linux-kernel - [PATCH 2/3] Documentation: locking: Add local_lock_nested

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Message-ID: <20250715144127.697787-3-bigeasy@linutronix.de>
Date: Tue, 15 Jul 2025 16:41:26 +0200
From: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To: linux-doc@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	linux-rt-devel@...ts.linux.dev
Cc: Boqun Feng <boqun.feng@...il.com>,
	Clark Williams <clrkwllms@...nel.org>,
	Frederic Weisbecker <frederic@...nel.org>,
	Ingo Molnar <mingo@...hat.com>,
	John Ogness <john.ogness@...utronix.de>,
	Jonathan Corbet <corbet@....net>,
	Peter Zijlstra <peterz@...radead.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Valentin Schneider <vschneid@...hat.com>,
	Waiman Long <longman@...hat.com>,
	Will Deacon <will@...nel.org>,
	Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Subject: [PATCH 2/3] Documentation: locking: Add local_lock_nested_bh() to locktypes

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@...utronix.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