Re: [PATCH 1/3] watchdog: pretimeout: Protect governor access with RCU for NMI safety

[Guenter Roeck]

On 7/30/26 14:46, Doug Anderson wrote:
> Hi,
>
> On Thu, Jul 30, 2026 at 2:33 PM Mayank Rungta via B4 Relay
> <devnull+mrungta.google.com@xxxxxxxxxx> wrote:
> > From: Mayank Rungta <mrungta@xxxxxxxxxx>
> >
> > Currently, watchdog_notify_pretimeout() acquires pretimeout_lock using
> > spin_lock_irqsave() to safely dereference wdd->gov before invoking the
> > pretimeout callback.
> >
> > On architectures supporting pseudo-NMIs (ARM64 GICv3), watchdog drivers
> > may register their pretimeout warning interrupt (bark) as an NMI.
> > Because spin_lock_irqsave() disables regular interrupts but leaves NMIs
> > unmasked, if a pretimeout NMI fires while pretimeout_lock is already
> > held by normal process context, attempting to re-acquire pretimeout_lock
> > triggers an unrecoverable deadlock.
> >
> > To make pretimeout notifications completely safe to execute from NMI,
> > convert read access to wdd->gov inside watchdog_notify_pretimeout() from
> > a spinlock to lockless RCU. Use rcu_assign_pointer() when modifying
> > wdd->gov and invoke synchronize_rcu() during governor unregister and
> > pretimeout unregister routines.
> >
> > Signed-off-by: Mayank Rungta <mrungta@xxxxxxxxxx>
> > ---
> >   drivers/watchdog/watchdog_pretimeout.c | 47 +++++++++++++++++++---------------
> >   include/linux/watchdog.h               |  2 +-
> >   2 files changed, 27 insertions(+), 22 deletions(-)
>
> I pre-reviewed this for Mayank. I'm not an RCU expert, but this looks
> right based on my understanding. My AI was also happy with it. ;-)

Sashiko doesn't like it, claiming that some "const" are missing. And, indeed,
after applying it, I get:

drivers/watchdog/watchdog_pretimeout.c: In function ‘watchdog_pretimeout_governor_get’:
drivers/watchdog/watchdog_pretimeout.c:75:13: error: assignment discards ‘const’ qualifier from pointer target type [-Werror=discarded-qualifiers]
   75 |         gov = rcu_dereference(wdd->gov);
      |             ^
drivers/watchdog/watchdog_pretimeout.c: In function ‘watchdog_notify_pretimeout’:
drivers/watchdog/watchdog_pretimeout.c:110:13: error: assignment discards ‘const’ qualifier from pointer target type [-Werror=discarded-qualifiers]
  110 |         gov = rcu_dereference(wdd->gov);
      |             ^

Please fix.

Thanks,
Guenter