Re: [PATCH v2 2/3] printk: nbcon: Introduce KDB helpers

[Marcos Paulo de Souza <mpdesouza@suse.com>]

On Thu, 2025-08-28 at 17:26 +0200, Petr Mladek wrote:
> On Mon 2025-08-11 10:32:46, Marcos Paulo de Souza wrote:
> > These helpers will be used when calling console->write_atomic on
> > KDB code in the next patch. It's basically the same implementaion
> > as nbcon_device_try_acquire, but using NBCON_PORIO_EMERGENCY when
> > acquiring the context.
> > 
> > For release, differently from nbcon_device_release, we don't need
> > to
> > flush the console, since all CPUs are stopped when KDB is active.
> 
> I thought this when we were discussion the code, especially the
> comment in
> 
> static void kdb_msg_write(const char *msg, int msg_len)
> {
> [...]
> 
> 	/*
> 	 * The console_srcu_read_lock() only provides safe console
> list
> 	 * traversal. The use of the ->write() callback relies on
> all other
> 	 * CPUs being stopped at the moment and console drivers
> being able to
> 	 * handle reentrance when @oops_in_progress is set.
> 
> 
> But I see that kdb_msg_write() is called from vkdb_printf() and
> there is the following synchronization:
> 
> int vkdb_printf(enum kdb_msgsrc src, const char *fmt, va_list ap)
> {
> [...]
> 
> 	/* Serialize kdb_printf if multiple cpus try to write at
> once.
> 	 * But if any cpu goes recursive in kdb, just print the
> output,
> 	 * even if it is interleaved with any other text.
> 	 */
> 	local_irq_save(flags);
> 	this_cpu = smp_processor_id();
> 	for (;;) {
> 		old_cpu = cmpxchg(&kdb_printf_cpu, -1, this_cpu);
> 		if (old_cpu == -1 || old_cpu == this_cpu)
> 			break;
> 
> 		cpu_relax();
> 	}
> 
> It suggests that the code might be used when other CPUs are still
> running.
> 
> And for example, kgdb_panic(buf) is called in vpanic() before
> the other CPUs are stopped.
> 
> 
> My opinion:
> 
> It might make sense to flush the console after all. But probably
> only the particular console, see below.

Thanks for being so meticulous and double checking the solution.

> 
> > --- a/kernel/printk/nbcon.c
> > +++ b/kernel/printk/nbcon.c
> > @@ -1855,3 +1855,29 @@ void nbcon_device_release(struct console
> > *con)
> >  	console_srcu_read_unlock(cookie);
> >  }
> >  EXPORT_SYMBOL_GPL(nbcon_device_release);
> > +
> 
> The function must be called only in the very specific kdb
> context, so it would deserve a comment. Inspired by
> nbcon_device_try_acquire():

Yeah... I planned to add documentation on the new functions, but
somehow I forgot until now. I'll take you suggestions and add to the
functions, thanks a lot!

> 
> /**
>  * nbcon_kdb_try_acquire - Try to acquire nbcon console, enter unsafe
>  *				section, and initialized nbcon write
> context
>  * @con:	The nbcon console to acquire
>  * @wctxt:	The nbcon write context to be used on success
>  *
>  * Context:	Under console_srcu_read_lock() for emiting a single
> kdb message
>  *		using the given con->write_atomic() callback. Can be
> called
>  *		only when the console is usable at the moment.
>  *
>  * Return:	True if the console was acquired. False otherwise.
>  *
>  * kdb emits messages on consoles registered for printk() without
>  * storing them into the ring buffer. It has to acquire the console
>  * ownerhip so that is could call con->write_atomic() callback a safe
> way.
>  *
>  * This function acquires the nbcon console using priority
> NBCON_PRIO_EMERGENCY
>  * and marks it unsafe for handover/takeover.
>  */
> 
> > +bool nbcon_kdb_try_acquire(struct console *con,
> > +			   struct nbcon_write_context *wctxt)
> > +{
> > +	struct nbcon_context *ctxt = &ACCESS_PRIVATE(wctxt, ctxt);
> > +
> > +	memset(ctxt, 0, sizeof(*ctxt));
> > +	ctxt->console = con;
> > +	ctxt->prio    = NBCON_PRIO_EMERGENCY;
> > +
> > +	if (!nbcon_context_try_acquire(ctxt, false))
> > +		return false;
> > +
> > +	if (!nbcon_context_enter_unsafe(ctxt))
> > +		return false;
> > +
> > +	return true;
> > +}
> > +
> 
> It deserves a comment as well, see below:
> 
> > +void nbcon_kdb_release(struct nbcon_write_context *wctxt)
> > +{
> > +	struct nbcon_context *ctxt = &ACCESS_PRIVATE(wctxt, ctxt);
> > +
> > +	nbcon_context_exit_unsafe(ctxt);
> > +	nbcon_context_release(ctxt);
> 
> I agree with John that the _release() should be called only when
> exit_unsafe() succeeded.

Done.

> 
> Also it might make sense to flush the console. I would do something
> like:
> 
> 
> /**
>  * nbcon_kdb_release - Exit unsafe section and release the nbcon
> console
>  *
>  * @wctxt:	The nbcon write context intialized by a succesful
>  *	nbcon_kdb_try_acquire()
>  *
>  * Context:	Under console_srcu_read_lock() for emiting a single
> kdb message
>  *		using the given con->write_atomic() callback. Can be
> called
>  *		only when the console is usable at the moment.
>  */
> void nbcon_kdb_release(struct nbcon_write_context *wctxt)
> {
> 	struct nbcon_context *ctxt = &ACCESS_PRIVATE(wctxt, ctxt);
> 
> 	nbcon_context_exit_unsafe(ctxt);
> 	nbcon_context_release(ctxt);
> 
> 	if (!nbcon_context_exit_unsafe(ctxt))
> 		return;
> 
> 	nbcon_context_release(ctxt);
> 
> 	/*
> 	 * Flush any new printk() messages added when the console
> was blocked.
> 	 * Only the console used by the given write context
> was	blocked.
> 	 * The console was locked only when the write_atomic()
> callback
> 	 * was usable.
> 	 */
> 	__nbcon_atomic_flush_pending_con(ctxt->console,
> prb_next_reserve_seq(prb), false);

Fixed locally, thanks a lot!

> 
> 
> Best Regards,
> Petr