Merge tag 'rcu.2024.07.12a' of git://git.kernel.org/pub/scm/linux/kernel/git/paulmck/linux-rcu

Pull RCU updates from Paul McKenney:

 - Update Tasks RCU and Tasks Rude RCU description in Requirements.rst
   and clarify rcu_assign_pointer() and rcu_dereference() ordering
   properties

 - Add lockdep assertions for RCU readers, limit inline wakeups for
   callback-bypass synchronize_rcu(), add an
   rcutree.nohz_full_patience_delay to reduce nohz_full OS jitter, add
   Uladzislau Rezki as RCU maintainer, and fix a subtle
   callback-migration memory-ordering issue

 - Remove a number of redundant memory barriers

 - Remove unnecessary bypass-list lock-contention mitigation, use
   parking API instead of open-coded ad-hoc equivalent, and upgrade
   obsolete comments

 - Revert avoidance of a deadlock that can no longer occur and properly
   synchronize Tasks Trace RCU checking of runqueues

 - Add tests for handling of double-call_rcu() bug, add missing
   MODULE_DESCRIPTION, and add a script that histograms the number of
   calls to RCU updaters

 - Fill out SRCU polled-grace-period API

* tag 'rcu.2024.07.12a' of git://git.kernel.org/pub/scm/linux/kernel/git/paulmck/linux-rcu: (29 commits)
  rcu: Fix rcu_barrier() VS post CPUHP_TEARDOWN_CPU invocation
  rcu: Eliminate lockless accesses to rcu_sync->gp_count
  MAINTAINERS: Add Uladzislau Rezki as RCU maintainer
  rcu: Add rcutree.nohz_full_patience_delay to reduce nohz_full OS jitter
  rcu/exp: Remove redundant full memory barrier at the end of GP
  rcu: Remove full memory barrier on RCU stall printout
  rcu: Remove full memory barrier on boot time eqs sanity check
  rcu/exp: Remove superfluous full memory barrier upon first EQS snapshot
  rcu: Remove superfluous full memory barrier upon first EQS snapshot
  rcu: Remove full ordering on second EQS snapshot
  srcu: Fill out polled grace-period APIs
  srcu: Update cleanup_srcu_struct() comment
  srcu: Add NUM_ACTIVE_SRCU_POLL_OLDSTATE
  srcu: Disable interrupts directly in srcu_gp_end()
  rcu: Disable interrupts directly in rcu_gp_init()
  rcu/tree: Reduce wake up for synchronize_rcu() common case
  rcu/tasks: Fix stale task snaphot for Tasks Trace
  tools/rcu: Add rcu-updaters.sh script
  rcutorture: Add missing MODULE_DESCRIPTION() macros
  rcutorture: Fix rcu_torture_fwd_cb_cr() data race
  ...

4 files changed, 46 insertions, 14 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 5750f125361b..728b1e690c64 100644
--- a/Documentation/RCU/Design/Memory-Ordering/Tree-RCU-Memory-Ordering.rst
+++ b/Documentation/RCU/Design/Memory-Ordering/Tree-RCU-Memory-Ordering.rst
@@ -149,9 +149,9 @@ This case is handled by calls to the strongly ordered
 ``atomic_add_return()`` read-modify-write atomic operation that
 is invoked within ``rcu_dynticks_eqs_enter()`` at idle-entry
 time and within ``rcu_dynticks_eqs_exit()`` at idle-exit time.
-The grace-period kthread invokes ``rcu_dynticks_snap()`` and
-``rcu_dynticks_in_eqs_since()`` (both of which invoke
-an ``atomic_add_return()`` of zero) to detect idle CPUs.
+The grace-period kthread invokes first ``ct_dynticks_cpu_acquire()``
+(preceded by a full memory barrier) and ``rcu_dynticks_in_eqs_since()``
+(both of which rely on acquire semantics) to detect idle CPUs.
 
 +-----------------------------------------------------------------------+
 | **Quick Quiz**:                                                       |
diff --git a/Documentation/RCU/Design/Requirements/Requirements.rst b/Documentation/RCU/Design/Requirements/Requirements.rst
index cccafdaa1f84..f511476b4550 100644
--- a/Documentation/RCU/Design/Requirements/Requirements.rst
+++ b/Documentation/RCU/Design/Requirements/Requirements.rst
@@ -2357,6 +2357,7 @@ section.
 #. `Sched Flavor (Historical)`_
 #. `Sleepable RCU`_
 #. `Tasks RCU`_
+#. `Tasks Trace RCU`_
 
 Bottom-Half Flavor (Historical)
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
@@ -2610,6 +2611,16 @@ critical sections that are delimited by voluntary context switches, that
 is, calls to schedule(), cond_resched(), and
 synchronize_rcu_tasks(). In addition, transitions to and from
 userspace execution also delimit tasks-RCU read-side critical sections.
+Idle tasks are ignored by Tasks RCU, and Tasks Rude RCU may be used to
+interact with them.
+
+Note well that involuntary context switches are *not* Tasks-RCU quiescent
+states.  After all, in preemptible kernels, a task executing code in a
+trampoline might be preempted.  In this case, the Tasks-RCU grace period
+clearly cannot end until that task resumes and its execution leaves that
+trampoline.  This means, among other things, that cond_resched() does
+not provide a Tasks RCU quiescent state.  (Instead, use rcu_softirq_qs()
+from softirq or rcu_tasks_classic_qs() otherwise.)
 
 The tasks-RCU API is quite compact, consisting only of
 call_rcu_tasks(), synchronize_rcu_tasks(), and
@@ -2632,6 +2643,11 @@ moniker.  And this operation is considered to be quite rude by real-time
 workloads that don't want their ``nohz_full`` CPUs receiving IPIs and
 by battery-powered systems that don't want their idle CPUs to be awakened.
 
+Once kernel entry/exit and deep-idle functions have been properly tagged
+``noinstr``, Tasks RCU can start paying attention to idle tasks (except
+those that are idle from RCU's perspective) and then Tasks Rude RCU can
+be removed from the kernel.
+
 The tasks-rude-RCU API is also reader-marking-free and thus quite compact,
 consisting of call_rcu_tasks_rude(), synchronize_rcu_tasks_rude(),
 and rcu_barrier_tasks_rude().
diff --git a/Documentation/RCU/whatisRCU.rst b/Documentation/RCU/whatisRCU.rst
index 94838c65c7d9..d585a5490aee 100644
--- a/Documentation/RCU/whatisRCU.rst
+++ b/Documentation/RCU/whatisRCU.rst
@@ -250,21 +250,25 @@ rcu_assign_pointer()
 ^^^^^^^^^^^^^^^^^^^^
 	void rcu_assign_pointer(p, typeof(p) v);
 
-	Yes, rcu_assign_pointer() **is** implemented as a macro, though it
-	would be cool to be able to declare a function in this manner.
-	(Compiler experts will no doubt disagree.)
+	Yes, rcu_assign_pointer() **is** implemented as a macro, though
+	it would be cool to be able to declare a function in this manner.
+	(And there has been some discussion of adding overloaded functions
+	to the C language, so who knows?)
 
 	The updater uses this spatial macro to assign a new value to an
 	RCU-protected pointer, in order to safely communicate the change
 	in value from the updater to the reader.  This is a spatial (as
 	opposed to temporal) macro.  It does not evaluate to an rvalue,
-	but it does execute any memory-barrier instructions required
-	for a given CPU architecture.  Its ordering properties are that
-	of a store-release operation.
-
-	Perhaps just as important, it serves to document (1) which
-	pointers are protected by RCU and (2) the point at which a
-	given structure becomes accessible to other CPUs.  That said,
+	but it does provide any compiler directives and memory-barrier
+	instructions required for a given compile or CPU architecture.
+	Its ordering properties are that of a store-release operation,
+	that is, any prior loads and stores required to initialize the
+	structure are ordered before the store that publishes the pointer
+	to that structure.
+
+	Perhaps just as important, rcu_assign_pointer() serves to document
+	(1) which pointers are protected by RCU and (2) the point at which
+	a given structure becomes accessible to other CPUs.  That said,
 	rcu_assign_pointer() is most frequently used indirectly, via
 	the _rcu list-manipulation primitives such as list_add_rcu().
 
@@ -283,7 +287,11 @@ rcu_dereference()
 	executes any needed memory-barrier instructions for a given
 	CPU architecture.  Currently, only Alpha needs memory barriers
 	within rcu_dereference() -- on other CPUs, it compiles to a
-	volatile load.
+	volatile load.	However, no mainstream C compilers respect
+	address dependencies, so rcu_dereference() uses volatile casts,
+	which, in combination with the coding guidelines listed in
+	rcu_dereference.rst, prevent current compilers from breaking
+	these dependencies.
 
 	Common coding practice uses rcu_dereference() to copy an
 	RCU-protected pointer to a local variable, then dereferences
diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index 27ec49af1bf2..fc01da74cf99 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -5015,6 +5015,14 @@
 			the ->nocb_bypass queue.  The definition of "too
 			many" is supplied by this kernel boot parameter.
 
+	rcutree.nohz_full_patience_delay= [KNL]
+			On callback-offloaded (rcu_nocbs) CPUs, avoid
+			disturbing RCU unless the grace period has
+			reached the specified age in milliseconds.
+			Defaults to zero.  Large values will be capped
+			at five seconds.  All values will be rounded down
+			to the nearest value representable by jiffies.
+
 	rcutree.qhimark= [KNL]
 			Set threshold of queued RCU callbacks beyond which
 			batch limiting is disabled.