Merge tag 'docs-6.18' of git://git.lwn.net/linux

Pull documentation updates from Jonathan Corbet:
 "It has been a relatively busy cycle in docsland, with changes all
  over:

   - Bring the kernel memory-model docs into the Sphinx build in the
     "literal include" mode.

   - Lots of build-infrastructure work, further cleaning up long-term
     kernel-doc technical debt. The sphinx-pre-install tool has been
     converted to Python and updated for current systems.

   - A new tool to detect when documents have been moved and generate
     HTML redirects; this can be used on kernel.org (or any other site
     hosting the rendered docs) to avoid breaking links.

   - Automated processing of the YAML files describing the netlink
     protocol.

   - A significant update of the maintainer's PGP guide.

  ... and a seemingly endless series of typo fixes, build-problem fixes,
  etc"

* tag 'docs-6.18' of git://git.lwn.net/linux: (193 commits)
  Documentation/features: Update feature lists for 6.17-rc7
  docs: remove cdomain.py
  Documentation/process: submitting-patches: fix typo in "were do"
  docs: dev-tools/lkmm: Fix typo of missing file extension
  Documentation: trace: histogram: Convert ftrace docs cross-reference
  Documentation: trace: histogram-design: Wrap introductory note in note:: directive
  Documentation: trace: historgram-design: Separate sched_waking histogram section heading and the following diagram
  Documentation: trace: histogram-design: Trim trailing vertices in diagram explanation text
  Documentation: trace: histogram: Fix histogram trigger subsection number order
  docs: driver-api: fix spelling of "buses".
  Documentation: fbcon: Use admonition directives
  Documentation: fbcon: Reindent 8th step of attach/detach/unload
  Documentation: fbcon: Add boot options and attach/detach/unload section headings
  docs: filesystems: sysfs: add remaining top level sysfs directory descriptions
  docs: filesystems: sysfs: clarify symlink destinations in dev and bus/devices descriptions
  docs: filesystems: sysfs: remove top level sysfs net directory
  docs: maintainer: Fix ambiguous subheading formatting
  docs: kdoc: a few more dump_typedef() tweaks
  docs: kdoc: remove redundant comment stripping in dump_typedef()
  docs: kdoc: remove some dead code in dump_typedef()
  ...

1 files changed, 21 insertions, 0 deletions

diff --git a/Documentation/locking/locktypes.rst b/Documentation/locking/locktypes.rst
index 80c914f6eae7..37b6a5670c2f 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.
+
+When 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
 =============================