linux-kernel - Re: [RFC][PATCH 1/2] printk: lock console_sem before we unregister boot consoles

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Date:   Thu, 25 Apr 2019 12:52:33 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
To:     Petr Mladek <pmladek@...e.com>
Cc:     Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-kernel@...r.kernel.org,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>
Subject: Re: [RFC][PATCH 1/2] printk: lock console_sem before we unregister
 boot consoles

On (04/24/19 16:49), Petr Mladek wrote:
> > +	if (bcon && (newcon->flags & (CON_CONSDEV|CON_BOOT)) == CON_CONSDEV) {
> > +		console_lock();
> > +		/*
> > +		 * We need to iterate through all boot consoles, to make
> >  		 * sure we print everything out, before we unregister them.
> >  		 */
> 
> I wondered if moving the console locking could break the above
> statement.
> 
> It seems that the comment has been invalid since the commit
> 8259cf4342029aad37660e ("printk: Ensure that "console
> enabled" messages are printed on the console").

That's very interesting. Yes, you are right, the comment is a
leftover. printk used to iterate consoles twice before
8259cf4342029aad37660e

               /* we need to iterate through twice, to make sure we print
                * everything out, before we unregister the console(s)
                */
               printk(KERN_INFO "console handover:");
               for_each_console(bcon)
                       printk("boot [%s%d] ", bcon->name, bcon->index);

               printk(" -> real [%s%d]\n", newcon->name, newcon->index);
               for_each_console(bcon)
                       unregister_console(bcon);

But 8259cf4342029aad37660e has changed that and has made comment
invalid.

> Could we remove it in this patch? It touches it indirectly anyway.

Sure we can.

We also can take extra care of pr_info("%sconsole [%s%d] enabled\n".
Right now we do

	...
	console_unlock();
	console_sysfs_notify();

	pr_info("%sconsole [%s%d] enabled\n",....


But we can simply move that pr_info() a bit up:

	pr_info("%sconsole [%s%d] enabled\n",
	console_unlock();
	console_sysfs_notify();


So the message will be printed on all consoles.

---

diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index c2bccf58d03e..981eb6c27cdb 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -2813,8 +2813,6 @@ void register_console(struct console *newcon)
 		exclusive_console_stop_seq = console_seq;
 		logbuf_unlock_irqrestore(flags);
 	}
-	console_unlock();
-	console_sysfs_notify();
 
 	/*
 	 * By unregistering the bootconsoles after we enable the real console
@@ -2827,6 +2825,9 @@ void register_console(struct console *newcon)
 		(newcon->flags & CON_BOOT) ? "boot" : "" ,
 		newcon->name, newcon->index);
 
+	console_unlock();
+	console_sysfs_notify();
+
 	if (keep_bootcon)
 		return;
 
---

> Otherwise, the patch looks fine to me:
> 
> Reviewed-by: Petr Mladek <pmladek@...e.com>

Thanks!

	-ss