Re: [PATCH 0/2] printk: Prevent printk kthreads from blocking direct console handling

[Paul E. McKenney]

On Wed, Jun 15, 2022 at 06:28:03PM +0200, Petr Mladek wrote:
> There are reports that console kthreads prevented printing
> messages during panic() or shutdown(), see
> BugLink: https://lore.kernel.org/r/20220610205038.GA3050413@paulmck-ThinkPad-P17-Gen-1
> BugLink: https://lore.kernel.org/r/CAMdYzYpF4FNTBPZsEFeWRuEwSies36QM_As8osPWZSr2q-viEA@xxxxxxxxxxxxxx
> 
> In both situations, printk() correctly tries to flush the consoles
> directly but it fails to get the global console_lock(). Both
> problems went away with these patches:
> 
> 1st patch blocks console kthreads so that they do not start
> handling new messages when the direct printing is preferred
> by the system state. It is probably enough on its own. It
> solves the problem when the kthreads actively did
> the wrong thing.
> 
> 2nd patch allows to wait for the console kthreads to release
> the lock in any context. It should make it more reliable.
> It would have been useful even for the legacy code.

Thank you!

For the series:

Tested-by: Paul E. McKenney <paulmck@xxxxxxxxxx>

> More possible improvements:
> 
>   +  the waiting might be necessary also in the suspend code paths
> 
>   + convert con->mutex to con->spinlock to avoid blocking
>     the global console_lock() when sleeping with con->lock
> 
>   + at least disable preemption around console_emit_next_record()
>     in console kthread to avoid sleeping in the console driver
>     code
> 
>   + somehow change the priority of the kthread so that it gets
>     scheduled immediately when the scheduler works
> 
> Petr Mladek (2):
>   printk: Block console kthreads when direct printing will be required
>   printk: Wait for the global console lock when the system is going down
> 
>  include/linux/printk.h      |  5 +++++
>  kernel/panic.c              |  2 ++
>  kernel/printk/internal.h    |  2 ++
>  kernel/printk/printk.c      |  8 +++++++-
>  kernel/printk/printk_safe.c | 32 ++++++++++++++++++++++++++++++++
>  kernel/reboot.c             |  2 ++
>  6 files changed, 50 insertions(+), 1 deletion(-)
> 
> -- 
> 2.35.3
>