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From:   John Ogness <john.ogness@linutronix.de>
To:     Petr Mladek <pmladek@suse.com>
Cc:     Sergey Senozhatsky <senozhatsky@chromium.org>,
        Steven Rostedt <rostedt@goodmis.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        linux-kernel@vger.kernel.org
Subject: Re: replay log: Re: [PATCH printk v4 38/39] printk: relieve
 console_lock of list synchronization duties
In-Reply-To: <Y3PZEbx+40ZyN/79@alley>
References: <20221114162932.141883-1-john.ogness@linutronix.de>
 <20221114162932.141883-39-john.ogness@linutronix.de>
 <Y3Oxck0/LAHFLYip@alley> <87tu30maqf.fsf@jogness.linutronix.de>
 <Y3PZEbx+40ZyN/79@alley>
Date:   Wed, 16 Nov 2022 09:58:28 +0106
Message-ID: <87y1sbjn77.fsf@jogness.linutronix.de>
MIME-Version: 1.0
Content-Type: text/plain
Precedence: bulk

On 2022-11-15, Petr Mladek <pmladek@suse.com> wrote:
> It is not that easy because console_flush_all() might handover the
> console_lock(). Also some new messages might appear so that we
> should re-read prb_next_seq().

OK. Taking all this into account, how about:

	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.
				 */
				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();
		}
	}

John Ogness