Re: [Xen-devel] [PATCH 4/7] xen/arm: When CPU dies, free percpu area immediatelly

[Mirela Simonovic <mirela.simonovic@xxxxxxxxxx>]

Hi Julien,

On Mon, Apr 16, 2018 at 5:21 PM, Julien Grall <julien.grall@xxxxxxx> wrote:
>
>
> On 16/04/18 14:41, Mirela Simonovic wrote:
>>
>> On Mon, Apr 16, 2018 at 3:14 PM, Julien Grall <julien.grall@xxxxxxx>
>> wrote:
>>>
>>> On 12/04/18 22:31, Stefano Stabellini wrote:
>>>>
>>>> On Thu, 12 Apr 2018, Julien Grall wrote:
>>>>>
>>>>> On 12/04/18 00:46, Stefano Stabellini wrote:
>>>>>>
>>>>>> On Wed, 11 Apr 2018, Julien Grall wrote:
>>>>>>>
>>>>>>> On 11/04/18 14:19, Mirela Simonovic wrote:
>>>
>>> I guess the rcu_barrier() in the function handling suspend/resume works.
>>> But
>>> that doesn't cover the hotplug case. Looking at x86, suspend/resume case.
>>> For the hotplug case, there are an rcu_barrier in cpu_{up,down}_helper
>>> but
>>> they are only present in the case of cpu_{up,down} failed. I am not
>>> entirely
>>> sure how this is handled in x86
>>>
>>> Andrew, Jan, do you know when the percpu will be free on hotplug? It is
>>> call
>>> to call_rcu(...) but I am not sure when this is going to be executed.
>>>
>>
>> AFAIK disable/enable_nonboot_cpus() is the only way to do the hotplug
>> and rcu_barrier() is not included in the flow.
>
>
> That's not the only way. I clearly specified one in my previous answer (see
> cpu_{up,down}_helper) and there are other place (look for cpu_up).
>

I've looked at cpu_{up,down}_helper and cpu_up and I'm convinced now
that adding rcu_barrier() prior to calling enable_nonboot_cpus() is
the right approch.

cpu_{up,down}_helper functions exist only for x86. cpu_up_helper()
does call rcu_barrier() prior to calling cpu_up().
So calling rcu_barrier() is expected to be done prior to calling
cpu_up() (or enable_nonboot_cpus(), which is just a wrapper for
cpu_up()).

I believe this is right way to do because cpu_up() is used for
enabling non-boot CPUs in both boot and suspend/hotplug scenarios,
while rcu_barrier() is not required in boot scenario.
Therefore, I'll add rcu_barrier() prior to calling
enable_nonboot_cpus(). If I missed something please let me know.

Thanks,
Mirela

> Cheers,
>
> --
> Julien Grall

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