Re: [PATCH v2] coresight: Fix possible deadlock in coresight_panic_cb

[Yeoreum Yun <yeoreum.yun@arm.com>]

Hi,

> > Hi,
> >
> >> Hi Sean,
> >>
> >> On Thu, Sep 11, 2025 at 11:33:15AM -0400, Sean Anderson wrote:
> >> > coresight_panic_cb is called with interrupts disabled during panics.
> >> > However, bus_for_each_dev calls bus_to_subsys which takes
> >> > bus_kset->list_lock without disabling IRQs. This will cause a deadlock
> >> > if a panic occurs while one of the other coresight functions that uses
> >> > bus_for_each_dev is running.
> >>
> >> The decription is a bit misleading. Even when IRQ is disabled, if an
> >> exception happens, a CPU still can be trapped for handling kernel panic.
> >>
> >> > Maintain a separate list of coresight devices to access during a panic.
> >>
> >> Rather than maintaining a separate list and introducing a new spinlock,
> >> I would argue if we can simply register panic notifier in TMC ETR and
> >> ETF drviers (see tmc_panic_sync_etr() and tmc_panic_sync_etf()).
> >>
> >> If there is no dependency between CoreSight modules in panic sync flow,
> >> it is not necessary to maintain list (and lock) for these modules.
>
> Yeah, I was thinking about this as I was preparing v2 of this patch.
>
> > +1 for this.
> > and using the spinlock in the panic_cb doesn't work on PREEMPT_RT side.
>
> What do you mean by this? I am using lockdep and it did not warn about this,
> so I assume that on PREEMPT_RT IRQs remain enabled in this path.

Hmm, I don't believe this.
When you see the panic(), it explicitly disable irq.
and preempt_disabled() before
calling atomic_notifier_call_chain(&panic_notifier_list, 0, buf);

also, atomic_nofier_call_chain() is rcu critical section.

As you know, since the spinlock becomes sleepable lock in PREEMPT_RT
this is problem.

The reason why lockdep doesn't report this problem since it was disabled
before panic notifier chain by calling debug_locks_off();

Thanks.

--
Sincerely,
Yeoreum Yun