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

Thomas Gleixner
2021-04-13T20:49:49Z

Paul,

On Mon, Apr 12 2021 at 16:18, Paul E. McKenney wrote:
> 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!

But it is too simple now :) ...

> 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);
> + }

because that list is _NOT_ protected by the clocksource_mutex as you
noticed yourself already.

But you don't have to walk that list at all because the only interesting
thing is the currently active clocksource, which is about to be changed
in case the watchdog marked it unstable and cannot be changed by any
other code concurrently because clocksource_mutex is held.

So all you need is:

if (curr_clocksource &&
curr_clocksource->flags & CLOCK_SOURCE_UNSTABLE &&
curr_clocksource->flags & CLOCK_SOURCE_VERIFY_PERCPU)
clocksource_verify_percpu_wreckage(curr_clocksource);

Hmm?

Thanks,

tglx