Re: [PATCH] timekeeping: Add a lockdep override in tick_freeze().

[Mateusz Jończyk]

W dniu 4.04.2025 o 15:34, Sebastian Andrzej Siewior pisze:
> tick_freeze() acquires a raw_spinlock_t (tick_freeze_lock). Later in the
> callchain (timekeeping_suspend() -> mc146818_avoid_UIP()) the RTC driver
> can acquire a spinlock_t which becomes a sleeping lock on PREEMPT_RT.
> Lockdep complains about this lock nesting.
>
> Add a lockdep override for this special case and a comment explaining
> why it is okay.
>
> Reported-by: Borislav Petkov <bp@xxxxxxxxx>
> Closes: https://lore.kernel.org/all/20250330113202.GAZ-krsjAnurOlTcp-@fat_crate.local/
> Reported-by: Chris Bainbridge <chris.bainbridge@xxxxxxxxx>
> Closes: https://lore.kernel.org/all/CAP-bSRZ0CWyZZsMtx046YV8L28LhY0fson2g4EqcwRAVN1Jk+Q@xxxxxxxxxxxxxx/
> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx>
> ---
>   kernel/time/tick-common.c | 20 ++++++++++++++++++++
>   1 file changed, 20 insertions(+)
>
> diff --git a/kernel/time/tick-common.c b/kernel/time/tick-common.c
> index a47bcf71defcf..8fd8e2ee09fa1 100644
> --- a/kernel/time/tick-common.c
> +++ b/kernel/time/tick-common.c
> @@ -509,6 +509,7 @@ void tick_resume(void)
>   
>   #ifdef CONFIG_SUSPEND
>   static DEFINE_RAW_SPINLOCK(tick_freeze_lock);
> +static DEFINE_WAIT_OVERRIDE_MAP(tick_freeze_map, LD_WAIT_SLEEP);
>   static unsigned int tick_freeze_depth;
>   
>   /**
> @@ -528,9 +529,20 @@ void tick_freeze(void)
>   	if (tick_freeze_depth == num_online_cpus()) {
>   		trace_suspend_resume(TPS("timekeeping_freeze"),
>   				     smp_processor_id(), true);
> +		/*
> +		 * All other CPUs have their interrupts disabled and are
> +		 * suspended to idle. Other tasks have been frozen so there is
> +		 * no scheduling happening. This means that there is no
> +		 * concurrency in the system at this point. Therefore it is okay
> +		 * to acquire a sleeping lock on PREEMPT_RT, such as spinlock_t,
> +		 * because the lock can not be acquired and can not block.

because the lock cannot be held by other CPUs / threads and
acquiring it cannot block.

> +		 * Inform lockdep about the situation.
> +		 */

Greetings,

Mateusz