Merge branch 'core-rcu-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip

Pull RCU updates from Ingo Molnar:
 "The main changes in this cycle were:

   - Make kfree_rcu() use kfree_bulk() for added performance

   - RCU updates

   - Callback-overload handling updates

   - Tasks-RCU KCSAN and sparse updates

   - Locking torture test and RCU torture test updates

   - Documentation updates

   - Miscellaneous fixes"

* 'core-rcu-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip: (74 commits)
  rcu: Make rcu_barrier() account for offline no-CBs CPUs
  rcu: Mark rcu_state.gp_seq to detect concurrent writes
  Documentation/memory-barriers: Fix typos
  doc: Add rcutorture scripting to torture.txt
  doc/RCU/rcu: Use https instead of http if possible
  doc/RCU/rcu: Use absolute paths for non-rst files
  doc/RCU/rcu: Use ':ref:' for links to other docs
  doc/RCU/listRCU: Update example function name
  doc/RCU/listRCU: Fix typos in a example code snippets
  doc/RCU/Design: Remove remaining HTML tags in ReST files
  doc: Add some more RCU list patterns in the kernel
  rcutorture: Set KCSAN Kconfig options to detect more data races
  rcutorture: Manually clean up after rcu_barrier() failure
  rcutorture: Make rcu_torture_barrier_cbs() post from corresponding CPU
  rcuperf: Measure memory footprint during kfree_rcu() test
  rcutorture: Annotation lockless accesses to rcu_torture_current
  rcutorture: Add READ_ONCE() to rcu_torture_count and rcu_torture_batch
  rcutorture: Fix stray access to rcu_fwd_cb_nodelay
  rcutorture: Fix rcu_torture_one_read()/rcu_torture_writer() data race
  rcutorture: Make kvm-find-errors.sh abort on bad directory
  ...

34 files changed, 1015 insertions, 294 deletions

diff --git a/Documentation/RCU/Design/Memory-Ordering/Tree-RCU-Memory-Ordering.rst b/Documentation/RCU/Design/Memory-Ordering/Tree-RCU-Memory-Ordering.rst
index 1a8b129cfc04..83ae3b79a643 100644
--- a/Documentation/RCU/Design/Memory-Ordering/Tree-RCU-Memory-Ordering.rst
+++ b/Documentation/RCU/Design/Memory-Ordering/Tree-RCU-Memory-Ordering.rst
@@ -4,7 +4,7 @@ A Tour Through TREE_RCU's Grace-Period Memory Ordering
 
 August 8, 2017
 
-This article was contributed by Paul E. McKenney
+This article was contributed by Paul E. McKenney
 
 Introduction
 ============
@@ -48,7 +48,7 @@ Tree RCU Grace Period Memory Ordering Building Blocks
 
 The workhorse for RCU's grace-period memory ordering is the
 critical section for the ``rcu_node`` structure's
-``->lock``. These critical sections use helper functions for lock
+``->lock``. These critical sections use helper functions for lock
 acquisition, including ``raw_spin_lock_rcu_node()``,
 ``raw_spin_lock_irq_rcu_node()``, and ``raw_spin_lock_irqsave_rcu_node()``.
 Their lock-release counterparts are ``raw_spin_unlock_rcu_node()``,
@@ -102,9 +102,9 @@ lock-acquisition and lock-release functions::
    23   r3 = READ_ONCE(x);
    24 }
    25
-   26 WARN_ON(r1 == 0 && r2 == 0 && r3 == 0);
+   26 WARN_ON(r1 == 0 && r2 == 0 && r3 == 0);
 
-The ``WARN_ON()`` is evaluated at “the end of time”,
+The ``WARN_ON()`` is evaluated at "the end of time",
 after all changes have propagated throughout the system.
 Without the ``smp_mb__after_unlock_lock()`` provided by the
 acquisition functions, this ``WARN_ON()`` could trigger, for example
diff --git a/Documentation/RCU/listRCU.rst b/Documentation/RCU/listRCU.rst
index 7956ff33042b..2a643e293fb4 100644
--- a/Documentation/RCU/listRCU.rst
+++ b/Documentation/RCU/listRCU.rst
@@ -4,12 +4,61 @@ Using RCU to Protect Read-Mostly Linked Lists
 =============================================
 
 One of the best applications of RCU is to protect read-mostly linked lists
-("struct list_head" in list.h).  One big advantage of this approach
+(``struct list_head`` in list.h).  One big advantage of this approach
 is that all of the required memory barriers are included for you in
 the list macros.  This document describes several applications of RCU,
 with the best fits first.
 
-Example 1: Read-Side Action Taken Outside of Lock, No In-Place Updates
+
+Example 1: Read-mostly list: Deferred Destruction
+-------------------------------------------------
+
+A widely used usecase for RCU lists in the kernel is lockless iteration over
+all processes in the system. ``task_struct::tasks`` represents the list node that
+links all the processes. The list can be traversed in parallel to any list
+additions or removals.
+
+The traversal of the list is done using ``for_each_process()`` which is defined
+by the 2 macros::
+
+	#define next_task(p) \
+		list_entry_rcu((p)->tasks.next, struct task_struct, tasks)
+
+	#define for_each_process(p) \
+		for (p = &init_task ; (p = next_task(p)) != &init_task ; )
+
+The code traversing the list of all processes typically looks like::
+
+	rcu_read_lock();
+	for_each_process(p) {
+		/* Do something with p */
+	}
+	rcu_read_unlock();
+
+The simplified code for removing a process from a task list is::
+
+	void release_task(struct task_struct *p)
+	{
+		write_lock(&tasklist_lock);
+		list_del_rcu(&p->tasks);
+		write_unlock(&tasklist_lock);
+		call_rcu(&p->rcu, delayed_put_task_struct);
+	}
+
+When a process exits, ``release_task()`` calls ``list_del_rcu(&p->tasks)`` under
+``tasklist_lock`` writer lock protection, to remove the task from the list of
+all tasks. The ``tasklist_lock`` prevents concurrent list additions/removals
+from corrupting the list. Readers using ``for_each_process()`` are not protected
+with the ``tasklist_lock``. To prevent readers from noticing changes in the list
+pointers, the ``task_struct`` object is freed only after one or more grace
+periods elapse (with the help of call_rcu()). This deferring of destruction
+ensures that any readers traversing the list will see valid ``p->tasks.next``
+pointers and deletion/freeing can happen in parallel with traversal of the list.
+This pattern is also called an **existence lock**, since RCU pins the object in
+memory until all existing readers finish.
+
+
+Example 2: Read-Side Action Taken Outside of Lock: No In-Place Updates
 ----------------------------------------------------------------------
 
 The best applications are cases where, if reader-writer locking were
@@ -26,7 +75,7 @@ added or deleted, rather than being modified in place.
 
 A straightforward example of this use of RCU may be found in the
 system-call auditing support.  For example, a reader-writer locked
-implementation of audit_filter_task() might be as follows::
+implementation of ``audit_filter_task()`` might be as follows::
 
 	static enum audit_state audit_filter_task(struct task_struct *tsk)
 	{
@@ -34,7 +83,7 @@ implementation of audit_filter_task() might be as follows::
 		enum audit_state   state;
 
 		read_lock(&auditsc_lock);
-		/* Note: audit_netlink_sem held by caller. */
+		/* Note: audit_filter_mutex held by caller. */
 		list_for_each_entry(e, &audit_tsklist, list) {
 			if (audit_filter_rules(tsk, &e->rule, NULL, &state)) {
 				read_unlock(&auditsc_lock);
@@ -58,7 +107,7 @@ This means that RCU can be easily applied to the read side, as follows::
 		enum audit_state   state;
 
 		rcu_read_lock();
-		/* Note: audit_netlink_sem held by caller. */
+		/* Note: audit_filter_mutex held by caller. */
 		list_for_each_entry_rcu(e, &audit_tsklist, list) {
 			if (audit_filter_rules(tsk, &e->rule, NULL, &state)) {
 				rcu_read_unlock();
@@ -69,18 +118,18 @@ This means that RCU can be easily applied to the read side, as follows::
 		return AUDIT_BUILD_CONTEXT;
 	}
 
-The read_lock() and read_unlock() calls have become rcu_read_lock()
+The ``read_lock()`` and ``read_unlock()`` calls have become rcu_read_lock()
 and rcu_read_unlock(), respectively, and the list_for_each_entry() has
-become list_for_each_entry_rcu().  The _rcu() list-traversal primitives
+become list_for_each_entry_rcu().  The **_rcu()** list-traversal primitives
 insert the read-side memory barriers that are required on DEC Alpha CPUs.
 
-The changes to the update side are also straightforward.  A reader-writer
-lock might be used as follows for deletion and insertion::
+The changes to the update side are also straightforward. A reader-writer lock
+might be used as follows for deletion and insertion::
 
 	static inline int audit_del_rule(struct audit_rule *rule,
 					 struct list_head *list)
 	{
-		struct audit_entry  *e;
+		struct audit_entry *e;
 
 		write_lock(&auditsc_lock);
 		list_for_each_entry(e, list, list) {
@@ -113,9 +162,9 @@ Following are the RCU equivalents for these two functions::
 	static inline int audit_del_rule(struct audit_rule *rule,
 					 struct list_head *list)
 	{
-		struct audit_entry  *e;
+		struct audit_entry *e;
 
-		/* Do not use the _rcu iterator here, since this is the only
+		/* No need to use the _rcu iterator here, since this is the only
 		 * deletion routine. */
 		list_for_each_entry(e, list, list) {
 			if (!audit_compare_rule(rule, &e->rule)) {
@@ -139,45 +188,45 @@ Following are the RCU equivalents for these two functions::
 		return 0;
 	}
 
-Normally, the write_lock() and write_unlock() would be replaced by
-a spin_lock() and a spin_unlock(), but in this case, all callers hold
-audit_netlink_sem, so no additional locking is required.  The auditsc_lock
-can therefore be eliminated, since use of RCU eliminates the need for
-writers to exclude readers.  Normally, the write_lock() calls would
-be converted into spin_lock() calls.
+Normally, the ``write_lock()`` and ``write_unlock()`` would be replaced by a
+spin_lock() and a spin_unlock(). But in this case, all callers hold
+``audit_filter_mutex``, so no additional locking is required. The
+``auditsc_lock`` can therefore be eliminated, since use of RCU eliminates the
+need for writers to exclude readers.
 
 The list_del(), list_add(), and list_add_tail() primitives have been
 replaced by list_del_rcu(), list_add_rcu(), and list_add_tail_rcu().
-The _rcu() list-manipulation primitives add memory barriers that are
-needed on weakly ordered CPUs (most of them!).  The list_del_rcu()
-primitive omits the pointer poisoning debug-assist code that would
-otherwise cause concurrent readers to fail spectacularly.
+The **_rcu()** list-manipulation primitives add memory barriers that are needed on
+weakly ordered CPUs (most of them!).  The list_del_rcu() primitive omits the
+pointer poisoning debug-assist code that would otherwise cause concurrent
+readers to fail spectacularly.
 
-So, when readers can tolerate stale data and when entries are either added
-or deleted, without in-place modification, it is very easy to use RCU!
+So, when readers can tolerate stale data and when entries are either added or
+deleted, without in-place modification, it is very easy to use RCU!
 
-Example 2: Handling In-Place Updates
+
+Example 3: Handling In-Place Updates
 ------------------------------------
 
-The system-call auditing code does not update auditing rules in place.
-However, if it did, reader-writer-locked code to do so might look as
-follows (presumably, the field_count is only permitted to decrease,
-otherwise, the added fields would need to be filled in)::
+The system-call auditing code does not update auditing rules in place.  However,
+if it did, the reader-writer-locked code to do so might look as follows
+(assuming only ``field_count`` is updated, otherwise, the added fields would
+need to be filled in)::
 
 	static inline int audit_upd_rule(struct audit_rule *rule,
 					 struct list_head *list,
 					 __u32 newaction,
 					 __u32 newfield_count)
 	{
-		struct audit_entry  *e;
-		struct audit_newentry *ne;
+		struct audit_entry *e;
+		struct audit_entry *ne;
 
 		write_lock(&auditsc_lock);
-		/* Note: audit_netlink_sem held by caller. */
+		/* Note: audit_filter_mutex held by caller. */
 		list_for_each_entry(e, list, list) {
 			if (!audit_compare_rule(rule, &e->rule)) {
 				e->rule.action = newaction;
-				e->rule.file_count = newfield_count;
+				e->rule.field_count = newfield_count;
 				write_unlock(&auditsc_lock);
 				return 0;
 			}
@@ -188,16 +237,16 @@ otherwise, the added fields would need to be filled in)::
 
 The RCU version creates a copy, updates the copy, then replaces the old
 entry with the newly updated entry.  This sequence of actions, allowing
-concurrent reads while doing a copy to perform an update, is what gives
-RCU ("read-copy update") its name.  The RCU code is as follows::
+concurrent reads while making a copy to perform an update, is what gives
+RCU (*read-copy update*) its name.  The RCU code is as follows::
 
 	static inline int audit_upd_rule(struct audit_rule *rule,
 					 struct list_head *list,
 					 __u32 newaction,
 					 __u32 newfield_count)
 	{
-		struct audit_entry  *e;
-		struct audit_newentry *ne;
+		struct audit_entry *e;
+		struct audit_entry *ne;
 
 		list_for_each_entry(e, list, list) {
 			if (!audit_compare_rule(rule, &e->rule)) {
@@ -206,7 +255,7 @@ RCU ("read-copy update") its name.  The RCU code is as follows::
 					return -ENOMEM;
 				audit_copy_rule(&ne->rule, &e->rule);
 				ne->rule.action = newaction;
-				ne->rule.file_count = newfield_count;
+				ne->rule.field_count = newfield_count;
 				list_replace_rcu(&e->list, &ne->list);
 				call_rcu(&e->rcu, audit_free_rule);
 				return 0;
@@ -215,34 +264,45 @@ RCU ("read-copy update") its name.  The RCU code is as follows::
 		return -EFAULT;		/* No matching rule */
 	}
 
-Again, this assumes that the caller holds audit_netlink_sem.  Normally,
-the reader-writer lock would become a spinlock in this sort of code.
+Again, this assumes that the caller holds ``audit_filter_mutex``.  Normally, the
+writer lock would become a spinlock in this sort of code.
 
-Example 3: Eliminating Stale Data
+Another use of this pattern can be found in the openswitch driver's *connection
+tracking table* code in ``ct_limit_set()``.  The table holds connection tracking
+entries and has a limit on the maximum entries.  There is one such table
+per-zone and hence one *limit* per zone.  The zones are mapped to their limits
+through a hashtable using an RCU-managed hlist for the hash chains. When a new
+limit is set, a new limit object is allocated and ``ct_limit_set()`` is called
+to replace the old limit object with the new one using list_replace_rcu().
+The old limit object is then freed after a grace period using kfree_rcu().
+
+
+Example 4: Eliminating Stale Data
 ---------------------------------
 
-The auditing examples above tolerate stale data, as do most algorithms
+The auditing example above tolerates stale data, as do most algorithms
 that are tracking external state.  Because there is a delay from the
 time the external state changes before Linux becomes aware of the change,
-additional RCU-induced staleness is normally not a problem.
+additional RCU-induced staleness is generally not a problem.
 
 However, there are many examples where stale data cannot be tolerated.
-One example in the Linux kernel is the System V IPC (see the ipc_lock()
-function in ipc/util.c).  This code checks a "deleted" flag under a
-per-entry spinlock, and, if the "deleted" flag is set, pretends that the
+One example in the Linux kernel is the System V IPC (see the shm_lock()
+function in ipc/shm.c).  This code checks a *deleted* flag under a
+per-entry spinlock, and, if the *deleted* flag is set, pretends that the
 entry does not exist.  For this to be helpful, the search function must
-return holding the per-entry spinlock, as ipc_lock() does in fact do.
+return holding the per-entry spinlock, as shm_lock() does in fact do.
+
+.. _quick_quiz:
 
 Quick Quiz:
-	Why does the search function need to return holding the per-entry lock for
-	this deleted-flag technique to be helpful?
+	For the deleted-flag technique to be helpful, why is it necessary
+	to hold the per-entry lock while returning from the search function?
 
-:ref:`Answer to Quick Quiz <answer_quick_quiz_list>`
+:ref:`Answer to Quick Quiz <quick_quiz_answer>`
 
-If the system-call audit module were to ever need to reject stale data,
-one way to accomplish this would be to add a "deleted" flag and a "lock"
-spinlock to the audit_entry structure, and modify audit_filter_task()
-as follows::
+If the system-call audit module were to ever need to reject stale data, one way
+to accomplish this would be to add a ``deleted`` flag and a ``lock`` spinlock to the
+audit_entry structure, and modify ``audit_filter_task()`` as follows::
 
 	static enum audit_state audit_filter_task(struct task_struct *tsk)
 	{
@@ -267,20 +327,20 @@ as follows::
 	}
 
 Note that this example assumes that entries are only added and deleted.
-Additional mechanism is required to deal correctly with the
-update-in-place performed by audit_upd_rule().  For one thing,
-audit_upd_rule() would need additional memory barriers to ensure
-that the list_add_rcu() was really executed before the list_del_rcu().
+Additional mechanism is required to deal correctly with the update-in-place
+performed by ``audit_upd_rule()``.  For one thing, ``audit_upd_rule()`` would
+need additional memory barriers to ensure that the list_add_rcu() was really
+executed before the list_del_rcu().
 
-The audit_del_rule() function would need to set the "deleted"
-flag under the spinlock as follows::
+The ``audit_del_rule()`` function would need to set the ``deleted`` flag under the
+spinlock as follows::
 
 	static inline int audit_del_rule(struct audit_rule *rule,
 					 struct list_head *list)
 	{
-		struct audit_entry  *e;
+		struct audit_entry *e;
 
-		/* Do not need to use the _rcu iterator here, since this
+		/* No need to use the _rcu iterator here, since this
 		 * is the only deletion routine. */
 		list_for_each_entry(e, list, list) {
 			if (!audit_compare_rule(rule, &e->rule)) {
@@ -295,6 +355,91 @@ flag under the spinlock as follows::
 		return -EFAULT;		/* No matching rule */
 	}
 
+This too assumes that the caller holds ``audit_filter_mutex``.
+
+
+Example 5: Skipping Stale Objects
+---------------------------------
+
+For some usecases, reader performance can be improved by skipping stale objects
+during read-side list traversal if the object in concern is pending destruction
+after one or more grace periods. One such example can be found in the timerfd
+subsystem. When a ``CLOCK_REALTIME`` clock is reprogrammed - for example due to
+setting of the system time, then all programmed timerfds that depend on this
+clock get triggered and processes waiting on them to expire are woken up in
+advance of their scheduled expiry. To facilitate this, all such timers are added
+to an RCU-managed ``cancel_list`` when they are setup in
+``timerfd_setup_cancel()``::
+
+	static void timerfd_setup_cancel(struct timerfd_ctx *ctx, int flags)
+	{
+		spin_lock(&ctx->cancel_lock);
+		if ((ctx->clockid == CLOCK_REALTIME &&
+		    (flags & TFD_TIMER_ABSTIME) && (flags & TFD_TIMER_CANCEL_ON_SET)) {
+			if (!ctx->might_cancel) {
+				ctx->might_cancel = true;
+				spin_lock(&cancel_lock);
+				list_add_rcu(&ctx->clist, &cancel_list);
+				spin_unlock(&cancel_lock);
+			}
+		}
+		spin_unlock(&ctx->cancel_lock);
+	}
+
+When a timerfd is freed (fd is closed), then the ``might_cancel`` flag of the
+timerfd object is cleared, the object removed from the ``cancel_list`` and
+destroyed::
+
+	int timerfd_release(struct inode *inode, struct file *file)
+	{
+		struct timerfd_ctx *ctx = file->private_data;
+
+		spin_lock(&ctx->cancel_lock);
+		if (ctx->might_cancel) {
+			ctx->might_cancel = false;
+			spin_lock(&cancel_lock);
+			list_del_rcu(&ctx->clist);
+			spin_unlock(&cancel_lock);
+		}
+		spin_unlock(&ctx->cancel_lock);
+
+		hrtimer_cancel(&ctx->t.tmr);
+		kfree_rcu(ctx, rcu);
+		return 0;
+	}
+
+If the ``CLOCK_REALTIME`` clock is set, for example by a time server, the
+hrtimer framework calls ``timerfd_clock_was_set()`` which walks the
+``cancel_list`` and wakes up processes waiting on the timerfd. While iterating
+the ``cancel_list``, the ``might_cancel`` flag is consulted to skip stale
+objects::
+
+	void timerfd_clock_was_set(void)
+	{
+		struct timerfd_ctx *ctx;
+		unsigned long flags;
+
+		rcu_read_lock();
+		list_for_each_entry_rcu(ctx, &cancel_list, clist) {
+			if (!ctx->might_cancel)
+				continue;
+			spin_lock_irqsave(&ctx->wqh.lock, flags);
+			if (ctx->moffs != ktime_mono_to_real(0)) {
+				ctx->moffs = KTIME_MAX;
+				ctx->ticks++;
+				wake_up_locked_poll(&ctx->wqh, EPOLLIN);
+			}
+			spin_unlock_irqrestore(&ctx->wqh.lock, flags);
+		}
+		rcu_read_unlock();
+	}
+
+The key point here is, because RCU-traversal of the ``cancel_list`` happens
+while objects are being added and removed to the list, sometimes the traversal
+can step on an object that has been removed from the list. In this example, it
+is seen that it is better to skip such objects using a flag.
+
+
 Summary
 -------
 
@@ -303,19 +448,21 @@ the most amenable to use of RCU.  The simplest case is where entries are
 either added or deleted from the data structure (or atomically modified
 in place), but non-atomic in-place modifications can be handled by making
 a copy, updating the copy, then replacing the original with the copy.
-If stale data cannot be tolerated, then a "deleted" flag may be used
+If stale data cannot be tolerated, then a *deleted* flag may be used
 in conjunction with a per-entry spinlock in order to allow the search
 function to reject newly deleted data.
 
-.. _answer_quick_quiz_list:
+.. _quick_quiz_answer:
 
 Answer to Quick Quiz:
-	Why does the search function need to return holding the per-entry
-	lock for this deleted-flag technique to be helpful?
+	For the deleted-flag technique to be helpful, why is it necessary
+	to hold the per-entry lock while returning from the search function?
 
 	If the search function drops the per-entry lock before returning,
 	then the caller will be processing stale data in any case.  If it
 	is really OK to be processing stale data, then you don't need a
-	"deleted" flag.  If processing stale data really is a problem,
+	*deleted* flag.  If processing stale data really is a problem,
 	then you need to hold the per-entry lock across all of the code
 	that uses the value that was returned.
+
+:ref:`Back to Quick Quiz <quick_quiz>`
diff --git a/Documentation/RCU/rcu.rst b/Documentation/RCU/rcu.rst
index 8dfb437dacc3..0e03c6ef3147 100644
--- a/Documentation/RCU/rcu.rst
+++ b/Documentation/RCU/rcu.rst
@@ -11,8 +11,8 @@ must be long enough that any readers accessing the item being deleted have
 since dropped their references.  For example, an RCU-protected deletion
 from a linked list would first remove the item from the list, wait for
 a grace period to elapse, then free the element.  See the
-Documentation/RCU/listRCU.rst file for more information on using RCU with
-linked lists.
+:ref:`Documentation/RCU/listRCU.rst <list_rcu_doc>` for more information on
+using RCU with linked lists.
 
 Frequently Asked Questions
 --------------------------
@@ -50,7 +50,7 @@ Frequently Asked Questions
 - If I am running on a uniprocessor kernel, which can only do one
   thing at a time, why should I wait for a grace period?
 
-  See the Documentation/RCU/UP.rst file for more information.
+  See :ref:`Documentation/RCU/UP.rst <up_doc>` for more information.
 
 - How can I see where RCU is currently used in the Linux kernel?
 
@@ -68,18 +68,18 @@ Frequently Asked Questions
 
 - Why the name "RCU"?
 
-  "RCU" stands for "read-copy update".  The file Documentation/RCU/listRCU.rst
-  has more information on where this name came from, search for
-  "read-copy update" to find it.
+  "RCU" stands for "read-copy update".
+  :ref:`Documentation/RCU/listRCU.rst <list_rcu_doc>` has more information on where
+  this name came from, search for "read-copy update" to find it.
 
 - I hear that RCU is patented?  What is with that?
 
   Yes, it is.  There are several known patents related to RCU,
-  search for the string "Patent" in RTFP.txt to find them.
+  search for the string "Patent" in Documentation/RCU/RTFP.txt to find them.
   Of these, one was allowed to lapse by the assignee, and the
   others have been contributed to the Linux kernel under GPL.
   There are now also LGPL implementations of user-level RCU
-  available (http://liburcu.org/).
+  available (https://liburcu.org/).
 
 - I hear that RCU needs work in order to support realtime kernels?
 
@@ -88,5 +88,5 @@ Frequently Asked Questions
 
 - Where can I find more information on RCU?
 
-  See the RTFP.txt file in this directory.
+  See the Documentation/RCU/RTFP.txt file.
   Or point your browser at (http://www.rdrop.com/users/paulmck/RCU/).
diff --git a/Documentation/RCU/torture.txt b/Documentation/RCU/torture.txt
index a41a0384d20c..af712a3c5b6a 100644
--- a/Documentation/RCU/torture.txt
+++ b/Documentation/RCU/torture.txt
@@ -124,9 +124,14 @@ using a dynamically allocated srcu_struct (hence "srcud-" rather than
 debugging.  The final "T" entry contains the totals of the counters.
 
 
-USAGE
+USAGE ON SPECIFIC KERNEL BUILDS
 
-The following script may be used to torture RCU:
+It is sometimes desirable to torture RCU on a specific kernel build,
+for example, when preparing to put that kernel build into production.
+In that case, the kernel should be built with CONFIG_RCU_TORTURE_TEST=m
+so that the test can be started using modprobe and terminated using rmmod.
+
+For example, the following script may be used to torture RCU:
 
 	#!/bin/sh
 
@@ -142,8 +147,136 @@ checked for such errors.  The "rmmod" command forces a "SUCCESS",
 two are self-explanatory, while the last indicates that while there
 were no RCU failures, CPU-hotplug problems were detected.
 
-However, the tools/testing/selftests/rcutorture/bin/kvm.sh script
-provides better automation, including automatic failure analysis.
-It assumes a qemu/kvm-enabled platform, and runs guest OSes out of initrd.
-See tools/testing/selftests/rcutorture/doc/initrd.txt for instructions
-on setting up such an initrd.
+
+USAGE ON MAINLINE KERNELS
+
+When using rcutorture to test changes to RCU itself, it is often
+necessary to build a number of kernels in order to test that change
+across a broad range of combinations of the relevant Kconfig options
+and of the relevant kernel boot parameters.  In this situation, use
+of modprobe and rmmod can be quite time-consuming and error-prone.
+
+Therefore, the tools/testing/selftests/rcutorture/bin/kvm.sh
+script is available for mainline testing for x86, arm64, and
+powerpc.  By default, it will run the series of tests specified by
+tools/testing/selftests/rcutorture/configs/rcu/CFLIST, with each test
+running for 30 minutes within a guest OS using a minimal userspace
+supplied by an automatically generated initrd.  After the tests are
+complete, the resulting build products and console output are analyzed
+for errors and the results of the runs are summarized.
+
+On larger systems, rcutorture testing can be accelerated by passing the
+--cpus argument to kvm.sh.  For example, on a 64-CPU system, "--cpus 43"
+would use up to 43 CPUs to run tests concurrently, which as of v5.4 would
+complete all the scenarios in two batches, reducing the time to complete
+from about eight hours to about one hour (not counting the time to build
+the sixteen kernels).  The "--dryrun sched" argument will not run tests,
+but rather tell you how the tests would be scheduled into batches.  This
+can be useful when working out how many CPUs to specify in the --cpus
+argument.
+
+Not all changes require that all scenarios be run.  For example, a change
+to Tree SRCU might run only the SRCU-N and SRCU-P scenarios using the
+--configs argument to kvm.sh as follows:  "--configs 'SRCU-N SRCU-P'".
+Large systems can run multiple copies of of the full set of scenarios,
+for example, a system with 448 hardware threads can run five instances
+of the full set concurrently.  To make this happen:
+
+	kvm.sh --cpus 448 --configs '5*CFLIST'
+
+Alternatively, such a system can run 56 concurrent instances of a single
+eight-CPU scenario:
+
+	kvm.sh --cpus 448 --configs '56*TREE04'
+
+Or 28 concurrent instances of each of two eight-CPU scenarios:
+
+	kvm.sh --cpus 448 --configs '28*TREE03 28*TREE04'
+
+Of course, each concurrent instance will use memory, which can be
+limited using the --memory argument, which defaults to 512M.  Small
+values for memory may require disabling the callback-flooding tests
+using the --bootargs parameter discussed below.
+
+Sometimes additional debugging is useful, and in such cases the --kconfig
+parameter to kvm.sh may be used, for example, "--kconfig 'CONFIG_KASAN=y'".
+
+Kernel boot arguments can also be supplied, for example, to control
+rcutorture's module parameters.  For example, to test a change to RCU's
+CPU stall-warning code, use "--bootargs 'rcutorture.stall_cpu=30'".
+This will of course result in the scripting reporting a failure, namely
+the resuling RCU CPU stall warning.  As noted above, reducing memory may
+require disabling rcutorture's callback-flooding tests:
+
+	kvm.sh --cpus 448 --configs '56*TREE04' --memory 128M \
+		--bootargs 'rcutorture.fwd_progress=0'
+
+Sometimes all that is needed is a full set of kernel builds.  This is
+what the --buildonly argument does.
+
+Finally, the --trust-make argument allows each kernel build to reuse what
+it can from the previous kernel build.
+
+There are additional more arcane arguments that are documented in the
+source code of the kvm.sh script.
+
+If a run contains failures, the number of buildtime and runtime failures
+is listed at the end of the kvm.sh output, which you really should redirect
+to a file.  The build products and console output of each run is kept in
+tools/testing/selftests/rcutorture/res in timestamped directories.  A
+given directory can be supplied to kvm-find-errors.sh in order to have
+it cycle you through summaries of errors and full error logs.  For example:
+
+	tools/testing/selftests/rcutorture/bin/kvm-find-errors.sh \
+		tools/testing/selftests/rcutorture/res/2020.01.20-15.54.23
+
+However, it is often more convenient to access the files directly.
+Files pertaining to all scenarios in a run reside in the top-level
+directory (2020.01.20-15.54.23 in the example above), while per-scenario
+files reside in a subdirectory named after the scenario (for example,
+"TREE04").  If a given scenario ran more than once (as in "--configs
+'56*TREE04'" above), the directories corresponding to the second an