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

On 7/30/26 14:46, Doug Anderson wrote:
> Hi,
> 
> On Thu, Jul 30, 2026 at 2:33 PM Mayank Rungta via B4 Relay
> <[email protected]> wrote:
>>
>> From: Mayank Rungta <[email protected]>
>>
>> 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 <[email protected]>
>> ---
>>   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