linux-kernel - Re: [PATCH printk 4/5] printk: remove logbuf_lock, add syslog

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Message-ID: <20200924084541.GA541@jagdpanzerIV.localdomain>
Date:   Thu, 24 Sep 2020 17:45:41 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky@...il.com>
To:     John Ogness <john.ogness@...utronix.de>
Cc:     Petr Mladek <pmladek@...e.com>,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH printk 4/5] printk: remove logbuf_lock, add syslog_lock

On (20/09/22 17:44), John Ogness wrote:
[..]
>  void printk_safe_flush_on_panic(void)
>  {
> -	/*
> -	 * Make sure that we could access the main ring buffer.
> -	 * Do not risk a double release when more CPUs are up.
> -	 */
> -	if (raw_spin_is_locked(&logbuf_lock)) {
> -		if (num_online_cpus() > 1)
> -			return;
> -
> -		debug_locks_off();
> -		raw_spin_lock_init(&logbuf_lock);

If we panic() with syslog_lock locked then we need to zap syslog_lock
and disable debug_locks for kmsg_dump() to succeed. Otherwise panic()
may deadlock:

	syslog_lock_irq()
	-> NMI
		panic()
		...
			printk_safe_flush_on_panic();
			kmsg_dump(KMSG_DUMP_PANIC);	// syslog_lock_irqsave(flags)
		...

Did I miss the point where we make sure that kmsg_dump() will not
deadlock?

	-ss