Re: CSD lockup during kexec due to unbounded busy-wait in pl011_console_write_atomic (arm64)

[John Ogness <john.ogness@linutronix.de>]

On 2025-12-01, John Ogness <john.ogness@linutronix.de> wrote:
>> diff --git a/kernel/printk/nbcon.c b/kernel/printk/nbcon.c
>> index 3fa403f9831f..6b8becb6ecd9 100644
>> --- a/kernel/printk/nbcon.c
>> +++ b/kernel/printk/nbcon.c
>> @@ -1549,6 +1549,7 @@ static int __nbcon_atomic_flush_pending_con(struct console *con, u64 stop_seq)
>>  {
>>  	struct nbcon_write_context wctxt = { };
>>  	struct nbcon_context *ctxt = &ACCESS_PRIVATE(&wctxt, ctxt);
>> +	unsigned long flags;
>>  	int err = 0;
>>  
>>  	ctxt->console			= con;
>> @@ -1557,18 +1558,31 @@ static int __nbcon_atomic_flush_pending_con(struct console *con, u64 stop_seq)
>>  	ctxt->allow_unsafe_takeover	= nbcon_allow_unsafe_takeover();
>>  
>>  	while (nbcon_seq_read(con) < stop_seq) {
>> -		if (!nbcon_context_try_acquire(ctxt, false))
>> +		/*
>> +		 * Atomic flushing does not use console driver synchronization
>> +		 * (i.e. it does not hold the port lock for uart consoles).
>> +		 * Therefore IRQs must be disabled to avoid being interrupted
>> +		 * and then calling into a driver that will deadlock trying
>> +		 * to acquire console ownership.
>> +		 */
>> +		local_irq_save(flags);
>> +		if (!nbcon_context_try_acquire(ctxt, false)) {
>> +			local_irq_restore(flags);
>>  			return -EPERM;
>> +		}
>>  
>>  		/*
>>  		 * nbcon_emit_next_record() returns false when the console was
>>  		 * handed over or taken over. In both cases the context is no
>>  		 * longer valid.
>>  		 */
>> -		if (!nbcon_emit_next_record(&wctxt, true))
>> +		if (!nbcon_emit_next_record(&wctxt, true)) {
>> +			local_irq_restore(flags);
>>  			return -EAGAIN;
>> +		}
>>  
>>  		nbcon_context_release(ctxt);
>> +		local_irq_restore(flags);
>
> Using local_irq_save()/_restore() here is not acceptable for PREEMPT_RT
> because __nbcon_atomic_flush_pending_con() is also used by
> nbcon_device_release().

After thinking about this more, this would be acceptable. If
printk_get_console_flush_type() is reporting nbcon_atomic==true, then
the system is in a state where latencies are irrelevant.

John Ogness