Re: [PATCH v4 11/13] firmware: arm_sdei: add support for CPU private events

James Morse
2017-11-01T15:59:52Z

Hi Will,

On 24/10/17 18:34, James Morse wrote:
> On 18/10/17 18:19, Will Deacon wrote:
>> On Tue, Oct 17, 2017 at 06:44:30PM +0100, James Morse wrote:
>>> Private SDE events are per-cpu, and need to be registered and enabled
>>> on each CPU.
>>>
>>> Hide this detail from the caller by adapting our {,un}register and
>>> {en,dis}able calls to send an IPI to each CPU if the event is private.
>>>
>>> CPU private events are unregistered when the CPU is powered-off, and
>>> re-registered when the CPU is brought back online. This saves bringing
>>> secondary cores back online to call private_reset() on shutdown, kexec
>>> and resume from hibernate.
>
>>> diff --git a/drivers/firmware/arm_sdei.c b/drivers/firmware/arm_sdei.c
>>> index 28e4c4cbb16d..5598d9ba8b5d 100644
>>> --- a/drivers/firmware/arm_sdei.c
>>> +++ b/drivers/firmware/arm_sdei.c
>>> @@ -610,6 +813,7 @@ static int sdei_device_freeze(struct device *dev)
>>> {
>>> int err;
>>>
>>> + frozen = true;
>>> err = sdei_event_unregister_all();
>
>> Are the release semantics from spin_unlock in sdei_event_unregister_all
>> sufficient for the ordering guarantees you need?
>
> ... ordering ...
>
> The hotplug notifiers don't touch that lock, so at the first level: no.
>
> It looks like I was relying on the cpu-hotplug code using stop-machine for its
> work, and the spinlocks changing the pre-empt count for this to work. Which is
> not something I want to debug if it changes!
>
> I'll post a patch changing this bool to a more sensible atomic type.

Here I show my ignorance: atomic_t didn't do what I thought. But I do take the
spinlock inside the hotplug callback, so I'll move the variable in there.

Thanks,

James