linux-kernel - Re: [ BUG: Invalid wait context ] rtc_lock at: mc146818_avoid

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Message-ID: <20250403135031.giGKVTEO@linutronix.de>
Date: Thu, 3 Apr 2025 15:50:31 +0200
From: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To: Thomas Gleixner <tglx@...utronix.de>
Cc: Borislav Petkov <bp@...en8.de>, linux-rtc@...r.kernel.org,
	Alexandre Belloni <alexandre.belloni@...tlin.com>,
	Mateusz Jończyk <mat.jonczyk@...pl>,
	lkml <linux-kernel@...r.kernel.org>,
	Anna-Maria Behnsen <anna-maria@...utronix.de>,
	Frederic Weisbecker <frederic@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>
Subject: Re: [ BUG: Invalid wait context ] rtc_lock at: mc146818_avoid_UIP

On 2025-04-03 15:12:08 [+0200], Thomas Gleixner wrote:
> 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???

I sure haven't seen it. But it has to.

> Suspend, whether it's suspend to idle or regular suspend, are weird
> contexts as there is no concurrency possible because interrupts are
> disabled and it is guaranteed that there is only a single CPU which is
> operational. The other CPUs are either in a deep idle state or when they
> are on the way back, they serialize on tick_freeze_lock.
> 
> So taking the non-raw spinlock in that context is safe, but obviously
> lockdep does not know about that.
> 
> Peter, any ideas?

We can tell lockdep that. I will look into this.

> Thanks,
> 
>         tglx

Sebastian