linux-kernel - Re: [BUG v6.3-rc4+] WARNING: CPU: 0 PID: 1 at drivers/thermal/thermal_sysfs.c:879 cooling_device_stats

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Date:   Thu, 30 Mar 2023 09:57:26 +0000
From:   "Zhang, Rui" <rui.zhang@...el.com>
To:     "Torvalds, Linus" <torvalds@...ux-foundation.org>,
        "rostedt@...dmis.org" <rostedt@...dmis.org>
CC:     "Wysocki, Rafael J" <rafael.j.wysocki@...el.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [BUG v6.3-rc4+] WARNING: CPU: 0 PID: 1 at
 drivers/thermal/thermal_sysfs.c:879 cooling_device_stats_setup+0xac/0xc0

Hi, Linus,

On Wed, 2023-03-29 at 15:52 -0700, Linus Torvalds wrote:
> On Wed, Mar 29, 2023 at 1:58 PM Steven Rostedt <rostedt@...dmis.org>
> wrote:
> > In preparation to adding my patch that checks for some kinds of
> > bugs in
> > trace events, I decided to run it on the Linus's latest branch, to
> > see if
> > there's any other trace events that may cause issues. But instead I
> > hit
> > this unrelated bug. Looks to be triggering an added
> > lockdep_assert() on
> > boot up.
> 
> So I think that lockdep assert is likely bogus.
> 
> It was added in commit 790930f44289 ("thermal: core: Introduce
> thermal_cooling_device_update()") but the reason I say it's bogus is
> that I don't think it has ever been tested:
> 
> > static void cooling_device_stats_setup(struct
> > thermal_cooling_device *cdev)
> > {
> >         lockdep_assert_held(&cdev->lock); <<<---- line 879
> 
> Yeah, so cooling_device_stats_setup() is called from two places:
> 
>  - thermal_cooling_device_setup_sysfs()
> 
>  - thermal_cooling_device_stats_reinit()
> 
> and that first place is when that cdev is created, before it's
> registered anywhere. It's not locked in that case, and yes, the
> lockdep_assert_held() will trigger.
> 
> As far as I can tell it will always trigger, and this
> lockdep_assert()
> has thus never been tested with lockdep enabled.
> 
> The "stats_reinit" case seems to also be called from only one place
> (thermal_cooling_device_update()), and that path does indeed hold the
> cdev->lock.
> 
> That lockdep could be made happy by having
> thermal_cooling_device_setup_sysfs() create that device with the cdev
> lock held. I guess that's easy enough, although somewhat annoyingly
> there is no "mutex_init_locked()", you have to actually do
> "mutex_init()" followed by a "mutex_lock()". And obviously unlock it
> after doing the setup_sysfs().
> 
> But I question whether the lockdep test should be done at all. I find
> it distasteful that it was added with absolutely zero testing.
> 
> 
I just realized why I cannot reproduce this problem on my testbox.

In order to test the original patch with ACPI passive cooling enabled,
I rebuild the kernel with customized DSDT.
This taints the kernel, and clears the debug_locks, thus I didn't get
any lockdep warnings...

thanks,
rui