Merge tag 'rcu.2022.12.02a' of git://git.kernel.org/pub/scm/linux/kernel/git/paulmck/linux-rcu

Pull RCU updates from Paul McKenney:

 - Documentation updates. This is the second in a series from an ongoing
   review of the RCU documentation.

 - Miscellaneous fixes.

 - Introduce a default-off Kconfig option that depends on RCU_NOCB_CPU
   that, on CPUs mentioned in the nohz_full or rcu_nocbs boot-argument
   CPU lists, causes call_rcu() to introduce delays.

   These delays result in significant power savings on nearly idle
   Android and ChromeOS systems. These savings range from a few percent
   to more than ten percent.

   This series also includes several commits that change call_rcu() to a
   new call_rcu_hurry() function that avoids these delays in a few
   cases, for example, where timely wakeups are required. Several of
   these are outside of RCU and thus have acks and reviews from the
   relevant maintainers.

 - Create an srcu_read_lock_nmisafe() and an srcu_read_unlock_nmisafe()
   for architectures that support NMIs, but which do not provide
   NMI-safe this_cpu_inc(). These NMI-safe SRCU functions are required
   by the upcoming lockless printk() work by John Ogness et al.

 - Changes providing minor but important increases in torture test
   coverage for the new RCU polled-grace-period APIs.

 - Changes to torturescript that avoid redundant kernel builds, thus
   providing about a 30% speedup for the torture.sh acceptance test.

* tag 'rcu.2022.12.02a' of git://git.kernel.org/pub/scm/linux/kernel/git/paulmck/linux-rcu: (49 commits)
  net: devinet: Reduce refcount before grace period
  net: Use call_rcu_hurry() for dst_release()
  workqueue: Make queue_rcu_work() use call_rcu_hurry()
  percpu-refcount: Use call_rcu_hurry() for atomic switch
  scsi/scsi_error: Use call_rcu_hurry() instead of call_rcu()
  rcu/rcutorture: Use call_rcu_hurry() where needed
  rcu/rcuscale: Use call_rcu_hurry() for async reader test
  rcu/sync: Use call_rcu_hurry() instead of call_rcu
  rcuscale: Add laziness and kfree tests
  rcu: Shrinker for lazy rcu
  rcu: Refactor code a bit in rcu_nocb_do_flush_bypass()
  rcu: Make call_rcu() lazy to save power
  rcu: Implement lockdep_rcu_enabled for !CONFIG_DEBUG_LOCK_ALLOC
  srcu: Debug NMI safety even on archs that don't require it
  srcu: Explain the reason behind the read side critical section on GP start
  srcu: Warn when NMI-unsafe API is used in NMI
  arch/s390: Add ARCH_HAS_NMI_SAFE_THIS_CPU_OPS Kconfig option
  arch/loongarch: Add ARCH_HAS_NMI_SAFE_THIS_CPU_OPS Kconfig option
  rcu: Fix __this_cpu_read() lockdep warning in rcu_force_quiescent_state()
  rcu-tasks: Make grace-period-age message human-readable
  ...

1 files changed, 4 insertions, 1 deletions

diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index e3142ee35fc6..7b0fe741a088 100644
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -1221,11 +1221,13 @@ static void rcu_spawn_one_boost_kthread(struct rcu_node *rnp)
  * We don't include outgoingcpu in the affinity set, use -1 if there is
  * no outgoing CPU.  If there are no CPUs left in the affinity set,
  * this function allows the kthread to execute on any CPU.
+ *
+ * Any future concurrent calls are serialized via ->boost_kthread_mutex.
  */
 static void rcu_boost_kthread_setaffinity(struct rcu_node *rnp, int outgoingcpu)
 {
 	struct task_struct *t = rnp->boost_kthread_task;
-	unsigned long mask = rcu_rnp_online_cpus(rnp);
+	unsigned long mask;
 	cpumask_var_t cm;
 	int cpu;
 
@@ -1234,6 +1236,7 @@ static void rcu_boost_kthread_setaffinity(struct rcu_node *rnp, int outgoingcpu)
 	if (!zalloc_cpumask_var(&cm, GFP_KERNEL))
 		return;
 	mutex_lock(&rnp->boost_kthread_mutex);
+	mask = rcu_rnp_online_cpus(rnp);
 	for_each_leaf_node_possible_cpu(rnp, cpu)
 		if ((mask & leaf_node_cpu_bit(rnp, cpu)) &&
 		    cpu != outgoingcpu)