linux-kernel - Re: [PATCH v2] lockdep: add lockdep_cleanup_dead

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Message-ID: <ZvYALG8PDrcwMSUb@boqun-archlinux>
Date: Thu, 26 Sep 2024 17:45:32 -0700
From: Boqun Feng <boqun.feng@...il.com>
To: David Woodhouse <dwmw2@...radead.org>
Cc: Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...hat.com>, Will Deacon <will@...nel.org>,
	Waiman Long <longman@...hat.com>,
	Kent Overstreet <kent.overstreet@...ux.dev>,
	Arnd Bergmann <arnd@...db.de>,
	Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] lockdep: add lockdep_cleanup_dead_cpu()

On Thu, Sep 26, 2024 at 05:37:12PM +0100, David Woodhouse wrote:
> On Thu, 2024-09-26 at 09:09 -0700, Boqun Feng wrote:
> > 
> > I won't call this a "perfectly harmless bug", safe_halt() also contains
> > tracepoints, which are not supposed to work in offline path IIUC, for
> > example, you may incorrectly use RCU when RCU is not watching, that
> > could mean reading garbage memory (surely it won't crash the system, but
> > I hope I never need to debug such a system ;-)).
> > 
> > Otherwise this patch looks good to me. Thanks!
> 
> Apart from the fact that I can't count. Apparently I got up to v3 of it
> last time, so this one should have been v4. I just mostly forgot all
> about it, and found it lying around in a git tree a year later, and it
> still seemed relevant. 

My point is calling a non-noinstr function in the offline path is not a
"perfectly harmless" bug, it can cause serious results, so that line in
the commit log is not true. Of course, lockdep should handle buggy code
gracefully, but buggy code is still buggy code.

Anyway, I've taken it into my tree (I removed the "perfectly harmless
bug" part because of the reason above):
	
	git://git.kernel.org/pub/scm/linux/kernel/git/boqun/linux.git lockdep-for-tip

and will send it in a PR to tip around -rc2 to -rc4, so it will goes
into v6.13 if things went well.

Feel free to send a new version, if the one in my tree needs some
changes. Again, thanks for the patch!

Regards,
Boqun

(you can refer some context here [1], in case you wonder who's this
Boqun guy and why is he doing this ;-))

[1]: https://lore.kernel.org/lkml/Zq5KmTEnalIOHf6a@boqun-archlinux/