linux-kernel - Re: [PATCH 2/7] printk: Use consoles_suspended flag when suspending/resuming all consoles

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Message-ID: <aFpkQHwNCslbKSP6@pathway.suse.cz>
Date: Tue, 24 Jun 2025 10:40:46 +0200
From: Petr Mladek <pmladek@...e.com>
To: John Ogness <john.ogness@...utronix.de>
Cc: Marcos Paulo de Souza <mpdesouza@...e.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Sergey Senozhatsky <senozhatsky@...omium.org>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Jiri Slaby <jirislaby@...nel.org>,
	Jason Wessel <jason.wessel@...driver.com>,
	Daniel Thompson <danielt@...nel.org>,
	Douglas Anderson <dianders@...omium.org>,
	Richard Weinberger <richard@....at>,
	Anton Ivanov <anton.ivanov@...bridgegreys.com>,
	Johannes Berg <johannes@...solutions.net>,
	linux-kernel@...r.kernel.org, linux-serial@...r.kernel.org,
	kgdb-bugreport@...ts.sourceforge.net, linux-um@...ts.infradead.org
Subject: Re: [PATCH 2/7] printk: Use consoles_suspended flag when
 suspending/resuming all consoles

On Fri 2025-06-20 16:49:07, John Ogness wrote:
> On 2025-06-13, Petr Mladek <pmladek@...e.com> wrote:
> >> diff --git a/kernel/printk/nbcon.c b/kernel/printk/nbcon.c
> >> index fd12efcc4aeda8883773d9807bc215f6e5cdf71a..72de12396e6f1bc5234acfdf6dcc393acf88d216 100644
> >> --- a/kernel/printk/nbcon.c
> >> +++ b/kernel/printk/nbcon.c
> >> @@ -1147,7 +1147,7 @@ static bool nbcon_kthread_should_wakeup(struct console *con, struct nbcon_contex
> >>  	cookie = console_srcu_read_lock();
> >>  
> >>  	flags = console_srcu_read_flags(con);
> >> -	if (console_is_usable(con, flags, false)) {
> >> +	if (console_is_usable(con, flags, false, consoles_suspended)) {
> >
> > The new global console_suspended value has the be synchronized the
> > same way as the current CON_SUSPENDED per-console flag.
> > It means that the value must be:
> >
> >   + updated only under console_list_lock together with
> >     synchronize_rcu().
> >
> >   + read using READ_ONCE() under console_srcu_read_lock()
> 
> Yes.
> 
> > I am going to propose more solutions because no one is obviously
> > the best one.
> 
> [...]
> 
> > Variant C:
> > ==========
> >
> > Remove even @flags parameter from console_is_usable() and read both
> > values there directly.
> >
> > Many callers read @flags only because they call console_is_usable().
> > The change would simplify the code.
> >
> > But there are few exceptions:
> >
> >    2. Another exception is __pr_flush() where console_is_usable() is
> >       called twice with @use_atomic set "true" and "false".
> >
> >       We would want to read "con->flags" only once here. A solution
> >       would be to add a parameter to check both con->write_atomic
> >       and con->write_thread in a single call.
> 
> Or it could become a bitmask of printing types to check:
> 
> #define ATOMIC_PRINTING 0x1
> #define NONATOMIC_PRINTING 0x2
> 
> and then __pr_flush() looks like:
> 
> if (!console_is_usable(c, flags, ATOMIC_PRINTING|NONATOMIC_PRINTING)

I like this. It will help even in all other cases when one mode is needed.
I mean that, for example:

   console_is_usable(c, flags, ATOMIC_PRINTING)

is more self-explaining than

   console_is_usable(c, flags, true)

Best Regards,
Petr