Re: [PATCH V3] firmware: google: Test spinlock on panic path to avoid lockups

Guilherme G. Piccoli
2022-09-01T17:47:50Z

On 01/09/2022 13:44, Greg KH wrote:
> [...]
>>> How are we supposed to know this here?
>>>
>>
>> Reading the code?
>> Or you mean, in the commit description this should be mentioned?
>
> Yes, and in the comment as this type of call is very rare and should
> almost never be used.

OK, I can add that, for sure.

>> [...]
>> I don't think it is so simple - we are in the panic path.
>
> Great, then the lock doesn't matter :)
>
>> So, imagine the lock was taken in CPU0, where GSMI is doing some
>> operation. During that operation, CPU1 panics!
>>
>> When that happens, panic() executes in CPU1, disabling CPU0 through
>> "strong" mechanisms (NMI). So, CPU0 had the lock, it is now off, and
>> when CPU1 goes through the panic notifiers, it'll eventually wait
>> forever for this lock in the GSMI handler, unless we have this patch
>> that would prevent the handler to run in such case.
>> Makes sense?
>
> I'm trying to say "if you are in panic, never grab the lock in the first
> place". So change the place when you grab the lock, not here.
>

Evan, any comment here?
I think the patch is still well suited for this case. Suggestions on how
to improve it are welcome, of course.

I honestly didn't understand exactly what you're suggesting Greg...
Mind clarifying?

Cheers,

Guilherme