Re: [PATCH printk v2 26/26] lockdep: Mark emergency section in lockdep splats

[Waiman Long <longman@redhat.com>]

On 2/18/24 13:57, John Ogness wrote:
> Mark an emergency section within print_usage_bug(), where
> lockdep bugs are printed. In this section, the CPU will not
> perform console output for the printk() calls. Instead, a
> flushing of the console output is triggered when exiting
> the emergency section.
>
> Signed-off-by: John Ogness <john.ogness@linutronix.de>
> ---
>   kernel/locking/lockdep.c | 5 +++++
>   1 file changed, 5 insertions(+)
>
> diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c
> index e85b5ad3e206..00465373d358 100644
> --- a/kernel/locking/lockdep.c
> +++ b/kernel/locking/lockdep.c
> @@ -56,6 +56,7 @@
>   #include <linux/kprobes.h>
>   #include <linux/lockdep.h>
>   #include <linux/context_tracking.h>
> +#include <linux/console.h>
>   
>   #include <asm/sections.h>
>   
> @@ -3970,6 +3971,8 @@ print_usage_bug(struct task_struct *curr, struct held_lock *this,
>   	if (!debug_locks_off() || debug_locks_silent)
>   		return;
>   
> +	nbcon_cpu_emergency_enter();
> +
>   	pr_warn("\n");
>   	pr_warn("================================\n");
>   	pr_warn("WARNING: inconsistent lock state\n");
> @@ -3998,6 +4001,8 @@ print_usage_bug(struct task_struct *curr, struct held_lock *this,
>   
>   	pr_warn("\nstack backtrace:\n");
>   	dump_stack();
> +
> +	nbcon_cpu_emergency_exit();
>   }
>   
>   /*

lockdep.c has multiple functions that print stuff to the console, like

  - print_circular_bug_header()
  - print_bad_irq_dependency()
  - print_deadlock_bug()
  - print_collision()
  - print_usage_bug()
  - print_irq_inversion_bug()
  - print_lock_invalid_wait_context()
  - print_lock_nested_lock_not_held()
  - print_unlock_imbalance_bug()
  - print_lock_contention_bug()
  - print_freed_lock_bug()
  - print_held_locks_bug()
  - lockdep_rcu_suspicious()

So what is special about print_usage_bug() that it needs this emergency 
treatment but not the other ones?

Cheers,
Longman