Re: [PATCH v2 3/3] kdb: Adapt kdb_msg_write to work with NBCON consoles

[John Ogness]

On 2025-08-29, Petr Mladek <pmladek@xxxxxxxx> wrote:
>      c) kdb_msg_write() also writes the message on all other consoles
> 	registered by printk. I guess that this is what John meant
> 	by mirroring.

Yes.

>> diff --git a/kernel/printk/nbcon.c b/kernel/printk/nbcon.c
>> index 79d8c74378061..2c168eaf378ed 100644
>> --- a/kernel/printk/nbcon.c
>> +++ b/kernel/printk/nbcon.c
>> @@ -10,6 +10,7 @@
>>  #include <linux/export.h>
>>  #include <linux/init.h>
>>  #include <linux/irqflags.h>
>> +#include <linux/kgdb.h>
>>  #include <linux/kthread.h>
>>  #include <linux/minmax.h>
>>  #include <linux/percpu.h>
>> @@ -247,6 +248,8 @@ static int nbcon_context_try_acquire_direct(struct nbcon_context *ctxt,
>>  		 * Panic does not imply that the console is owned. However,
>>  		 * since all non-panic CPUs are stopped during panic(), it
>>  		 * is safer to have them avoid gaining console ownership.
>> +		 * The only exception is if kgdb is active, which may print
>> +		 * from multiple CPUs during a panic.
>>  		 *
>>  		 * If this acquire is a reacquire (and an unsafe takeover
>>  		 * has not previously occurred) then it is allowed to attempt
>> @@ -255,6 +258,7 @@ static int nbcon_context_try_acquire_direct(struct nbcon_context *ctxt,
>>  		 * interrupted by the panic CPU while printing.
>>  		 */
>>  		if (other_cpu_in_panic() &&
>> +		    atomic_read(&kgdb_active) == -1 &&
>
> This would likely work for most kgdb_printk() calls. But what about
> the one called from kgdb_panic()?

Nice catch.

> Alternative solution would be to allow it only for the CPU locked
> by kdb, something like:
>
> 		    READ_ONCE(kdb_printf_cpu) != raw_smp_processor_id() &&

Yes, I like this.

> Note that I used READ_ONCE() to guarantee an atomic read. The
> condition will fail only when we are inside a code locked by
> the kdb_printf_cpu().

Neither the READ_ONCE() nor any memory barriers are needed because the
only interesting case is when the CPU sees that it is the one stored in
@kdb_printf_cpu. In which case it was the one that did the storing and
the value is always correctly loaded.

>> [0] https://lore.kernel.org/lkml/20210803131301.5588-4-john.ogness@xxxxxxxxxxxxx
>
> Sigh, I have already forgotten that we discussed this in the past.

After so many years, I do not think there is a printk scenario we have
not discussed. ;-)

John