Re: [ BUG: Invalid wait context ] rtc_lock at: mc146818_avoid_UIP

[Mateusz Jończyk]

W dniu 3.04.2025 o 15:12, Thomas Gleixner pisze:
> On Sun, Mar 30 2025 at 13:32, Borislav Petkov wrote:
> > while playing with suspend to RAM, I got this lockdep splat below.
> >
> > Poking around I found:
> >
> > ec5895c0f2d8 ("rtc: mc146818-lib: extract mc146818_avoid_UIP")
> >
> > which is doing this funky taking and dropping the rtc_lock and I guess that's
> > inherited from ye olde times.
> >
> > I "fixed" it so lockdeup doesn't warn by converting rtc_lock to a raw spinlock
> > but this is definitely not the right fix so let me bounce it off to the folks
> > on Cc who might have a better idea perhaps...
> Converting it to a raw lock "fixes" the problem, but RT people will hunt
> you down with a big latency bat.
>
> But this is not related to the commit above and not new.
>
> timekeeping_suspend() has always invoked mach_get_cmos_time() with the
> freeze lock held and mc146818_get_time() has always locked rtc_lock.
>
> I wonder, why this splat hasn't popped before. On RT lockdep should have
> complained forever. Sebastian???

Hello,

I was able to trigger the bug on Linux 6.1.0 with
CONFIG_PROVE_RAW_LOCK_NESTING enabled and I suspect it can be
triggered much earlier.

It is likely that only after

commit 560af5dc839eef ("lockdep: Enable PROVE_RAW_LOCK_NESTING with PROVE_LOCKING.")

people are seeing this simply because no one previously did
the test with CONFIG_PROVE_RAW_LOCK_NESTING=y.

See https://lore.kernel.org/all/CAP-bSRZ0CWyZZsMtx046YV8L28LhY0fson2g4EqcwRAVN1Jk+Q@xxxxxxxxxxxxxx/ :

> This splat happens on suspend/resume on a HP laptop. It doesn't appear
> to be a recent regression, as a bisect only leads to 560af5dc839e
> ("lockdep: Enable PROVE_RAW_LOCK_NESTING with PROVE_LOCKING.") -

Ccing Chris Bainbridge <chris.bainbridge@xxxxxxxxx>, author of the 
previous bug report.

Greetings,

Mateusz