Re: [PATCH printk v5 00/40] reduce console_lock scope

Greg Kroah-Hartman
2022-11-22T16:43:35Z

On Wed, Nov 16, 2022 at 05:27:12PM +0106, John Ogness wrote:
> This is v5 of a series to prepare for threaded/atomic
> printing. v4 is here [0]. This series focuses on reducing the
> scope of the BKL console_lock. It achieves this by switching to
> SRCU and a dedicated mutex for console list iteration and
> modification, respectively. The console_lock will no longer
> offer this protection.
>
> Also, during the review of v2 it came to our attention that
> many console drivers are checking CON_ENABLED to see if they
> are registered. Because this flag can change without
> unregistering and because this flag does not represent an
> atomic point when an (un)registration process is complete,
> a new console_is_registered() function is introduced. This
> function uses the console_list_lock to synchronize with the
> (un)registration process to provide a reliable status.
>
> All users of the console_lock for list iteration have been
> modified. For the call sites where the console_lock is still
> needed (for other reasons), comments are added to explain
> exactly why the console_lock is needed.
>
> All users of CON_ENABLED for registration status have been
> modified to use console_is_registered(). Note that there are
> still users of CON_ENABLED, but this is for legitimate purposes
> about a registered console being able to print.
>
> The base commit for this series is from Paul McKenney's RCU tree
> and provides an NMI-safe SRCU implementation [1]. Without the
> NMI-safe SRCU implementation, this series is not less safe than
> mainline. But we will need the NMI-safe SRCU implementation for
> atomic consoles anyway, so we might as well get it in
> now. Especially since it _does_ increase the reliability for
> mainline in the panic path.
>
> Changes since v4:
>
> printk:
>
> - Introduce console_init_seq() to handle the now rather complex
> procedure to find an appropriate start sequence number for a
> new console upon registration.
>
> - When registering a non-boot console and boot consoles are
> registered, try to flush all the consoles to get the next @seq
> value before falling back to use the @seq of the enabled boot
> console that is furthest behind.
>
> - For console_force_preferred_locked(), make the console the
> head of the console list.
>

Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>