Re: [PATCH v3 4/5] kernel/watchdog: Adapt the watchdog_hld interface for async model

Lecopzer Chen
2022-04-07T16:36:29Z

> On Tue 2022-04-05 21:35:03, Lecopzer Chen wrote:
> > > On Thu 2022-03-24 22:14:04, Lecopzer Chen wrote:
> > > > When lockup_detector_init()->watchdog_nmi_probe(), PMU may be not ready
> > > > yet. E.g. on arm64, PMU is not ready until
> > > > device_initcall(armv8_pmu_driver_init). And it is deeply integrated
> > > > with the driver model and cpuhp. Hence it is hard to push this
> > > > initialization before smp_init().
> > > >
> > > > But it is easy to take an opposite approach and try to initialize
> > > > the watchdog once again later.
> > > > The delayed probe is called using workqueues. It need to allocate
> > > > memory and must be proceed in a normal context.
> > > > The delayed probe is queued only when the early one returns -EBUSY.
> > > > It is the return code returned when PMU is not ready yet.
> > > >
> > > > Provide an API - retry_lockup_detector_init() for anyone who needs
> > > > to delayed init lockup detector.
> > > >
> > > > The original assumption is: nobody should use delayed probe after
> > > > lockup_detector_check() which has __init attribute.
> > > > That is, anyone uses this API must call between lockup_detector_init()
> > > > and lockup_detector_check(), and the caller must have __init attribute
> > > >
> > > > --- a/kernel/watchdog.c
> > > > +++ b/kernel/watchdog.c
> > > > +}
> > > > +
> > > > +/*
> > > > + * retry_lockup_detector_init - retry init lockup detector if possible.
> > > > + *
> > > > + * Only take effect when allow_lockup_detector_init_retry is true, which
> > > > + * means it must call between lockup_detector_init() and lockup_detector_check().
> > > > + * Be aware that caller must have __init attribute, relative functions
> > > > + * will be freed after kernel initialization.
> > > > + */
> > > > +void __init retry_lockup_detector_init(void)
> > > > +{
> > > > + if (!allow_lockup_detector_init_retry)
> > > > + return;
> > > > +
> > > > + queue_work_on(__smp_processor_id(), system_wq, &detector_work);
> > > > +}
> > > > +
> > > > +/* Ensure the check is called after the initialization of driver */
> > > > +static int __init lockup_detector_check(void)
> > > > +{
> > > > + /* Make sure no work is pending. */
> > > > + flush_work(&detector_work);
> > >
> > > This is racy. We should first disable
> > > "allow_lockup_detector_init_retry" to make sure
> > > that retry_lockup_detector_init() will not queue
> > > the work any longer.
> >
> > But disable before flush_work will make the
> > lockup_detector_delay_init() ->
> > watchdog_nmi_probe ->
> > + if (!allow_lockup_detector_init_retry)
> > + return -EBUSY;
>
> I see. It is exactly the reason why I suggest to remove the
> optimization and keep the code simple.
>
> > how about:
> > ...
> > static bool __init delayed_init_allowed = true;
> > ...
> > /*
> > * retry_lockup_detector_init - retry init lockup detector if possible.
> > *
> > * Only take effect when allow_lockup_detector_init_retry is true, which
> > * means it must call between lockup_detector_init() and lockup_detector_check().
> > * Be aware that caller must have __init attribute, relative functions
> > * will be freed after kernel initialization.
> > */
> > void __init retry_lockup_detector_init(void)
> > {
> > if (!allow_lockup_detector_init_retry || !delayed_init_allowed)
> > return;
> >
> > /*
> > * we shouldn't queue any delayed init work twice to avoid
> > * any unwanted racy.
> > */
> > delayed_init_allowed = false;
>
> Grrr, this is so complicated and confusing. It might be because of
> badly selected variable names or comments. But I think that it is
> simply a bad approach.
>
> OK, you suggest two variables. If I get it correctly:
>
> + The variable "delayed_init_allowed"
> tries to prevent the race in lockup_detector_check().
>
> It will make sure that the work could not be queued after
> flush_work() finishes.
>
> Is this obvious from the comment?
> Is this obvious from the variable name?
>
> I am sorry. But it is not obvious to me. I understand it only
> because I see it together in this mail. It will be pretty
> hard to get it from the code when I see it one year later.
>
>
> + The variable "allow_lockup_detector_init_retry" has an unclear
> meaning. It might mean:
>
> + watchdog_nmi_probe() ended with -EBUSY in
> lockup_detector_init() and we can try the delayed init.
>
> + but it also means that watchdog_nmi_probe() succeeded in
> lockup_detector_delay_init() and there is no need to
> try the delayed init any longer.
>
> Is this obvious from the variable name?
> Is it explained anywhere?
> Is it easy to understand?
>
> No, from my POV. It is really bad idea to have a single
> variable with so many meanings.
>
>
> And this is my problem with this approach. There was one variable with
> unclear meanting. And you are trying to fix it by two variables
> with unclear meaning.
>

I really apreciate for your reply, many thanks for it.

For my point of view, the naming for "delayed_init_allowed" is the
whole system state now is able to(allowed) do delayed init.
The "allow_lockup_detector_init_retry" is that delayed init is ready
to retry register NMI watchdog.

Thus the meaning of delayed_init_"allowed" is, we are allowed to
do delayed init including
1. initialization of delayed init
2. the retry of delayed init

I'm sorry for that I didn't express the meaning clearly.
I'll have definition in detail in the later version patch not only
a brief comment and I hope you can review much easier.
This only explain the thought how I made decision for the naming.

So let me back to the discussion below.

> > queue_work_on(__smp_processor_id(), system_wq, &detector_work);
> > }
> >
> >
> > /*
> > * Ensure the check is called after the initialization of driver
> > * and before removing init code.
> > */
> > static int __init lockup_detector_check(void)
> > {
> > delayed_init_allowed = false;
> > flush_work(&detector_work);
> > allow_lockup_detector_init_retry = false;
> >
> > return 0;
> > }
>
> No, please keep it simple. Just have one variable that will say
> whether we are allowed to queue the work:
>
> + It will be allowed when watchdog_nmi_probe() ended
> with -EBUSY in lockup_detector_init()
>
> + It will not longer be allowed when watchdog_nmi_probe()
> succeeded or when lockup_detector_check() flushes
> the pending works.
>

Okay, let me think about it. I'll try to find a better solution that
only uses one variable.
And it's strongly about how users use it in 5th patch, I'll give further
reply in 5th patch

thanks
BRs,
Lecopzer