Re: [PATCH] printk_legacy_map: use LD_WAIT_CONFIG instead of LD_WAIT_SLEEP

[Petr Mladek <pmladek@suse.com>]

On Wed 2025-10-22 17:41:15, Oleg Nesterov wrote:
> printk_legacy_map is used on !PREEMPT_RT to avoid false positives from
> CONFIG_PROVE_RAW_LOCK_NESTING about raw_spinlock/spinlock nesting.
> 
> However, LD_WAIT_SLEEP is not exactly right; it fools lockdep as if it
> is fine to acquire a sleeping lock.
> 
> Change DEFINE_WAIT_OVERRIDE_MAP(printk_legacy_map) to use LD_WAIT_CONFIG.
> 
> (We can also make printk_legacy_allow_spinlock_enter/exit() depend on
>  !PREEMPT_RT && CONFIG_PROVE_RAW_LOCK_NESTING)

I do not have strong opinion about adding (&& CONFIG_PROVE_RAW_LOCK_NESTING).
This dependency is already handled in LD_WAIT_CONFIG definition.

> Signed-off-by: Oleg Nesterov <oleg@redhat.com>

Anyway, the change makes sense to me. It seems that this better fits
the purpose.

Reviewed-by: Petr Mladek <pmladek@suse.com>

See a note below.

> ---
>  kernel/printk/printk.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
> index 5aee9ffb16b9..f11b2f31999b 100644
> --- a/kernel/printk/printk.c
> +++ b/kernel/printk/printk.c
> @@ -3007,7 +3007,7 @@ bool printk_get_next_message(struct printk_message *pmsg, u64 seq,
>   * false positive. For PREEMPT_RT the false positive condition does not
>   * occur.

From the comment, it was not obvious to me why the condition does not
occur for PREEMPT_RT. I had to check the commit message from the
commit daeed1595b4ddf314b ("printk: Avoid false positive lockdep
report for legacy printing").

<paste>
    However, on PREEMPT_RT the printing path from atomic context is
    always avoided and the console driver is always invoked from a
    dedicated thread. Thus the lockdep splat on !PREEMPT_RT is a
    false positive.
 </paste>

This is much more clear. It might make sense to improve the comment,
for example:

<proposal>
/*
 * Legacy console printing from printk() caller context does not respect
 * raw_spinlock/spinlock nesting. However, on PREEMPT_RT the printing
 * path from atomic context is always avoided and the console driver
 * is always invoked from a dedicated thread. Thus the lockdep splat
 * on !PREEMPT_RT is a false positive.
 *
 * This map is used to temporarily establish LD_WAIT_CONFIG context for the
 * console write() callback when legacy printing to avoid false positive
 * lockdep complaints, thus allowing lockdep to continue to function for
 * real issues.
 */
</proposal>

But it can be done in a separate patch...

>   *
> - * This map is used to temporarily establish LD_WAIT_SLEEP context for the
> + * This map is used to temporarily establish LD_WAIT_CONFIG context for the
>   * console write() callback when legacy printing to avoid false positive
>   * lockdep complaints, thus allowing lockdep to continue to function for
>   * real issues.
> @@ -3016,7 +3016,7 @@ bool printk_get_next_message(struct printk_message *pmsg, u64 seq,
>  static inline void printk_legacy_allow_spinlock_enter(void) { }
>  static inline void printk_legacy_allow_spinlock_exit(void) { }
>  #else
> -static DEFINE_WAIT_OVERRIDE_MAP(printk_legacy_map, LD_WAIT_SLEEP);
> +static DEFINE_WAIT_OVERRIDE_MAP(printk_legacy_map, LD_WAIT_CONFIG);
>  
>  static inline void printk_legacy_allow_spinlock_enter(void)
>  {

Best Regards,
Petr

PS: I would take this patch via the printk tree. But I am going to wait
    for feedback from others (John, Sebastian, ...).