linux-kernel.vger.kernel.org archive mirror
 help / color / mirror / Atom feed
From: Petr Mladek <pmladek@suse.com>
To: Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>
Cc: Konstantin Khlebnikov <koct9i@gmail.com>,
	Sergey Senozhatsky <sergey.senozhatsky@gmail.com>,
	Steven Rostedt <rostedt@goodmis.org>,
	Peter Zijlstra <peterz@infradead.org>,
	x86@kernel.org, John Ogness <john.ogness@linutronix.de>,
	Thomas Gleixner <tglx@linutronix.de>,
	Andrew Morton <akpm@linux-foundation.org>,
	Petr Tesarik <ptesarik@suse.cz>,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH 1/2] printk/panic: Access the main printk log in panic() only when safe
Date: Fri, 19 Jul 2019 14:57:53 +0200	[thread overview]
Message-ID: <20190719125753.miniwfq4nhicy76n@pathway.suse.cz> (raw)
In-Reply-To: <20190718094934.GA10041@jagdpanzerIV>

On Thu 2019-07-18 18:49:34, Sergey Senozhatsky wrote:
> On (07/18/19 10:36), Petr Mladek wrote:
> > On Wed 2019-07-17 18:56:15, Sergey Senozhatsky wrote:
> > > On (07/16/19 09:28), Petr Mladek wrote:
> > > > Kernel tries hard to store and show printk messages when panicking. Even
> > > > logbuf_lock gets re-initialized when only one CPU is running after
> > > > smp_send_stop().
> > > > 
> > > > Unfortunately, smp_send_stop() might fail on architectures that do not
> > > > use NMI as a fallback. Then printk log buffer might stay locked and
> > > > a deadlock is almost inevitable.
> > > 
> > > I'd say that deadlock is still almost inevitable.
> > > 
> > > panic-CPU syncs with the printing-CPU before it attempts to SMP_STOP.
> > > If there is an active printing-CPU, which is looping in console_unlock(),
> > > taking logbuf_lock in order to msg_print_text() and stuff, then panic-CPU
> > > will spin on console_owner waiting for that printing-CPU to handover
> > > printing duties.
> > > 
> > > 	pr_emerg("Kernel panic - not syncing");
> > > 	smp_send_stop();
> > 
> > Good point. I forgot the handover logic. Well, it is enabled only
> > around call_console_drivers(). Therefore it is not under
> > lockbuf_lock.
> > 
> > I had in mind some infinite loop or deadlock in vprintk_store().
> > There was at least one long time ago (warning triggered
> > by leap second).
> > 
> > 
> > > If printing-CPU goes nuts under logbuf_lock, has corrupted IDT or anything
> > > else, then we will not progress with panic(). panic-CPU will deadlock. If
> > > not on
> > > 	pr_emerg("Kernel panic - not syncing")
> > > 
> > > then on another pr_emerg(), right before the NMI-fallback.
> > 
> > Nested printk() should not be problem thanks to printk_safe.
> 
> Where do nested printk()-s come from? Which one of the following
> scenarios you cover in commit message:
> 
> scenario 1
> 
> - we have CPUB which holds logbuf_lock
> - we have CPUA which panic()-s the system, but can't bring CPUB down,
>   so logbuf_lock stays locked on remote CPU

No, this scenario is not affected by this patch. It would always lead to
a deadlock.

> scenario 2
> 
> - we have CPUA which holds logbuf_lock
> - we have panic() on CPUA, but it cannot bring down some other CPUB
>   so logbuf_lock stays locked on local CPU, and it cannot re-init
>   logbuf.

This scenario should get better handled by this patch. The difference
will be when smp_send_stop() is not able to stop all CPUs:

  + Before:
      + printk_safe_flush_on_panic() will keep logbuf_lock locked
	and do nothing.

      + kmsg_dump(), console_unblank(), or console_flush_on_panic()
	will deadlock when they try to get logbuf_lock(). They will
	not be able to process any single line.

  + After:
      + printk_bust_lock_safe() will keep logbuf_lock locked

      + All functions using logbuf_lock will not get called.
	We will not see the messages (as previously) but the
	system will not deadlock.


But there is one more scenario 3:

  - we have CPUB which loops or is deadlocked in IRQ context

  - we have CPUA which panic()-s the system, but can't bring CPUB down,
    so logbuf_lock might be takes and release from time to time
    by CPUB

Hmm, this scenario might be handled a bit _worse_ by this patch:

  + Before:
      + printk_safe_flush_on_panic() will not touch logbuf_lock
	The messages will get flushed according to the state of
	logbuf_lock at the moment when it is being checked.

      + kmsg_dump(), console_unblank(), or console_flush_on_panic()
	will be able to do their job.

  + After:
      +  printk_safe_flush_on_panic(), kmsg_dump(), console_unblank(),
	 and console_flush_on_panic() could finish the job. But they
	 will get called _only_ when logbuf_lock is released at
	 the moment when it is being checked by printk_bust_lock_safe().


Resume:

From my POV, the 3rd scenario is the most likely one. Therefore this
patch would make more bad than good.

It might be possible to somehow detect if lockbuf_lock is released
from time the time on the non-stopped CPU. But it would be hairy.
IMHO, it is not worth it.

Thanks a lot for helping me to sort the ideas. I suggest to forget
this patch and work on lockless ringbuffer.

Best Regards,
Petr

  reply	other threads:[~2019-07-19 12:57 UTC|newest]

Thread overview: 17+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-07-16  7:28 [PATCH 0/2] panic/printk/x86: Prevent some more printk-related deadlocks in panic() Petr Mladek
2019-07-16  7:28 ` [PATCH 1/2] printk/panic: Access the main printk log in panic() only when safe Petr Mladek
2019-07-17  9:56   ` Sergey Senozhatsky
2019-07-18  8:36     ` Petr Mladek
2019-07-18  9:49       ` Sergey Senozhatsky
2019-07-19 12:57         ` Petr Mladek [this message]
2019-07-23  3:13           ` Sergey Senozhatsky
2019-07-24 12:27             ` Petr Mladek
2019-07-31  6:08               ` Sergey Senozhatsky
2019-07-31  6:59                 ` Sergey Senozhatsky
2019-07-18 10:11   ` Sergey Senozhatsky
2019-07-16  7:28 ` [PATCH 2/2] printk/panic/x86: Allow to access printk log buffer after crash_smp_send_stop() Petr Mladek
2019-07-18 10:47   ` Sergey Senozhatsky
2019-07-18 11:07     ` Konstantin Khlebnikov
2019-07-18 11:29       ` Sergey Senozhatsky
2019-07-19 12:19         ` Petr Mladek
2019-07-18  9:59 ` [PATCH 0/2] panic/printk/x86: Prevent some more printk-related deadlocks in panic() Konstantin Khlebnikov

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=20190719125753.miniwfq4nhicy76n@pathway.suse.cz \
    --to=pmladek@suse.com \
    --cc=akpm@linux-foundation.org \
    --cc=john.ogness@linutronix.de \
    --cc=koct9i@gmail.com \
    --cc=linux-kernel@vger.kernel.org \
    --cc=peterz@infradead.org \
    --cc=ptesarik@suse.cz \
    --cc=rostedt@goodmis.org \
    --cc=sergey.senozhatsky.work@gmail.com \
    --cc=sergey.senozhatsky@gmail.com \
    --cc=tglx@linutronix.de \
    --cc=x86@kernel.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;
as well as URLs for NNTP newsgroup(s).