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

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Message-ID: <CANpmjNOsSmKUxrLxTWYMD3RKnzSw5dfM=7QNJ02GMFG7BMeOGA@mail.gmail.com>
Date: Wed, 26 Nov 2025 15:54:26 +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 15:13, Breno Leitao <leitao@...ian.org> wrote:
>
> On Tue, Nov 25, 2025 at 08:02:16AM -0800, Breno Leitao wrote:
> > 6. Meanwhile, kfence's toggle_allocation_gate() on another CPU attempts to
> > perform a synchronous operation across all CPUs, which correctly triggers a CSD
> > lock timeout because CPU#0 is stuck in the busy loop with IRQs disabled.
>
> I've hacked a patch to disable kfence IPIs during machine shutdown, and
> with it loaded, I don't reproduce the problem described in this thread.
>
>         Author: Breno Leitao <leitao@...ian.org>
>         Date:   Tue Nov 25 07:21:55 2025 -0800
>
>         mm/kfence: add reboot notifier to disable KFENCE on shutdown
>
>         Register a reboot notifier to disable KFENCE and cancel any pending
>         timer work during system shutdown. This prevents potential IPI
>         synchronization issues that can occur when KFENCE is active during
>         the reboot process.
>
>         The notifier runs with high priority (INT_MAX) to ensure KFENCE is
>         disabled early in the shutdown sequence.
>
>         Signed-off-by: Breno Leitao <leitao@...ian.org>
>
>         diff --git a/mm/kfence/core.c b/mm/kfence/core.c
>         index 727c20c94ac5..5810afaaf6b4 100644
>         --- a/mm/kfence/core.c
>         +++ b/mm/kfence/core.c
>         @@ -26,6 +26,7 @@
>         #include <linux/panic_notifier.h>
>         #include <linux/random.h>
>         #include <linux/rcupdate.h>
>         +#include <linux/reboot.h>
>         #include <linux/sched/clock.h>
>         #include <linux/seq_file.h>
>         #include <linux/slab.h>
>         @@ -819,6 +820,21 @@ static struct notifier_block kfence_check_canary_notifier = {
>
>         static struct delayed_work kfence_timer;
>
>         +static int kfence_reboot_callback(struct notifier_block *nb,
>         +                                 unsigned long action, void *data)
>         +{
>         +       /* Disable KFENCE to avoid IPI synchronization during shutdown */
>         +       WRITE_ONCE(kfence_enabled, false);
>         +       /* Cancel any pending timer work */
>         +       cancel_delayed_work_sync(&kfence_timer);
>         +       return NOTIFY_OK;
>         +}
>         +
>         +static struct notifier_block kfence_reboot_notifier = {
>         +       .notifier_call = kfence_reboot_callback,
>         +       .priority = INT_MAX, /* Run early to stop timers ASAP */
>         +};

Just place it under the #ifdef CONFIG_KFENCE_STATIS_KEYS below, I do
not think this is required if CONFIG_KFENCE_STATIC_KEYS is unset.

>         #ifdef CONFIG_KFENCE_STATIC_KEYS
>         /* Wait queue to wake up allocation-gate timer task. */
>         static DECLARE_WAIT_QUEUE_HEAD(allocation_wait);
>         @@ -901,6 +917,8 @@ static void kfence_init_enable(void)
>                 if (kfence_check_on_panic)
>                         atomic_notifier_chain_register(&panic_notifier_list, &kfence_check_canary_notifier);
>
>         +       register_reboot_notifier(&kfence_reboot_notifier);
>         +
>                 WRITE_ONCE(kfence_enabled, true);
>                 queue_delayed_work(system_unbound_wq, &kfence_timer, 0);
>
>
> 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.

> The primary motivation is to avoid triggering IPIs during the machine
> teardown process, mainly when the nbconsole is not running in threaded
> mode.

Thanks,
-- Marco