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From: Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>
To: Petr Mladek <pmladek@suse.com>
Cc: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>,
	Steven Rostedt <rostedt@goodmis.org>,
	Peter Zijlstra <peterz@infradead.org>,
	Andrew Morton <akpm@linux-foundation.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	John Ogness <john.ogness@linutronix.de>,
	Petr Tesarik <ptesarik@suse.cz>,
	Konstantin Khlebnikov <koct9i@gmail.com>,
	x86@kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH 1/2] printk/panic: Access the main printk log in panic() only when safe
Date: Wed, 17 Jul 2019 18:56:15 +0900	[thread overview]
Message-ID: <20190717095615.GD3664@jagdpanzerIV> (raw)
In-Reply-To: <20190716072805.22445-2-pmladek@suse.com>

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();


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.

	static void native_stop_other_cpus()
	{
	...
		pr_emerg("Shutting down cpus with NMI\n");
		           ^^ deadlock here
		apic->send_IPI_allbutself(NMI_VECTOR);
		           ^^ not going to happen
	...
	}

And it's not only x86. In many cases if we fail to SMP_STOP other
CPUs, and one of hem is holding logbuf_lock then we are done with
panic(). We will not return from smp_send_stop().

arm/kernel/smp.c

void smp_send_stop(void)
{
	...
	if (num_online_cpus() > 1)
		pr_warn("SMP: failed to stop secondary CPUs\n");
}

arm64/kernel/smp.c

void crash_smp_send_stop(void)
{
	...
	pr_crit("SMP: stopping secondary CPUs\n");
	smp_cross_call(&mask, IPI_CPU_CRASH_STOP);

	...
	if (atomic_read(&waiting_for_crash_ipi) > 0)
		pr_warning("SMP: failed to stop secondary CPUs %*pbl\n",
				cpumask_pr_args(&mask));
	...
}

arm64/kernel/smp.c

void smp_send_stop(void)
{
	...
	if (num_online_cpus() > 1)
		pr_warning("SMP: failed to stop secondary CPUs %*pbl\n",
				cpumask_pr_args(cpu_online_mask));
	...
}


riscv/kernel/smp.c

void smp_send_stop(void)
{
	...
	if (num_online_cpus() > 1)
		pr_warn("SMP: failed to stop secondary CPUs %*pbl\n",
			cpumask_pr_args(cpu_online_mask));
	...
}

And so on.

	-ss

  reply	other threads:[~2019-07-17  9:56 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 [this message]
2019-07-18  8:36     ` Petr Mladek
2019-07-18  9:49       ` Sergey Senozhatsky
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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