Re: [PATCH v2 3/3] printk/nmi: Prevent deadlock when accessing the main log buffer in NMI

From: Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>
To: Petr Mladek <pmladek@suse.com>, Steven Rostedt <rostedt@goodmis.org>
Cc: Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>,
	Sergey Senozhatsky <sergey.senozhatsky@gmail.com>,
	Peter Zijlstra <peterz@infradead.org>,
	Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>,
	linux-kernel@vger.kernel.org, stable@vger.kernel.org
Subject: Re: [PATCH v2 3/3] printk/nmi: Prevent deadlock when accessing the main log buffer in NMI
Date: Fri, 29 Jun 2018 10:47:03 +0900	[thread overview]
Message-ID: <20180629014703.GA614@jagdpanzerIV> (raw)
In-Reply-To: <20180628094125.cq5qunckjyxhbovb@pathway.suse.cz>

On (06/28/18 11:41), Petr Mladek wrote:
> > 
> > A side note: This nesting also handles recursive printk-s for us.
> > 
> > NMI:
> > 	printk_nmi_enter
> > 	ftrace_dump
> > 	 printk_nmi_direct_enter
> > 	  vprintk_func
> > 	   spin_lock(logbuf_lock)
> > 	    vprintk_store
> > 	     vsprintf
> > 	      WARN_ON
> > 	       vprintk_func
> > 	        vprintk_nmi
> 
> Uff, it seems that the current design is "good" at lest from some
> points of view.

yep yep

> > > +		len = vprintk_store(0, LOGLEVEL_DEFAULT, NULL, 0, fmt, args);
> > > +		raw_spin_unlock(&logbuf_lock);
> > > +		defer_console();
> > > +		return len;
> > > +	}
> > 
> > So, maybe, something a bit better than defer_console().
> 
> I am not super happy with the name either. But wakeup_console(),
> schedule_console(), or queue_console() looked confusing.

Hmm. defer_console() makes me think that we are dealing with that
fbcon=nodefer and deferred console takeover thing here.

So I summon Mr. Rostedt!

Does schedule_console_output() look bad?
What about defer_console_output()?
Any other ideas?

	-ss