Re: printk meeting at LPC

From: John Ogness <john.ogness@linutronix.de>
To: Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>
Cc: Steven Rostedt <rostedt@goodmis.org>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	Peter Zijlstra <peterz@infradead.org>,
	Petr Mladek <pmladek@suse.com>,
	Andrea Parri <parri.andrea@gmail.com>,
	Sergey Senozhatsky <sergey.senozhatsky@gmail.com>,
	Brendan Higgins <brendanhiggins@google.com>,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	LKML <linux-kernel@vger.kernel.org>,
	Theodore Ts'o <tytso@mit.edu>, Paul Turner <pjt@google.com>,
	Daniel Vetter <daniel.vetter@ffwll.ch>,
	Prarit Bhargava <prarit@redhat.com>
Subject: Re: printk meeting at LPC
Date: Wed, 18 Sep 2019 09:42:34 +0200	[thread overview]
Message-ID: <87h85anj85.fsf@linutronix.de> (raw)
In-Reply-To: <20190918051933.GA220683@jagdpanzerIV> (Sergey Senozhatsky's message of "Wed, 18 Sep 2019 14:19:33 +0900")

On 2019-09-18, Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com> wrote:
>> For instance, tty/sysrq must be able to switch printk emergency
>> on/off.
>
> How did we come up to that _sync() printk() emergency mode (when we
> make sure that there is no active printing kthread)? We had a number
> of cases (complaints) of lost kernel messages. There are scenarios in
> which we cannot offload to async preemptible printing kthread, because
> current control path is, for instance, going to reboot the kernel. In
> sync printk() mode we have some sort (!) of guarantees that when we do
>
> 		pr_emerg("Restarting system\n");
> 		kmsg_dump(KMSG_DUMP_RESTART);
> 		machine_restart(cmd);
>
> pr_emerg("Restarting system\n") is going to flush logbuf before the
> system will machine_restart().

Yes, this was why I asked Daniel how the bsod stuff will be
implemented. We don't want a bsod just because we are
restarting. Perhaps write_atomic() should also have a "reason" argument
like kmsg_dump does. I will keep in touch with Daniel to make sure we
are sync on this.

> It's going to be a bit harder when we have per-console kthread.

Each console has its own iterator. This iterators will need to advance,
regardless if the message was printed via write() or write_atomic().

John Ogness