linux-kernel - Re: [PATCH printk v6 11/40] printk: introduce console_list

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Message-ID: <Y3t+ztLCYj7uibD0@alley>
Date:   Mon, 21 Nov 2022 14:36:14 +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,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: Re: [PATCH printk v6 11/40] printk: introduce console_list_lock

On Mon 2022-11-21 12:16:12, John Ogness wrote:
> Currently there exist races in register_console(), where the types
> of registered consoles are checked (without holding the console_lock)
> and then after acquiring the console_lock, it is assumed that the list
> has not changed. Also, some code that performs console_unregister()
> make similar assumptions.
> 
> It might be possible to fix these races using the console_lock. But
> it would require a complex analysis of all console drivers to make
> sure that the console_lock is not taken in match() and setup()
> callbacks. And we really prefer to split up and reduce the
> responsibilities of console_lock rather than expand its complexity.
> Therefore, introduce a new console_list_lock to provide full
> synchronization for any console list changes.
> 
> In addition, also use console_list_lock for synchronization of
> console->flags updates. All flags are either static or modified only
> during the console registration. There are only two exceptions.
> 
> The first exception is CON_ENABLED, which is also modified by
> console_start()/console_stop(). Therefore, these functions must
> also take the console_list_lock.
> 
> The second exception is when the flags are modified by the console
> driver init code before the console is registered. These will be
> ignored because they are not visible to the rest of the system
> via the console_drivers list.
> 
> Note that one of the various responsibilities of the console_lock is
> also intended to provide console list and console->flags
> synchronization. Later changes will update call sites relying on the
> console_lock for these purposes. Once all call sites have been
> updated, the console_lock will be relieved of synchronizing
> console_list and console->flags updates.
> 
> Suggested-by: Thomas Gleixner <tglx@...utronix.de>
> Signed-off-by: John Ogness <john.ogness@...utronix.de>
> Reviewed-by: Petr Mladek <pmladek@...e.com>
> ---
>  Stephen reported [0] a build failure with linux-next. The problem was a
>  missing EXPORT of @lockdep_assert_console_list_lock_held for when
>  drivers are built as modules.
>  
>  [0] https://lore.kernel.org/lkml/20221121110041.1d2c635b@canb.auug.org.au

JFYI, the branch rework/console-list-lock in printk/linux.git has been
rebased using this version of the patch.

Thanks a lot for the fix.

Best Regards,
Petr