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

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Date:   Thu, 7 Mar 2019 16:30:29 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
To:     John Ogness <john.ogness@...utronix.de>
Cc:     linux-kernel@...r.kernel.org,
        Peter Zijlstra <peterz@...radead.org>,
        Petr Mladek <pmladek@...e.com>,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        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 (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.

Or am I missing something?

	-ss