RCU pull request for v6.8

This pull request contains the following branches:
 
 doc.2023.12.13a: Documentation and comment updates.
 
 torture.2023.11.23a: RCU torture, locktorture updates that include
         cleanups; nolibc init build support for mips, ppc and rv64;
         testing of mid stall duration scenario and fixing fqs task
         creation conditions.
 
 fixes.2023.12.13a: Misc fixes, most notably restricting usage of
         RCU CPU stall notifiers, to confine their usage primarily
         to debug kernels.
 
 rcu-tasks.2023.12.12b: RCU tasks minor fixes.
 
 srcu.2023.12.13a: lockdep annotation fix for NMI-safe accesses,
         callback advancing/acceleration cleanup and documentation
         improvements.
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Merge tag 'rcu.release.v6.8' of https://github.com/neeraju/linux

Pull RCU updates from Neeraj Upadhyay:

 - Documentation and comment updates

 - RCU torture, locktorture updates that include cleanups; nolibc init
   build support for mips, ppc and rv64; testing of mid stall duration
   scenario and fixing fqs task creation conditions

 - Misc fixes, most notably restricting usage of RCU CPU stall
   notifiers, to confine their usage primarily to debug kernels

 - RCU tasks minor fixes

 - lockdep annotation fix for NMI-safe accesses, callback
   advancing/acceleration cleanup and documentation improvements

* tag 'rcu.release.v6.8' of https://github.com/neeraju/linux:
  rcu: Force quiescent states only for ongoing grace period
  doc: Clarify historical disclaimers in memory-barriers.txt
  doc: Mention address and data dependencies in rcu_dereference.rst
  doc: Clarify RCU Tasks reader/updater checklist
  rculist.h: docs: Fix wrong function summary
  Documentation: RCU: Remove repeated word in comments
  srcu: Use try-lock lockdep annotation for NMI-safe access.
  srcu: Explain why callbacks invocations can't run concurrently
  srcu: No need to advance/accelerate if no callback enqueued
  srcu: Remove superfluous callbacks advancing from srcu_gp_start()
  rcu: Remove unused macros from rcupdate.h
  rcu: Restrict access to RCU CPU stall notifiers
  rcu-tasks: Mark RCU Tasks accesses to current->rcu_tasks_idle_cpu
  rcutorture: Add fqs_holdoff check before fqs_task is created
  rcutorture: Add mid-sized stall to TREE07
  rcutorture: add nolibc init support for mips, ppc and rv64
  locktorture: Increase Hamming distance between call_rcu_chain and rcu_call_chains

Documentation/RCU/checklist.rst

@@ -241,15 +241,22 @@ over a rather long period of time, but improvements are always welcome!
 	srcu_struct.  The rules for the expedited RCU grace-period-wait
 	primitives are the same as for their non-expedited counterparts.
 
-	If the updater uses call_rcu_tasks() or synchronize_rcu_tasks(),
+	Similarly, it is necessary to correctly use the RCU Tasks flavors:
-	then the readers must refrain from executing voluntary
+
-	context switches, that is, from blocking.  If the updater uses
+	a.	If the updater uses synchronize_rcu_tasks() or
-	call_rcu_tasks_trace() or synchronize_rcu_tasks_trace(), then
+		call_rcu_tasks(), then the readers must refrain from
-	the corresponding readers must use rcu_read_lock_trace() and
+		executing voluntary context switches, that is, from
-	rcu_read_unlock_trace().  If an updater uses call_rcu_tasks_rude()
+		blocking.
-	or synchronize_rcu_tasks_rude(), then the corresponding readers
+
-	must use anything that disables preemption, for example,
+	b.	If the updater uses call_rcu_tasks_trace()
-	preempt_disable() and preempt_enable().
+		or synchronize_rcu_tasks_trace(), then the
+		corresponding readers must use rcu_read_lock_trace()
+		and rcu_read_unlock_trace().
+
+	c.	If an updater uses call_rcu_tasks_rude() or
+		synchronize_rcu_tasks_rude(), then the corresponding
+		readers must use anything that disables preemption,
+		for example, preempt_disable() and preempt_enable().
 
 	Mixing things up will result in confusion and broken kernels, and
 	has even resulted in an exploitable security issue.  Therefore,

Documentation/RCU/rcu_dereference.rst

@@ -3,13 +3,26 @@
 PROPER CARE AND FEEDING OF RETURN VALUES FROM rcu_dereference()
 ===============================================================
 
-Most of the time, you can use values from rcu_dereference() or one of
+Proper care and feeding of address and data dependencies is critically
-the similar primitives without worries.  Dereferencing (prefix "*"),
+important to correct use of things like RCU.  To this end, the pointers
-field selection ("->"), assignment ("="), address-of ("&"), addition and
+returned from the rcu_dereference() family of primitives carry address and
-subtraction of constants, and casts all work quite naturally and safely.
+data dependencies.  These dependencies extend from the rcu_dereference()
+macro's load of the pointer to the later use of that pointer to compute
+either the address of a later memory access (representing an address
+dependency) or the value written by a later memory access (representing
+a data dependency).
 
-It is nevertheless possible to get into trouble with other operations.
+Most of the time, these dependencies are preserved, permitting you to
-Follow these rules to keep your RCU code working properly:
+freely use values from rcu_dereference().  For example, dereferencing
+(prefix "*"), field selection ("->"), assignment ("="), address-of
+("&"), casts, and addition or subtraction of constants all work quite
+naturally and safely.  However, because current compilers do not take
+either address or data dependencies into account it is still possible
+to get into trouble.
+
+Follow these rules to preserve the address and data dependencies emanating
+from your calls to rcu_dereference() and friends, thus keeping your RCU
+readers working properly:
 
 -	You must use one of the rcu_dereference() family of primitives
 	to load an RCU-protected pointer, otherwise CONFIG_PROVE_RCU

Documentation/RCU/torture.rst

@@ -185,7 +185,7 @@ 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,
+Large systems can run multiple copies 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::
 

Documentation/admin-guide/kernel-parameters.txt

@@ -5313,6 +5313,12 @@
 			Dump ftrace buffer after reporting RCU CPU
 			stall warning.
 
+	rcupdate.rcu_cpu_stall_notifiers= [KNL]
+			Provide RCU CPU stall notifiers, but see the
+			warnings in the RCU_CPU_STALL_NOTIFIER Kconfig
+			option's help text.  TL;DR:  You almost certainly
+			do not want rcupdate.rcu_cpu_stall_notifiers.
+
 	rcupdate.rcu_cpu_stall_suppress= [KNL]
 			Suppress RCU CPU stall warning messages.
 

Documentation/memory-barriers.txt

@@ -396,10 +396,11 @@ Memory barriers come in four basic varieties:
 
 
  (2) Address-dependency barriers (historical).
-     [!] This section is marked as HISTORICAL: For more up-to-date
+     [!] This section is marked as HISTORICAL: it covers the long-obsolete
-     information, including how compiler transformations related to pointer
+     smp_read_barrier_depends() macro, the semantics of which are now
-     comparisons can sometimes cause problems, see
+     implicit in all marked accesses.  For more up-to-date information,
-     Documentation/RCU/rcu_dereference.rst.
+     including how compiler transformations can sometimes break address
+     dependencies, see Documentation/RCU/rcu_dereference.rst.
 
      An address-dependency barrier is a weaker form of read barrier.  In the
      case where two loads are performed such that the second depends on the
@@ -560,9 +561,11 @@ There are certain things that the Linux kernel memory barriers do not guarantee:
 
 ADDRESS-DEPENDENCY BARRIERS (HISTORICAL)
 ----------------------------------------
-[!] This section is marked as HISTORICAL: For more up-to-date information,
+[!] This section is marked as HISTORICAL: it covers the long-obsolete
-including how compiler transformations related to pointer comparisons can
+smp_read_barrier_depends() macro, the semantics of which are now implicit
-sometimes cause problems, see Documentation/RCU/rcu_dereference.rst.
+in all marked accesses.  For more up-to-date information, including
+how compiler transformations can sometimes break address dependencies,
+see Documentation/RCU/rcu_dereference.rst.
 
 As of v4.15 of the Linux kernel, an smp_mb() was added to READ_ONCE() for
 DEC Alpha, which means that about the only people who need to pay attention

include/linux/rcu_notifier.h

@@ -13,7 +13,7 @@
 #define RCU_STALL_NOTIFY_NORM	1
 #define RCU_STALL_NOTIFY_EXP	2
 
-#ifdef CONFIG_RCU_STALL_COMMON
+#if defined(CONFIG_RCU_STALL_COMMON) && defined(CONFIG_RCU_CPU_STALL_NOTIFIER)
 
 #include <linux/notifier.h>
 #include <linux/types.h>
@@ -21,12 +21,12 @@
 int rcu_stall_chain_notifier_register(struct notifier_block *n);
 int rcu_stall_chain_notifier_unregister(struct notifier_block *n);
 
-#else // #ifdef CONFIG_RCU_STALL_COMMON
+#else // #if defined(CONFIG_RCU_STALL_COMMON) && defined(CONFIG_RCU_CPU_STALL_NOTIFIER)
 
 // No RCU CPU stall warnings in Tiny RCU.
 static inline int rcu_stall_chain_notifier_register(struct notifier_block *n) { return -EEXIST; }
 static inline int rcu_stall_chain_notifier_unregister(struct notifier_block *n) { return -ENOENT; }
 
-#endif // #else // #ifdef CONFIG_RCU_STALL_COMMON
+#endif // #else // #if defined(CONFIG_RCU_STALL_COMMON) && defined(CONFIG_RCU_CPU_STALL_NOTIFIER)
 
 #endif /* __LINUX_RCU_NOTIFIER_H */

include/linux/rculist.h

@@ -355,7 +355,7 @@ static inline void list_splice_tail_init_rcu(struct list_head *list,
 })
 
 /**
- * list_next_or_null_rcu - get the first element from a list
+ * list_next_or_null_rcu - get the next element from a list
  * @head:	the head for the list.
  * @ptr:        the list head to take the next element from.
  * @type:       the type of the struct this is embedded in.

include/linux/rcupdate.h

@@ -34,9 +34,6 @@
 
 #define ULONG_CMP_GE(a, b)	(ULONG_MAX / 2 >= (a) - (b))
 #define ULONG_CMP_LT(a, b)	(ULONG_MAX / 2 < (a) - (b))
-#define ulong2long(a)		(*(long *)(&(a)))
-#define USHORT_CMP_GE(a, b)	(USHRT_MAX / 2 >= (unsigned short)((a) - (b)))
-#define USHORT_CMP_LT(a, b)	(USHRT_MAX / 2 < (unsigned short)((a) - (b)))
 
 /* Exported common interfaces */
 void call_rcu(struct rcu_head *head, rcu_callback_t func);
@@ -301,6 +298,11 @@ static inline void rcu_lock_acquire(struct lockdep_map *map)
 	lock_acquire(map, 0, 0, 2, 0, NULL, _THIS_IP_);
 }
 
+static inline void rcu_try_lock_acquire(struct lockdep_map *map)
+{
+	lock_acquire(map, 0, 1, 2, 0, NULL, _THIS_IP_);
+}
+
 static inline void rcu_lock_release(struct lockdep_map *map)
 {
 	lock_release(map, _THIS_IP_);
@@ -315,6 +317,7 @@ int rcu_read_lock_any_held(void);
 #else /* #ifdef CONFIG_DEBUG_LOCK_ALLOC */
 
 # define rcu_lock_acquire(a)		do { } while (0)
+# define rcu_try_lock_acquire(a)	do { } while (0)
 # define rcu_lock_release(a)		do { } while (0)
 
 static inline int rcu_read_lock_held(void)

include/linux/srcu.h

@@ -229,7 +229,7 @@ static inline int srcu_read_lock_nmisafe(struct srcu_struct *ssp) __acquires(ssp
 
 	srcu_check_nmi_safety(ssp, true);
 	retval = __srcu_read_lock_nmisafe(ssp);
-	rcu_lock_acquire(&ssp->dep_map);
+	rcu_try_lock_acquire(&ssp->dep_map);
 	return retval;
 }
 

kernel/locking/locktorture.c

@@ -124,7 +124,7 @@ struct call_rcu_chain {
 	struct rcu_head crc_rh;
 	bool crc_stop;
 };
-struct call_rcu_chain *call_rcu_chain;
+struct call_rcu_chain *call_rcu_chain_list;
 
 /* Forward reference. */
 static void lock_torture_cleanup(void);
@@ -1074,12 +1074,12 @@ static int call_rcu_chain_init(void)
 
 	if (call_rcu_chains <= 0)
 		return 0;
-	call_rcu_chain = kcalloc(call_rcu_chains, sizeof(*call_rcu_chain), GFP_KERNEL);
+	call_rcu_chain_list = kcalloc(call_rcu_chains, sizeof(*call_rcu_chain_list), GFP_KERNEL);
-	if (!call_rcu_chain)
+	if (!call_rcu_chain_list)
 		return -ENOMEM;
 	for (i = 0; i < call_rcu_chains; i++) {
-		call_rcu_chain[i].crc_stop = false;
+		call_rcu_chain_list[i].crc_stop = false;
-		call_rcu(&call_rcu_chain[i].crc_rh, call_rcu_chain_cb);
+		call_rcu(&call_rcu_chain_list[i].crc_rh, call_rcu_chain_cb);
 	}
 	return 0;
 }
@@ -1089,13 +1089,13 @@ static void call_rcu_chain_cleanup(void)
 {
 	int i;
 
-	if (!call_rcu_chain)
+	if (!call_rcu_chain_list)
 		return;
 	for (i = 0; i < call_rcu_chains; i++)
-		smp_store_release(&call_rcu_chain[i].crc_stop, true);
+		smp_store_release(&call_rcu_chain_list[i].crc_stop, true);
 	rcu_barrier();
-	kfree(call_rcu_chain);
+	kfree(call_rcu_chain_list);
-	call_rcu_chain = NULL;
+	call_rcu_chain_list = NULL;
 }
 
 static void lock_torture_cleanup(void)

kernel/rcu/Kconfig.debug

@@ -105,6 +105,31 @@ config RCU_CPU_STALL_CPUTIME
 	  The boot option rcupdate.rcu_cpu_stall_cputime has the same function
 	  as this one, but will override this if it exists.
 
+config RCU_CPU_STALL_NOTIFIER
+	bool "Provide RCU CPU-stall notifiers"
+	depends on RCU_STALL_COMMON
+	depends on DEBUG_KERNEL
+	depends on RCU_EXPERT
+	default n
+	help
+	  WARNING:  You almost certainly do not want this!!!
+
+	  Enable RCU CPU-stall notifiers, which are invoked just before
+	  printing the RCU CPU stall warning.  As such, bugs in notifier
+	  callbacks can prevent stall warnings from being printed.
+	  And the whole reason that a stall warning is being printed is
+	  that something is hung up somewhere.	Therefore, the notifier
+	  callbacks must be written extremely carefully, preferably
+	  containing only lockless code.  After all, it is quite possible
+	  that the whole reason that the RCU CPU stall is happening in
+	  the first place is that someone forgot to release whatever lock
+	  that you are thinking of acquiring.  In which case, having your
+	  notifier callback acquire that lock will hang, preventing the
+	  RCU CPU stall warning from appearing.
+
+	  Say Y here if you want RCU CPU stall notifiers (you don't want them)
+	  Say N if you are unsure.
+
 config RCU_TRACE
 	bool "Enable tracing for RCU"
 	depends on DEBUG_KERNEL

kernel/rcu/rcu.h

@@ -262,6 +262,8 @@ static inline bool rcu_stall_is_suppressed_at_boot(void)
 	return rcu_cpu_stall_suppress_at_boot && !rcu_inkernel_boot_has_ended();
 }
 
+extern int rcu_cpu_stall_notifiers;
+
 #ifdef CONFIG_RCU_STALL_COMMON
 
 extern int rcu_cpu_stall_ftrace_dump;
@@ -659,10 +661,10 @@ static inline bool rcu_cpu_beenfullyonline(int cpu) { return true; }
 bool rcu_cpu_beenfullyonline(int cpu);
 #endif
 
-#ifdef CONFIG_RCU_STALL_COMMON
+#if defined(CONFIG_RCU_STALL_COMMON) && defined(CONFIG_RCU_CPU_STALL_NOTIFIER)
 int rcu_stall_notifier_call_chain(unsigned long val, void *v);
-#else // #ifdef CONFIG_RCU_STALL_COMMON
+#else // #if defined(CONFIG_RCU_STALL_COMMON) && defined(CONFIG_RCU_CPU_STALL_NOTIFIER)
 static inline int rcu_stall_notifier_call_chain(unsigned long val, void *v) { return NOTIFY_DONE; }
-#endif // #else // #ifdef CONFIG_RCU_STALL_COMMON
+#endif // #else // #if defined(CONFIG_RCU_STALL_COMMON) && defined(CONFIG_RCU_CPU_STALL_NOTIFIER)
 
 #endif /* __LINUX_RCU_H */

kernel/rcu/rcutorture.c

@@ -2450,10 +2450,12 @@ static int rcu_torture_stall(void *args)
 	unsigned long stop_at;
 
 	VERBOSE_TOROUT_STRING("rcu_torture_stall task started");
+	if (rcu_cpu_stall_notifiers) {
 		ret = rcu_stall_chain_notifier_register(&rcu_torture_stall_block);
 		if (ret)
 			pr_info("%s: rcu_stall_chain_notifier_register() returned %d, %sexpected.\n",
 				__func__, ret, !IS_ENABLED(CONFIG_RCU_STALL_COMMON) ? "un" : "");
+	}
 	if (stall_cpu_holdoff > 0) {
 		VERBOSE_TOROUT_STRING("rcu_torture_stall begin holdoff");
 		schedule_timeout_interruptible(stall_cpu_holdoff * HZ);
@@ -2497,7 +2499,7 @@ static int rcu_torture_stall(void *args)
 		cur_ops->readunlock(idx);
 	}
 	pr_alert("%s end.\n", __func__);
-	if (!ret) {
+	if (rcu_cpu_stall_notifiers && !ret) {
 		ret = rcu_stall_chain_notifier_unregister(&rcu_torture_stall_block);
 		if (ret)
 			pr_info("%s: rcu_stall_chain_notifier_unregister() returned %d.\n", __func__, ret);
@@ -3872,7 +3874,9 @@ rcu_torture_init(void)
 	}
 	if (fqs_duration < 0)
 		fqs_duration = 0;
-	if (fqs_duration) {
+	if (fqs_holdoff < 0)
+		fqs_holdoff = 0;
+	if (fqs_duration && fqs_holdoff) {
 		/* Create the fqs thread */
 		firsterr = torture_create_kthread(rcu_torture_fqs, NULL,
 						  fqs_task);

kernel/rcu/srcutree.c

@@ -772,20 +772,10 @@ EXPORT_SYMBOL_GPL(__srcu_read_unlock_nmisafe);
  */
 static void srcu_gp_start(struct srcu_struct *ssp)
 {
-	struct srcu_data *sdp;
 	int state;
 
-	if (smp_load_acquire(&ssp->srcu_sup->srcu_size_state) < SRCU_SIZE_WAIT_BARRIER)
-		sdp = per_cpu_ptr(ssp->sda, get_boot_cpu_id());
-	else
-		sdp = this_cpu_ptr(ssp->sda);
 	lockdep_assert_held(&ACCESS_PRIVATE(ssp->srcu_sup, lock));
 	WARN_ON_ONCE(ULONG_CMP_GE(ssp->srcu_sup->srcu_gp_seq, ssp->srcu_sup->srcu_gp_seq_needed));
-	spin_lock_rcu_node(sdp);  /* Interrupts already disabled. */
-	rcu_segcblist_advance(&sdp->srcu_cblist,
-			      rcu_seq_current(&ssp->srcu_sup->srcu_gp_seq));
-	WARN_ON_ONCE(!rcu_segcblist_segempty(&sdp->srcu_cblist, RCU_NEXT_TAIL));
-	spin_unlock_rcu_node(sdp);  /* Interrupts remain disabled. */
 	WRITE_ONCE(ssp->srcu_sup->srcu_gp_start, jiffies);
 	WRITE_ONCE(ssp->srcu_sup->srcu_n_exp_nodelay, 0);
 	smp_mb(); /* Order prior store to ->srcu_gp_seq_needed vs. GP start. */
@@ -1271,9 +1261,11 @@ static unsigned long srcu_gp_start_if_needed(struct srcu_struct *ssp,
 	 *     period (gp_num = X + 8). So acceleration fails.
 	 */
 	s = rcu_seq_snap(&ssp->srcu_sup->srcu_gp_seq);
+	if (rhp) {
 		rcu_segcblist_advance(&sdp->srcu_cblist,
 				      rcu_seq_current(&ssp->srcu_sup->srcu_gp_seq));
-	WARN_ON_ONCE(!rcu_segcblist_accelerate(&sdp->srcu_cblist, s) && rhp);
+		WARN_ON_ONCE(!rcu_segcblist_accelerate(&sdp->srcu_cblist, s));
+	}
 	if (ULONG_CMP_LT(sdp->srcu_gp_seq_needed, s)) {
 		sdp->srcu_gp_seq_needed = s;
 		needgp = true;
@@ -1723,6 +1715,11 @@ static void srcu_invoke_callbacks(struct work_struct *work)
 	WARN_ON_ONCE(!rcu_segcblist_segempty(&sdp->srcu_cblist, RCU_NEXT_TAIL));
 	rcu_segcblist_advance(&sdp->srcu_cblist,
 			      rcu_seq_current(&ssp->srcu_sup->srcu_gp_seq));
+	/*
+	 * Although this function is theoretically re-entrant, concurrent
+	 * callbacks invocation is disallowed to avoid executing an SRCU barrier
+	 * too early.
+	 */
 	if (sdp->srcu_cblist_invoking ||
 	    !rcu_segcblist_ready_cbs(&sdp->srcu_cblist)) {
 		spin_unlock_irq_rcu_node(sdp);
@@ -1753,6 +1750,7 @@ static void srcu_invoke_callbacks(struct work_struct *work)
 	sdp->srcu_cblist_invoking = false;
 	more = rcu_segcblist_ready_cbs(&sdp->srcu_cblist);
 	spin_unlock_irq_rcu_node(sdp);
+	/* An SRCU barrier or callbacks from previous nesting work pending */
 	if (more)
 		srcu_schedule_cbs_sdp(sdp, 0);
 }

kernel/rcu/tasks.h

@@ -975,7 +975,7 @@ static void check_holdout_task(struct task_struct *t,
 	    t->rcu_tasks_nvcsw != READ_ONCE(t->nvcsw) ||
 	    !rcu_tasks_is_holdout(t) ||
 	    (IS_ENABLED(CONFIG_NO_HZ_FULL) &&
-	     !is_idle_task(t) && t->rcu_tasks_idle_cpu >= 0)) {
+	     !is_idle_task(t) && READ_ONCE(t->rcu_tasks_idle_cpu) >= 0)) {
 		WRITE_ONCE(t->rcu_tasks_holdout, false);
 		list_del_init(&t->rcu_tasks_holdout_list);
 		put_task_struct(t);
@@ -993,7 +993,7 @@ static void check_holdout_task(struct task_struct *t,
 		 t, ".I"[is_idle_task(t)],
 		 "N."[cpu < 0 || !tick_nohz_full_cpu(cpu)],
 		 t->rcu_tasks_nvcsw, t->nvcsw, t->rcu_tasks_holdout,
-		 t->rcu_tasks_idle_cpu, cpu);
+		 data_race(t->rcu_tasks_idle_cpu), cpu);
 	sched_show_task(t);
 }
 

kernel/rcu/tree.c

@@ -2338,6 +2338,8 @@ void rcu_force_quiescent_state(void)
 	struct rcu_node *rnp;
 	struct rcu_node *rnp_old = NULL;
 
+	if (!rcu_gp_in_progress())
+		return;
 	/* Funnel through hierarchy to reduce memory contention. */
 	rnp = raw_cpu_read(rcu_data.mynode);
 	for (; rnp != NULL; rnp = rnp->parent) {

kernel/rcu/tree_stall.h

@@ -1061,6 +1061,7 @@ static int __init rcu_sysrq_init(void)
 }
 early_initcall(rcu_sysrq_init);
 
+#ifdef CONFIG_RCU_CPU_STALL_NOTIFIER
 
 //////////////////////////////////////////////////////////////////////////////
 //
@@ -1081,7 +1082,13 @@ static ATOMIC_NOTIFIER_HEAD(rcu_cpu_stall_notifier_list);
  */
 int rcu_stall_chain_notifier_register(struct notifier_block *n)
 {
+	int rcsn = rcu_cpu_stall_notifiers;
+
+	WARN(1, "Adding %pS() to RCU stall notifier list (%s).\n", n->notifier_call,
+	     rcsn ? "possibly suppressing RCU CPU stall warnings" : "failed, so all is well");
+	if (rcsn)
 		return atomic_notifier_chain_register(&rcu_cpu_stall_notifier_list, n);
+	return -EEXIST;
 }
 EXPORT_SYMBOL_GPL(rcu_stall_chain_notifier_register);
 
@@ -1115,3 +1122,5 @@ int rcu_stall_notifier_call_chain(unsigned long val, void *v)
 {
 	return atomic_notifier_call_chain(&rcu_cpu_stall_notifier_list, val, v);
 }
+
+#endif // #ifdef CONFIG_RCU_CPU_STALL_NOTIFIER

kernel/rcu/update.c

@@ -538,9 +538,15 @@ long torture_sched_setaffinity(pid_t pid, const struct cpumask *in_mask)
 EXPORT_SYMBOL_GPL(torture_sched_setaffinity);
 #endif
 
+int rcu_cpu_stall_notifiers __read_mostly; // !0 = provide stall notifiers (rarely useful)
+EXPORT_SYMBOL_GPL(rcu_cpu_stall_notifiers);
+
 #ifdef CONFIG_RCU_STALL_COMMON
 int rcu_cpu_stall_ftrace_dump __read_mostly;
 module_param(rcu_cpu_stall_ftrace_dump, int, 0644);
+#ifdef CONFIG_RCU_CPU_STALL_NOTIFIER
+module_param(rcu_cpu_stall_notifiers, int, 0444);
+#endif // #ifdef CONFIG_RCU_CPU_STALL_NOTIFIER
 int rcu_cpu_stall_suppress __read_mostly; // !0 = suppress stall warnings.
 EXPORT_SYMBOL_GPL(rcu_cpu_stall_suppress);
 module_param(rcu_cpu_stall_suppress, int, 0644);

tools/testing/selftests/rcutorture/bin/mkinitrd.sh

@@ -67,7 +67,10 @@ ___EOF___
 # build using nolibc on supported archs (smaller executable) and fall
 # back to regular glibc on other ones.
 if echo -e "#if __x86_64__||__i386__||__i486__||__i586__||__i686__" \
-	   "||__ARM_EABI__||__aarch64__||__s390x__||__loongarch__\nyes\n#endif" \
+	   "||__ARM_EABI__||__aarch64__||(__mips__ && _ABIO32)" \
+	   "||__powerpc__||(__riscv && __riscv_xlen == 64)" \
+	   "||__s390x__||__loongarch__" \
+	   "\nyes\n#endif" \
    | ${CROSS_COMPILE}gcc -E -nostdlib -xc - \
    | grep -q '^yes'; then
 	# architecture supported by nolibc

tools/testing/selftests/rcutorture/configs/rcu/TREE07.boot

@@ -1 +1,4 @@
 nohz_full=2-9
+rcutorture.stall_cpu=14
+rcutorture.stall_cpu_holdoff=90
+rcutorture.fwd_progress=0