linux-kernel - Re: [PATCH v7 clocksource 3/5] clocksource: Check per-CPU clock synchronization when marked unstable

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Date:   Sat, 10 Apr 2021 17:20:20 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     linux-kernel@...r.kernel.org, john.stultz@...aro.org,
        sboyd@...nel.org, corbet@....net, Mark.Rutland@....com,
        maz@...nel.org, kernel-team@...com, neeraju@...eaurora.org,
        ak@...ux.intel.com
Subject: Re: [PATCH v7 clocksource 3/5] clocksource: Check per-CPU clock
 synchronization when marked unstable

On Sat, Apr 10, 2021 at 11:00:25AM +0200, Thomas Gleixner wrote:
> On Fri, Apr 02 2021 at 15:49, paulmck wrote:
> >
> > +static void clocksource_verify_percpu_wq(struct work_struct *unused)
> > +{
> > +	int cpu;
> > +	struct clocksource *cs;
> > +	int64_t cs_nsec;
> > +	u64 csnow_begin;
> > +	u64 csnow_end;
> > +	u64 delta;
> 
> Please use reverse fir tree ordering and stick variables of the same
> type together:
> 
> 	u64 csnow_begin, csnow_end, delta;
> 	struct clocksource *cs;
> 	s64 cs_nsec;
>         int cpu;

Will do.

> > +
> > +	cs = smp_load_acquire(&clocksource_verify_work_cs); // pairs with release
> 
> Please don't use tail comments. They are a horrible distraction.

I will remove it.

> > +	if (WARN_ON_ONCE(!cs))
> > +		return;
> > +	pr_warn("Checking clocksource %s synchronization from CPU %d.\n",
> > +		cs->name, smp_processor_id());
> > +	cpumask_clear(&cpus_ahead);
> > +	cpumask_clear(&cpus_behind);
> > +	csnow_begin = cs->read(cs);
> 
> So this is invoked via work and the actual clocksource change is done
> via work too. Once the clocksource is not longer actively used for
> timekeeping it can go away. What's guaranteeing that this runs prior to
> the clocksource change and 'cs' is valid throughout this function?

>From what I can see, cs->read() doesn't care whether or not the
clocksource has been marked unstable.  So it should be OK to call
cs->read() before, during, or after the call to __clocksource_unstable().

Also, this is only done on clocksources marked CLOCK_SOURCE_VERIFY_PERCPU,
so any clocksource that did not like cs->read() being invoked during
or after the call to __clocksource_unstable() should leave off the
CLOCK_SOURCE_VERIFY_PERCPU bit.

Or did I take a wrong turn somewhere in the pointers to functions?

> > +	queue_work(system_highpri_wq, &clocksource_verify_work);
> 
> This does not guarantee anything. So why does this need an extra work
> function which is scheduled seperately?

Because I was concerned about doing smp_call_function() while holding
watchdog_lock, which is also acquired elsewhere using spin_lock_irqsave().
And it still looks like on x86 that spin_lock_irqsave() spins with irqs
disabled, which could result in deadlock.  The smp_call_function_single()
would wait for the target CPU to enable interrupts, which would not
happen until after the smp_call_function_single() returned due to its
caller holding watchdog_lock.

Or is there something that I am missing that prevents this deadlock
from occurring?

							Thanx, Paul