Re: [Xen-devel] [PATCH v8]xen: sched: convert RTDS from time to event driven model

[Meng Xu <mengxu@xxxxxxxxxxxxx>]

On Mon, Mar 14, 2016 at 12:35 PM, Dario Faggioli
<dario.faggioli@xxxxxxxxxx> wrote:
> On Mon, 2016-03-14 at 12:03 -0400, Meng Xu wrote:
>> On Mon, Mar 14, 2016 at 11:38 AM, Meng Xu <mengxu@xxxxxxxxxxxxx>
>> wrote:
>> >
>> > I'm ok that we keep using spin_lock_irqsave() for now. But maybe
>> > later, it will be a better idea to explore if spin_lock_irq() can
>> > replace all spin_lock_irqsave() in the RTDS scheduler?
>> >
>> I rethink about the choice of replacing spin_lock_irqsave with
>> spin_lock_irq().
>> If in the future ,we will introduce new locks and there may exit the
>> situaiton when we want to lock two locks in the same function. In
>> that
>> case, we won't use spin_lock_irq() but have to use
>> spin_lock_irqsave(). If we can mix up spin_lock_irq() with
>> spin_lock_irqsave() in different fucntiosn for the same lock, which I
>> think we can (right?), we should be fine. Otherwise, we will have to
>> keep using spin_lock_irqsave().
>>
> Mixing per se is not a problem, it's how you mix...
>
> If you call spin_unlock_irq() within a critical section protected by
> either spin_lock_irq() or spin_lock_irqsave(), that is not a good mix!
> :-)
>
> if you call _irqsave() inside a critical section protected by either
> _irq() or _irqsave(), that's what should be done (it's the purpose of
> _irqsave(), actually!).
>
> Actually, in case of nesting, most of the time the inner lock can be
> taken by just spin_lock(). Look, for instance, at csched2_dump_pcpu().

Yes. I totally agree. Then we can use _irq() lock in this patch first.
Then we can try to replace _irqsave() lock with irq() lock in another
patch for the RTDS scheduler and see if it works. (It should work if I
didn't miss something in the corner case.)

As to the nested lock issues, we can handle it later when we are
facing the issue, as long as we can mix the lock in a correct way. :-D

Thanks,

Meng

-- 
Meng Xu
PhD Student in Computer and Information Science
University of Pennsylvania
http://www.cis.upenn.edu/~mengxu/

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