Re: [PATCH 1/2] printk/panic: Access the main printk log in panic() only when safe

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: Thu, 18 Jul 2019 10:36:29 +0200	[thread overview]
Message-ID: <20190718083629.nso3vwbvmankqgks@pathway.suse.cz> (raw)
In-Reply-To: <20190717095615.GD3664@jagdpanzerIV>

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.

Also printk_safe_flush_on_panic() is safe because it checks whether
the lock is available.

The problem is kmsg_dump() and console_unlock() called from
console_unblank() and console_flush_on_panic(). They do not
check whether the lock is available.

This patch does not help in all possible scenarios. But I still
believe that it will help in some.

Well, I am primary interested into the 2nd patch. It fixes
a real life bug report.

Best Regards,
Petr