linux-kernel - Re: NMI watchdog dump does not print on hard lockup

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Date:   Mon, 16 Oct 2017 10:15:47 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Sergey Senozhatsky <sergey.senozhatsky@...il.com>
Cc:     Petr Mladek <pmladek@...e.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>
Subject: Re: NMI watchdog dump does not print on hard lockup

On Mon, 16 Oct 2017 22:13:05 +0900
Sergey Senozhatsky <sergey.senozhatsky@...il.com> wrote:

> just "brainstorming" it... with some silly ideas.
> 
> pushing the data from NMI panic might look like we are replacing one
> deadlock scenario with another deadlock scenario. some of the console
> drivers are soooo complex internally. so I have been thinking about...
> may be we can extend struct console and add ->write_on_panic() and that
> handler must be as lockless as possible; so lockless that calling it
> from anything that is not panic() is a severe bug.

This may not be a bad idea. And make it so it can't be called unless we
are in panic mode (or at least "oops in progress").

If oops_in_progress is set, and the console has a "write_on_panic"
handler, then just call that.

Heck, if it doesn't have one, and early_printk is defined, then perhaps
that should be the default "write_on_panic" output?

-- Steve


> 
> an absolutely trivial case,
> if serial console does
> 
> 	console_write_cb(struct console *co, const char *s, unsigned int count)
> 	{
> 		spin_lock_irqsave(&port->lock, flags);
> 		uart_console_write(s, count, console_putchar);
> 		spin_unlock_irqrestore(&port->lock, flags);
> 	}
> 
> then panic callback can look like
> 
> 	console_write_on_panic_cb(struct console *co, const char *s, unsigned int count)
> 	{
> 		/* no, we don't take the port lock here */
> 		uart_console_write(s, count, console_putchar);
> 	}
> 
> a less trivial case might look more involved. but in general that
> write_on_panic() callback must do the absolute minimum of work. so
> it's sort of a early console, but as part of normal console driver.
> 
> I also got some other serial console crazy ideas, but they are not
> related to this topic.
> 
> 	-ss