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

[Sean Anderson <sean.anderson@linux.dev>]

On 9/13/25 00:30, Yeoreum Yun wrote:
> 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();

Ah, that makes sense.

--Sean