linux-kernel - Re: replay log: Re: [PATCH printk v4 38/39] printk: relieve console

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Date:   Wed, 16 Nov 2022 10:48:54 +0100
From:   Petr Mladek <pmladek@...e.com>
To:     John Ogness <john.ogness@...utronix.de>
Cc:     Sergey Senozhatsky <senozhatsky@...omium.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-kernel@...r.kernel.org
Subject: Re: replay log: Re: [PATCH printk v4 38/39] printk: relieve
 console_lock of list synchronization duties

On Wed 2022-11-16 10:14:35, John Ogness wrote:
> Hi Petr,
> 
> Sorry, console_flush_all() only loses the console_lock if there was a
> handover. Here is a new complete suggestion from me.
> 
> 	if (newcon->flags & (CON_PRINTBUFFER | CON_BOOT)) {
> 		/* Get a consistent copy of @syslog_seq. */
> 		mutex_lock(&syslog_lock);
> 		newcon->seq = syslog_seq;
> 		mutex_unlock(&syslog_lock);
> 	} else {
> 		/* Begin with next message added to ringbuffer. */
> 		newcon->seq = prb_next_seq(prb);
> 
> 		/*
> 		 * If any enabled boot consoles are due to be unregistered
> 		 * shortly, some may not be caught up and may be the same
> 		 * device as @newcon. Since it is not known which boot console
> 		 * is the same device, flush all consoles and, if necessary,
> 		 * start with the message of the enabled boot console that is
> 		 * the furthest behind.
> 		 */
> 		if (bootcon_registered && !keep_bootcon) {
> 			bool handover;
> 
> 			/*
> 			 * Hold the console_lock to guarantee safe access to
> 			 * console->seq.
> 			 */
> 			console_lock();
> 
> 			/*
> 			 * Flush all consoles and set the console to start at
> 			 * the next unprinted sequence number.
> 			 */
> 			if (!console_flush_all(true, &newcon->seq, &handover)) {
> 				/*
> 				 * Flushing failed. Just choose the lowest
> 				 * sequence of the enabled boot consoles.
> 				 */
> 
> 				/*
> 				 * If there was a handover, this context no
> 				 * longer holds the console_lock.
> 				 */
> 				if (handover)
> 					console_lock();
> 
> 				newcon->seq = prb_next_seq(prb);
> 				for_each_console(con) {
> 					if ((con->flags & CON_BOOT) &&
> 					    (con->flags & CON_ENABLED) &&
> 					    con->seq < newcon->seq) {
> 						newcon->seq = con->seq;
> 					}
> 				}
> 			}
> 
> 			console_unlock();
> 		}

It looks good to me.

Now, we just need to agree how to add this into the patchset.
My proposal is to:

1. patch: hide the original code into a function, .e.g. console_init_seq()
2. patch: move console_init_seq() and add the above trickery

Do both before the 3rd patch in this patchset. It moved the code
outside console_lock() guarded section.

Best Regards,
Petr