linux-kernel - Re: [RFC PATCH v1 19/25] printk: introduce emergency messages

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Date:   Fri, 8 Mar 2019 11:31:27 +0100
From:   Petr Mladek <pmladek@...e.com>
To:     Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
Cc:     John Ogness <john.ogness@...utronix.de>,
        linux-kernel@...r.kernel.org,
        Peter Zijlstra <peterz@...radead.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Daniel Wang <wonderfly@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Alan Cox <gnomes@...rguk.ukuu.org.uk>,
        Jiri Slaby <jslaby@...e.com>,
        Peter Feiner <pfeiner@...gle.com>,
        linux-serial@...r.kernel.org,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>
Subject: Re: [RFC PATCH v1 19/25] printk: introduce emergency messages

On Thu 2019-03-07 16:30:29, Sergey Senozhatsky wrote:
> On (02/12/19 15:29), John Ogness wrote:
> [..]
> > +static bool console_can_emergency(int level)
> > +{
> > +	struct console *con;
> > +
> > +	for_each_console(con) {
> > +		if (!(con->flags & CON_ENABLED))
> > +			continue;
> > +		if (con->write_atomic && level < emergency_console_loglevel)
> > +			return true;
> > +		if (con->write && (con->flags & CON_BOOT))
> > +			return true;
> > +	}
> > +	return false;
> > +}
> > +
> > +static void call_emergency_console_drivers(int level, const char *text,
> > +					   size_t text_len)
> > +{
> > +	struct console *con;
> > +
> > +	for_each_console(con) {
> > +		if (!(con->flags & CON_ENABLED))
> > +			continue;
> > +		if (con->write_atomic && level < emergency_console_loglevel) {
> > +			con->write_atomic(con, text, text_len);
> > +			continue;
> > +		}
> > +		if (con->write && (con->flags & CON_BOOT)) {
> > +			con->write(con, text, text_len);
> > +			continue;
> > +		}
> > +	}
> > +}
> > +
> > +static void printk_emergency(char *buffer, int level, u64 ts_nsec, u16 cpu,
> > +			     char *text, u16 text_len)
> > +{
> > +	struct printk_log msg;
> > +	size_t prefix_len;
> > +
> > +	if (!console_can_emergency(level))
> > +		return;
> > +
> > +	msg.level = level;
> > +	msg.ts_nsec = ts_nsec;
> > +	msg.cpu = cpu;
> > +	msg.facility = 0;
> > +
> > +	/* "text" must have PREFIX_MAX preceding bytes available */
> > +
> > +	prefix_len = print_prefix(&msg,
> > +				  console_msg_format & MSG_FORMAT_SYSLOG,
> > +				  printk_time, buffer);
> > +	/* move the prefix forward to the beginning of the message text */
> > +	text -= prefix_len;
> > +	memmove(text, buffer, prefix_len);
> > +	text_len += prefix_len;
> > +
> > +	text[text_len++] = '\n';
> > +
> > +	call_emergency_console_drivers(level, text, text_len);
> 
> So this iterates the console list and calls consoles' callbacks, but what
> prevents console driver to be rmmod-ed under us?
> 
> 	CPU0						CPU1
> 
> 	printk_emergency()				rmmod netcon
> 	 call_emergency_console_drivers()		
> 	  con_foo->flags & CON_ENABLED == 1
> 							unregister_console(con_foo)
> 							con_foo->flags &= ~CON_ENABLED
> 							__exit // con_foo gone ?
> 	  con_foo->write()
> 
> We use console_lock()/console_trylock() in order to protect the list and
> console drivers; but this brings scheduler to the picture, with all its
> locks.

Great catch!

I think that it is doable to guard the list using RCU.

Best Regards,
Petr