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

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Date:   Mon, 12 Apr 2021 16:18:09 -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 Mon, Apr 12, 2021 at 10:37:10PM +0200, Thomas Gleixner wrote:
> On Mon, Apr 12 2021 at 12:57, Paul E. McKenney wrote:
> > On Mon, Apr 12, 2021 at 08:54:03PM +0200, Thomas Gleixner wrote:
> >> > I will send a new series out later today, Pacific Time.
> >> 
> >> Can you do me a favour and send it standalone and not as yet another
> >> reply to this existing thread maze. A trivial lore link to the previous
> >> version gives enough context.
> >
> > Will do!
> >
> > Of course, it turns out that lockdep also doesn't like waited-on
> > smp_call_function_single() invocations from timer handlers,
> > so I am currently looking at other options for dealing with that
> > potential use-after-free.  I am starting to like the looks of "only set
> > CLOCK_SOURCE_VERIFY_PERCPU on statically allocated clocksource structures
> > and let KASAN enforce this restriction", but I have not quite given up
> > on making it more general.
> 
> The simplest point is in the thread under the clocksource_mutex which
> prevents anything from vanishing under your feet.

And lockdep is -much- happier with the setup shown below, so thank
you again!

							Thanx, Paul

------------------------------------------------------------------------

diff --git a/kernel/time/clocksource.c b/kernel/time/clocksource.c
index f047c6cb056c..34dc38b6b923 100644
--- a/kernel/time/clocksource.c
+++ b/kernel/time/clocksource.c
@@ -519,6 +515,13 @@ static int __clocksource_watchdog_kthread(void)
 	unsigned long flags;
 	int select = 0;
 
+	/* Do any required per-CPU skew verification. */
+	list_for_each_entry(cs, &watchdog_list, wd_list) {
+		if ((cs->flags & (CLOCK_SOURCE_UNSTABLE | CLOCK_SOURCE_VERIFY_PERCPU)) ==
+		    (CLOCK_SOURCE_UNSTABLE | CLOCK_SOURCE_VERIFY_PERCPU))
+			clocksource_verify_percpu(cs);
+	}
+
 	spin_lock_irqsave(&watchdog_lock, flags);
 	list_for_each_entry_safe(cs, tmp, &watchdog_list, wd_list) {
 		if (cs->flags & CLOCK_SOURCE_UNSTABLE) {