Merge branch 'doc.2025.08.20a' into HEAD

RCU documentation updates:
* Update whatisRCU.rst for recent RCU API additions
* Add RCU guards to checklist.rst
* Requirements.rst: Abide by conventions of kernel documentation
* Fix formatting/typo issues in torture.rst and index.rst
* Fix dead URLs in RTFP.txt

Author: paulmckrcu

Documentation/RCU/Design/Requirements/Requirements.rst

@@ -1973,9 +1973,7 @@ code, and the FQS loop, all of which refer to or modify this bookkeeping.
 Note that grace period initialization (rcu_gp_init()) must carefully sequence
 CPU hotplug scanning with grace period state changes. For example, the
 following race could occur in rcu_gp_init() if rcu_seq_start() were to happen
-after the CPU hotplug scanning.
-
-.. code-block:: none
+after the CPU hotplug scanning::
 
    CPU0 (rcu_gp_init)                   CPU1                          CPU2
    ---------------------                ----                          ----
@@ -2008,22 +2006,22 @@ after the CPU hotplug scanning.
                                                                       kfree(r1);
                                         r2 = *r0; // USE-AFTER-FREE!
 
-By incrementing gp_seq first, CPU1's RCU read-side critical section
+By incrementing ``gp_seq`` first, CPU1's RCU read-side critical section
 is guaranteed to not be missed by CPU2.
 
-**Concurrent Quiescent State Reporting for Offline CPUs**
+Concurrent Quiescent State Reporting for Offline CPUs
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 RCU must ensure that CPUs going offline report quiescent states to avoid
 blocking grace periods. This requires careful synchronization to handle
 race conditions
 
-**Race condition causing Offline CPU to hang GP**
-
-A race between CPU offlining and new GP initialization (gp_init) may occur
-because `rcu_report_qs_rnp()` in `rcutree_report_cpu_dead()` must temporarily
-release the `rcu_node` lock to wake the RCU grace-period kthread:
+Race condition causing Offline CPU to hang GP
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
-.. code-block:: none
+A race between CPU offlining and new GP initialization (gp_init()) may occur
+because rcu_report_qs_rnp() in rcutree_report_cpu_dead() must temporarily
+release the ``rcu_node`` lock to wake the RCU grace-period kthread::
 
    CPU1 (going offline)                 CPU0 (GP kthread)
    --------------------                 -----------------
@@ -2044,15 +2042,14 @@ release the `rcu_node` lock to wake the RCU grace-period kthread:
        // Reacquire lock (but too late)
      rnp->qsmaskinitnext &= ~mask       // Finally clears bit
 
-Without `ofl_lock`, the new grace period includes the offline CPU and waits
+Without ``ofl_lock``, the new grace period includes the offline CPU and waits
 forever for its quiescent state causing a GP hang.
 
-**A solution with ofl_lock**
+A solution with ofl_lock
+^^^^^^^^^^^^^^^^^^^^^^^^
 
-The `ofl_lock` (offline lock) prevents `rcu_gp_init()` from running during
-the vulnerable window when `rcu_report_qs_rnp()` has released `rnp->lock`:
-
-.. code-block:: none
+The ``ofl_lock`` (offline lock) prevents rcu_gp_init() from running during
+the vulnerable window when rcu_report_qs_rnp() has released ``rnp->lock``::
 
    CPU0 (rcu_gp_init)                   CPU1 (rcutree_report_cpu_dead)
    ------------------                   ------------------------------
@@ -2065,21 +2062,20 @@ the vulnerable window when `rcu_report_qs_rnp()` has released `rnp->lock`:
        arch_spin_unlock(&ofl_lock) ---> // Now CPU1 can proceed
    }                                    // But snapshot already taken
 
-**Another race causing GP hangs in rcu_gpu_init(): Reporting QS for Now-offline CPUs**
+Another race causing GP hangs in rcu_gpu_init(): Reporting QS for Now-offline CPUs
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 After the first loop takes an atomic snapshot of online CPUs, as shown above,
-the second loop in `rcu_gp_init()` detects CPUs that went offline between
-releasing `ofl_lock` and acquiring the per-node `rnp->lock`. This detection is
-crucial because:
+the second loop in rcu_gp_init() detects CPUs that went offline between
+releasing ``ofl_lock`` and acquiring the per-node ``rnp->lock``.
+This detection is crucial because:
 
 1. The CPU might have gone offline after the snapshot but before the second loop
 2. The offline CPU cannot report its own QS if it's already dead
 3. Without this detection, the grace period would wait forever for CPUs that
    are now offline.
 
-The second loop performs this detection safely:
-
-.. code-block:: none
+The second loop performs this detection safely::
 
    rcu_for_each_node_breadth_first(rnp) {
        raw_spin_lock_irqsave_rcu_node(rnp, flags);
@@ -2093,10 +2089,10 @@ The second loop performs this detection safely:
    }
 
 This approach ensures atomicity: quiescent state reporting for offline CPUs
-happens either in `rcu_gp_init()` (second loop) or in `rcutree_report_cpu_dead()`,
-never both and never neither. The `rnp->lock` held throughout the sequence
-prevents races - `rcutree_report_cpu_dead()` also acquires this lock when
-clearing `qsmaskinitnext`, ensuring mutual exclusion.
+happens either in rcu_gp_init() (second loop) or in rcutree_report_cpu_dead(),
+never both and never neither. The ``rnp->lock`` held throughout the sequence
+prevents races - rcutree_report_cpu_dead() also acquires this lock when
+clearing ``qsmaskinitnext``, ensuring mutual exclusion.
 
 Scheduler and RCU
 ~~~~~~~~~~~~~~~~~

Documentation/RCU/RTFP.txt

@@ -641,7 +641,7 @@ Orran Krieger and Rusty Russell and Dipankar Sarma and Maneesh Soni"
 ,Month="July"
 ,Year="2001"
 ,note="Available:
-\url{http://www.linuxsymposium.org/2001/abstracts/readcopy.php}
+\url{https://kernel.org/doc/ols/2001/read-copy.pdf}
 \url{http://www.rdrop.com/users/paulmck/RCU/rclock_OLS.2001.05.01c.pdf}
 [Viewed June 23, 2004]"
 ,annotation={
@@ -1480,7 +1480,7 @@ Suparna Bhattacharya"
 ,Year="2006"
 ,pages="v2 123-138"
 ,note="Available:
-\url{http://www.linuxsymposium.org/2006/view_abstract.php?content_key=184}
+\url{https://kernel.org/doc/ols/2006/ols2006v2-pages-131-146.pdf}
 \url{http://www.rdrop.com/users/paulmck/RCU/OLSrtRCU.2006.08.11a.pdf}
 [Viewed January 1, 2007]"
 ,annotation={
@@ -1511,7 +1511,7 @@ Canis Rufus and Zoicon5 and Anome and Hal Eisen"
 ,Year="2006"
 ,pages="v2 249-254"
 ,note="Available:
-\url{http://www.linuxsymposium.org/2006/view_abstract.php?content_key=184}
+\url{https://kernel.org/doc/ols/2006/ols2006v2-pages-249-262.pdf}
 [Viewed January 11, 2009]"
 ,annotation={
 	Uses RCU-protected radix tree for a lockless page cache.

Documentation/RCU/checklist.rst

@@ -69,7 +69,13 @@ over a rather long period of time, but improvements are always welcome!
 	Explicit disabling of preemption (preempt_disable(), for example)
 	can serve as rcu_read_lock_sched(), but is less readable and
 	prevents lockdep from detecting locking issues.  Acquiring a
-	spinlock also enters an RCU read-side critical section.
+	raw spinlock also enters an RCU read-side critical section.
+
+	The guard(rcu)() and scoped_guard(rcu) primitives designate
+	the remainder of the current scope or the next statement,
+	respectively, as the RCU read-side critical section.  Use of
+	these guards can be less error-prone than rcu_read_lock(),
+	rcu_read_unlock(), and friends.
 
 	Please note that you *cannot* rely on code known to be built
 	only in non-preemptible kernels.  Such code can and will break,
@@ -405,9 +411,11 @@ over a rather long period of time, but improvements are always welcome!
 13.	Unlike most flavors of RCU, it *is* permissible to block in an
 	SRCU read-side critical section (demarked by srcu_read_lock()
 	and srcu_read_unlock()), hence the "SRCU": "sleepable RCU".
-	Please note that if you don't need to sleep in read-side critical
-	sections, you should be using RCU rather than SRCU, because RCU
-	is almost always faster and easier to use than is SRCU.
+	As with RCU, guard(srcu)() and scoped_guard(srcu) forms are
+	available, and often provide greater ease of use.  Please note
+	that if you don't need to sleep in read-side critical sections,
+	you should be using RCU rather than SRCU, because RCU is almost
+	always faster and easier to use than is SRCU.
 
 	Also unlike other forms of RCU, explicit initialization and
 	cleanup is required either at build time via DEFINE_SRCU()
@@ -443,10 +451,13 @@ over a rather long period of time, but improvements are always welcome!
 	real-time workloads than is synchronize_rcu_expedited().
 
 	It is also permissible to sleep in RCU Tasks Trace read-side
-	critical section, which are delimited by rcu_read_lock_trace() and
-	rcu_read_unlock_trace().  However, this is a specialized flavor
-	of RCU, and you should not use it without first checking with
-	its current users.  In most cases, you should instead use SRCU.
+	critical section, which are delimited by rcu_read_lock_trace()
+	and rcu_read_unlock_trace().  However, this is a specialized
+	flavor of RCU, and you should not use it without first checking
+	with its current users.  In most cases, you should instead
+	use SRCU.  As with RCU and SRCU, guard(rcu_tasks_trace)() and
+	scoped_guard(rcu_tasks_trace) are available, and often provide
+	greater ease of use.
 
 	Note that rcu_assign_pointer() relates to SRCU just as it does to
 	other forms of RCU, but instead of rcu_dereference() you should

Documentation/RCU/index.rst

@@ -1,13 +1,13 @@
 .. SPDX-License-Identifier: GPL-2.0
 
-.. _rcu_concepts:
+.. _rcu_handbook:
 
 ============
-RCU concepts
+RCU Handbook
 ============
 
 .. toctree::
-   :maxdepth: 3
+   :maxdepth: 2
 
    checklist
    lockdep

Documentation/RCU/torture.rst

@@ -344,7 +344,7 @@ painstaking and error-prone.
 
 And this is why the kvm-remote.sh script exists.
 
-If you the following command works::
+If the following command works::
 
 	ssh system0 date
 
@@ -364,7 +364,7 @@ systems must come first.
 The kvm.sh ``--dryrun scenarios`` argument is useful for working out
 how many scenarios may be run in one batch across a group of systems.
 
-You can also re-run a previous remote run in a manner similar to kvm.sh:
+You can also re-run a previous remote run in a manner similar to kvm.sh::
 
 	kvm-remote.sh "system0 system1 system2 system3 system4 system5" \
 		tools/testing/selftests/rcutorture/res/2022.11.03-11.26.28-remote \