linux-kernel - Re: CSD lockup during kexec due to unbounded busy-wait in pl011_console_write

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Message-ID: <CANpmjNMnKJGvneNDOCFRfC8xUWq-uuXVjLRQZsPYo86Xau5UHw@mail.gmail.com>
Date: Wed, 26 Nov 2025 17:08:59 +0100
From: Marco Elver <elver@...gle.com>
To: Breno Leitao <leitao@...ian.org>
Cc: glider@...gle.com, dvyukov@...gle.com, usamaarif642@...il.com, 
	leo.yan@....com, linux-arm-kernel@...ts.infradead.org, 
	linux-kernel@...r.kernel.org, kernel-team@...a.com, rmikey@...a.com, 
	john.ogness@...utronix.de, pmladek@...e.com, linux@...linux.org.uk, 
	paulmck@...nel.org, kasan-dev@...glegroups.com
Subject: Re: CSD lockup during kexec due to unbounded busy-wait in
 pl011_console_write_atomic (arm64)

On Wed, 26 Nov 2025 at 16:54, Breno Leitao <leitao@...ian.org> wrote:
[..]
> > > Alexander, Marco and Kasan maintainers:
> > >
> > > What is the potential impact of disabling KFENCE during reboot
> > > procedures?
> >
> > But only if CONFIG_KFENCE_STATIC_KEYS is enabled?
> > That would be reasonable, given our recommendation has been to disable
> > CONFIG_KFENCE_STATIC_KEYS since
> > https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=4f612ed3f748962cbef1316ff3d323e2b9055b6e
> > in most cases.
> >
> > I believe some low-CPU count systems are still benefiting from it, but
> > in general, I'd advise against it.
>
> Thanks for your review and guidance.
>
> Just to confirm my understanding: You’re okay with me adding this
> notifier specifically for CONFIG_KFENCE_STATIC_KEYS (which is what
> I need), but you would not support adding it for the general case where
> !CONFIG_KFENCE_STATIC_KEYS, correct?

Yes, correct. If there's a real issue with CONFIG_KFENCE_STATIC_KEYS,
it's worth fixing if there are still valid uses for it. But I wouldn't
pessimize the now default mode, which is !CONFIG_KFENCE_STATIC_KEYS,
as it doesn't appear to have this problem.

Thanks,
-- Marco