From: Petr Mladek <pmladek@suse.com>
To: John Ogness <john.ogness@linutronix.de>,
Linus Torvalds <torvalds@linuxfoundation.org>
Cc: Sergey Senozhatsky <senozhatsky@chromium.org>,
Steven Rostedt <rostedt@goodmis.org>,
Thomas Gleixner <tglx@linutronix.de>,
linux-kernel@vger.kernel.org,
Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Subject: panic context: was: Re: [PATCH printk v2 04/11] printk: nbcon: Provide functions to mark atomic write sections
Date: Fri, 6 Oct 2023 14:53:36 +0200 [thread overview]
Message-ID: <ZSADUKp8oJ2Ws2vC@alley> (raw)
In-Reply-To: <87h6n5teos.fsf@jogness.linutronix.de>
(2nd attempt with with Linus really in Cc).
Adding Linus into Cc. I would like to be sure about the flushing
of atomic consoles in panic context.
> During the demo at LPC2022 we had the situation that there was a large
> backlog when a WARN was hit. With current mainline the first line of the
> WARN is put into the ringbuffer and then the entire backlog is flushed
> before storing the rest of the WARN into the ringbuffer. At the time it
> was obvious that we should finish storing the WARN message and then
> start flushing the backlog.
This talks about the "emergency" context (WARN/OOPS/watchdog).
The system might be in big troubles but it would still try to continue.
Do we really want to defer the flush also for panic() context?
I ask because I was not on LPC 2022 in person and I do not remember
all details.
Anyway, the deferred flush works relatively well for the "emergency" context:
+ flushed from nbcon_atomic_exit()
+ printk kthread might emit the messages while they are being added
But it is tricky in panic(), see 8th patch at
https://lore.kernel.org/r/20230919230856.661435-9-john.ogness@linutronix.de
+ nbcon_atomic_exit() is called only in one code path.
+ nbcon_atomic_flush_all() is used in other paths. It looks like
a "Whack a mole" game to me.
+ messages are never emitted by printk kthread either because
CPUs are stopped or the kthread is not allowed to get the lock[*]
I see only one positive of the explicit flush. The consoles would
not delay crash_exec() and the crash dump might be closer to
the point where panic() was called.
Otherwise I see only negatives => IMHO, we want to flush atomic
consoles synchronously from printk() in panic().
Does anyone really want explicit flushes in panic()?
[*] Emitting messages is explicitly blocked on non-panic CPUs. It
increases the change that panic-CPU would be able to take
the console lock the safe way.
Best Regards,
Petr
next prev parent reply other threads:[~2023-10-06 12:53 UTC|newest]
Thread overview: 42+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-09-19 23:08 [PATCH printk v2 00/11] wire up nbcon atomic printing John Ogness
2023-09-19 23:08 ` [PATCH printk v2 01/11] printk: Make console_is_usable() available to nbcon John Ogness
2023-09-22 8:33 ` Petr Mladek
2023-09-19 23:08 ` [PATCH printk v2 02/11] printk: Let console_is_usable() handle nbcon John Ogness
2023-09-22 8:37 ` Petr Mladek
2023-09-19 23:08 ` [PATCH printk v2 03/11] printk: Add @flags argument for console_is_usable() John Ogness
2023-09-22 8:41 ` Petr Mladek
2023-09-19 23:08 ` [PATCH printk v2 04/11] printk: nbcon: Provide functions to mark atomic write sections John Ogness
2023-09-22 9:33 ` Petr Mladek
2023-09-25 9:25 ` John Ogness
2023-09-25 16:04 ` Petr Mladek
2023-10-05 12:51 ` John Ogness
2023-10-06 12:51 ` panic context: was: " Petr Mladek
2023-10-06 12:53 ` Petr Mladek [this message]
2023-10-08 10:13 ` John Ogness
2023-10-09 15:32 ` Petr Mladek
2023-10-10 16:02 ` John Ogness
2023-10-16 8:58 ` Dave Young
2023-10-16 10:09 ` John Ogness
2023-10-06 15:52 ` Petr Mladek
2023-09-19 23:08 ` [PATCH printk v2 05/11] printk: nbcon: Provide function for atomic flushing John Ogness
2023-09-22 12:32 ` Petr Mladek
2023-09-25 11:11 ` John Ogness
2023-09-19 23:08 ` [PATCH printk v2 06/11] printk: nbcon: Wire up nbcon console " John Ogness
2023-09-22 17:41 ` Petr Mladek
2023-09-25 13:37 ` John Ogness
2023-09-26 12:14 ` Petr Mladek
2023-10-05 13:59 ` John Ogness
2023-09-19 23:08 ` [PATCH printk v2 07/11] printk: nbcon: Wire up nbcon into console_flush_all() John Ogness
2023-09-26 11:34 ` Petr Mladek
2023-09-19 23:08 ` [PATCH printk v2 08/11] panic: Add atomic write enforcement to warn/panic John Ogness
2023-09-27 12:02 ` Petr Mladek
2023-09-19 23:08 ` [PATCH printk v2 09/11] panic: Add atomic write enforcement to oops John Ogness
2023-09-19 23:36 ` John Ogness
2023-09-20 13:28 ` Andy Shevchenko
2023-09-20 14:20 ` John Ogness
2023-09-20 14:45 ` Andy Shevchenko
2023-09-27 13:15 ` Petr Mladek
2023-09-19 23:08 ` [PATCH printk v2 10/11] rcu: Add atomic write enforcement for rcu stalls John Ogness
2023-09-27 15:00 ` Petr Mladek
2023-09-19 23:08 ` [PATCH printk v2 11/11] lockdep: Add atomic write enforcement for lockdep splats John Ogness
2023-09-29 8:31 ` Petr Mladek
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=ZSADUKp8oJ2Ws2vC@alley \
--to=pmladek@suse.com \
--cc=gregkh@linuxfoundation.org \
--cc=john.ogness@linutronix.de \
--cc=linux-kernel@vger.kernel.org \
--cc=rostedt@goodmis.org \
--cc=senozhatsky@chromium.org \
--cc=tglx@linutronix.de \
--cc=torvalds@linuxfoundation.org \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox