday, September 1, 2016 4:49 PM
>> >>> On 31.08.16 at 05:56, <feng.wu@xxxxxxxxx> wrote:
>> > +        new_cpu = cpumask_any(&cpu_online_map);
>> > +        new_lock = &per_cpu(vmx_pi_blocking, new_cpu).lock;
>> > +
>> > +        spin_lock(new_lock);
>> > +
>> > +        /*
>> > +         * If the new_cpu is not online, that means it became offline 
>> > between
>> > +         * we got 'new_cpu' and acquiring its lock above, we need to find
>> > +         * another online cpu instead. Such as, this fucntion is being 
>> > called
>> > +         * on 'new_cpu' at the same time. Can this happen??
>> > +         */
>> > +        if ( !cpu_online(new_cpu) )
>> > +        {
>> > +            spin_unlock(new_lock);
>> > +            goto restart;
>> > +        }
>> 
>> ... I think this too has been discussed before: Is this case really
>> possible? You're in the context of a CPU_DEAD or CPU_UP_CANCELED
>> notification, which both get issued with cpu_add_remove_lock held.
>> How can a second CPU go down in parallel?
> 
> Here is the call chain:
> 
> cpu_down() ->
>       stop_machine_run() ->
>               get_cpu_maps()  /* Try to hold the cpu_add_remove_lock */
>               ......
>               put_cpu_maps()  /* Release the lock */
>       notifier_call_chain(..., CPU_DEAD, ...) ->
>               vmx_vcpu_dead() ->
>                       vmx_pi_desc_fixup()
> 
> Seems vmx_pi_desc_fixup() is not calling with holding cpu_add_remove_lock?
> Or do I miss something? Thanks for further comments in advance!

The only place I see CPU_DEAD being passed to the notifier is
right in cpu_down(), with cpu_hotplug_done() (which is basically an
alias of put_cpu_maps()) clearly called a few lines later.

Jan


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