local_lock_nested_bh() is used within networking where applicable.
Document why it is used and how it behaves.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
---
 Documentation/locking/locktypes.rst | 21 +++++++++++++++++++++
 1 file changed, 21 insertions(+)

diff --git a/Documentation/locking/locktypes.rst b/Documentation/locking/locktypes.rst
index 80c914f6eae7a..4851ec1232058 100644
--- a/Documentation/locking/locktypes.rst
+++ b/Documentation/locking/locktypes.rst
@@ -204,6 +204,27 @@ per-CPU data structures on a non PREEMPT_RT kernel.
 local_lock is not suitable to protect against preemption or interrupts on a
 PREEMPT_RT kernel due to the PREEMPT_RT specific spinlock_t semantics.
 
+CPU local scope and bottom-half
+-------------------------------
+
+Per-CPU variables that are accessed only in softirq context should not rely on
+the assumption that this context is implicitly protected due to being
+non-preemptible. In a PREEMPT_RT kernel, softirq context is preemptible, and
+synchronizing every bottom-half-disabled section via implicit context results
+in an implicit per-CPU "big kernel lock."
+
+A local_lock_t together with local_lock_nested_bh() and
+local_unlock_nested_bh() for locking operations help to identify the locking
+scope.
+
+With lockdep is enabled, these functions verify that data structure access
+occurs within softirq context.
+Unlike local_lock(), local_unlock_nested_bh() does not disable preemption and
+does not add overhead when used without lockdep.
+
+On a PREEMPT_RT kernel, local_lock_t behaves as a real lock and
+local_unlock_nested_bh() serializes access to the data structure, which allows
+removal of serialization via local_bh_disable().
 
 raw_spinlock_t and spinlock_t
 =============================
-- 
2.50.0

Sebastian Andrzej Siewior <bigeasy@linutronix.de> writes:

> local_lock_nested_bh() is used within networking where applicable.
> Document why it is used and how it behaves.
>
> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
> ---
>  Documentation/locking/locktypes.rst | 21 +++++++++++++++++++++
>  1 file changed, 21 insertions(+)
>
> diff --git a/Documentation/locking/locktypes.rst b/Documentation/locking/locktypes.rst
> index 80c914f6eae7a..4851ec1232058 100644
> --- a/Documentation/locking/locktypes.rst
> +++ b/Documentation/locking/locktypes.rst
> @@ -204,6 +204,27 @@ per-CPU data structures on a non PREEMPT_RT kernel.
>  local_lock is not suitable to protect against preemption or interrupts on a
>  PREEMPT_RT kernel due to the PREEMPT_RT specific spinlock_t semantics.
>  
> +CPU local scope and bottom-half
> +-------------------------------
> +
> +Per-CPU variables that are accessed only in softirq context should not rely on
> +the assumption that this context is implicitly protected due to being
> +non-preemptible. In a PREEMPT_RT kernel, softirq context is preemptible, and
> +synchronizing every bottom-half-disabled section via implicit context results
> +in an implicit per-CPU "big kernel lock."
> +
> +A local_lock_t together with local_lock_nested_bh() and
> +local_unlock_nested_bh() for locking operations help to identify the locking
> +scope.
> +
> +With lockdep is enabled, these functions verify that data structure access

Did you mean *When* lockdep is enabled?

Otherwise no complaints here.

jon

On 2025-07-15 14:10:23 [-0600], Jonathan Corbet wrote:
…
> > +A local_lock_t together with local_lock_nested_bh() and
> > +local_unlock_nested_bh() for locking operations help to identify the locking
> > +scope.
> > +
> > +With lockdep is enabled, these functions verify that data structure access
> 
> Did you mean *When* lockdep is enabled?

Lets do "When". I most likely should have removed the "is".

> Otherwise no complaints here.
Thanks.

> 
> jon

Sebastian