Re: [PATCH v2] printk/console: Check consistent sequence number when handling race in console_unlock()

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, Petr Mladek <pmladek@suse.com>,
	kernel test robot <lkp@intel.com>,
	stable@vger.kernel.org
Subject: Re: [PATCH v2] printk/console: Check consistent sequence number when handling race in console_unlock()
Date: Sat, 03 Jul 2021 08:32:02 +0206	[thread overview]
Message-ID: <87y2an7w1x.fsf@jogness.linutronix.de> (raw)
In-Reply-To: <20210702150657.26760-1-pmladek@suse.com>

On 2021-07-02, Petr Mladek <pmladek@suse.com> wrote:
> The standard printk() tries to flush the message to the console
> immediately. It tries to take the console lock. If the lock is
> already taken then the current owner is responsible for flushing
> even the new message.
>
> There is a small race window between checking whether a new message is
> available and releasing the console lock. It is solved by re-checking
> the state after releasing the console lock. If the check is positive
> then console_unlock() tries to take the lock again and process the new
> message as well.
>
> The commit 996e966640ddea7b535c ("printk: remove logbuf_lock") causes that
> console_seq is not longer read atomically. As a result, the re-check might
> be done with an inconsistent 64-bit index.
>
> Solve it by using the last sequence number that has been checked under
> the console lock. In the worst case, it will take the lock again only
> to realized that the new message has already been proceed. But it
> was possible even before.
>
> The variable next_seq is marked as __maybe_unused to call down compiler
> warning when CONFIG_PRINTK is not defined.

As Sergey already pointed out, this patch is not fixing a real
problem. An inconsistent value (or an increased consistent value) would
mean that another printer is actively printing, and thus a retry is not
necessary anyway. But this patch will avoid a KASAN message about an
unmarked (although safe) data race.

> Fixes: commit 996e966640ddea7b535c ("printk: remove logbuf_lock")
> Reported-by: kernel test robot <lkp@intel.com>  # unused next_seq warning
> Cc: stable@vger.kernel.org # 5.13
> Signed-off-by: Petr Mladek <pmladek@suse.com>
> Acked-by: Sergey Senozhatsky <senozhatsky@chromium.org>

Reviewed-by: John Ogness <john.ogness@linutronix.de>