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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: <Y3PBNJRvE6tU8ct+@alley>
References: <20221114162932.141883-1-john.ogness@linutronix.de>
 <20221114162932.141883-39-john.ogness@linutronix.de>
 <Y3Oxck0/LAHFLYip@alley> <Y3PBNJRvE6tU8ct+@alley>
Date:   Tue, 15 Nov 2022 18:21:34 +0106
Message-ID: <87r0y4m955.fsf@jogness.linutronix.de>
MIME-Version: 1.0
Content-Type: text/plain
Precedence: bulk

On 2022-11-15, Petr Mladek <pmladek@suse.com> wrote:
>>> --- a/kernel/printk/printk.c
>>> +++ b/kernel/printk/printk.c
>>> @@ -3334,6 +3330,11 @@ void register_console(struct console *newcon)
>>>  		 * boot console that is the furthest behind.
>>>  		 */
>>>  		if (bootcon_registered && !keep_bootcon) {
>>> +			/*
>>> +			 * Hold the console_lock to guarantee safe access to
>>> +			 * console->seq.
>>> +			 */
>>> +			console_lock();
>>>  			for_each_console(con) {
>>>  				if ((con->flags & CON_BOOT) &&
>>>  				    (con->flags & CON_ENABLED) &&
>>> @@ -3341,6 +3342,7 @@ void register_console(struct console *newcon)
>>>  					newcon->seq = con->seq;
>>>  				}
>>>  			}
>>> +			console_unlock();
>
> So, without the above two hunks:
>
> Reviewed-by: Petr Mladek <pmladek@suse.com>

Note that we actually need those hunks to guarantee a consistent @seq
value. The console_lock is the only synchronization mechanism available
to read console->seq.

John