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From: Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>
To: Petr Mladek <pmladek@suse.com>
Cc: Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>,
	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: Thu, 18 Jul 2019 18:49:34 +0900	[thread overview]
Message-ID: <20190718094934.GA10041@jagdpanzerIV> (raw)
In-Reply-To: <20190718083629.nso3vwbvmankqgks@pathway.suse.cz>

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

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.

	-ss

  reply	other threads:[~2019-07-18  9:49 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 [this message]
2019-07-19 12:57         ` Petr Mladek
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

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