rcu: Add RCU CPU stall notifier

It is sometimes helpful to have a way for the subsystem causing
the stall to dump its state when an RCU CPU stall occurs.  This
commit therefore bases rcu_stall_chain_notifier_register() and
rcu_stall_chain_notifier_unregister() on atomic notifiers in order to
provide this functionality.
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Cc: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Frederic Weisbecker <frederic@kernel.org>

include/linux/rcu_notifier.h

+/* SPDX-License-Identifier: GPL-2.0+ */
+/*
+ * Read-Copy Update notifiers, initially RCU CPU stall notifier.
+ * Separate from rcupdate.h to avoid #include loops.
+ *
+ * Copyright (C) 2023 Paul E. McKenney.
+ */
+
+#ifndef __LINUX_RCU_NOTIFIER_H
+#define __LINUX_RCU_NOTIFIER_H
+
+// Actions for RCU CPU stall notifier calls.
+#define RCU_STALL_NOTIFY_NORM	1
+#define RCU_STALL_NOTIFY_EXP	2
+
+#ifdef CONFIG_RCU_STALL_COMMON
+
+#include <linux/notifier.h>
+#include <linux/types.h>
+
+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
+
+// 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 /* __LINUX_RCU_NOTIFIER_H */

kernel/rcu/rcu.h

@@ -654,4 +654,10 @@ static inline bool rcu_cpu_beenfullyonline(int cpu) { return true; }
 bool rcu_cpu_beenfullyonline(int cpu);
 #endif
 
+#ifdef CONFIG_RCU_STALL_COMMON
+int rcu_stall_notifier_call_chain(unsigned long val, void *v);
+#else // #ifdef CONFIG_RCU_STALL_COMMON
+static inline int rcu_stall_notifier_call_chain(unsigned long val, void *v) { return NOTIFY_DONE; }
+#endif // #else // #ifdef CONFIG_RCU_STALL_COMMON
+
 #endif /* __LINUX_RCU_H */

kernel/rcu/tree_exp.h

@@ -621,10 +621,14 @@ static void synchronize_rcu_expedited_wait(void)
 	}
 
 	for (;;) {
+		unsigned long j;
+
 		if (synchronize_rcu_expedited_wait_once(jiffies_stall))
 			return;
 		if (rcu_stall_is_suppressed())
 			continue;
+		j = jiffies;
+		rcu_stall_notifier_call_chain(RCU_STALL_NOTIFY_EXP, (void *)(j - jiffies_start));
 		trace_rcu_stall_warning(rcu_state.name, TPS("ExpeditedStall"));
 		pr_err("INFO: %s detected expedited stalls on CPUs/tasks: {",
 		       rcu_state.name);
@@ -647,7 +651,7 @@ static void synchronize_rcu_expedited_wait(void)
 			}
 		}
 		pr_cont(" } %lu jiffies s: %lu root: %#lx/%c\n",
-			jiffies - jiffies_start, rcu_state.expedited_sequence,
+			j - jiffies_start, rcu_state.expedited_sequence,
 			data_race(rnp_root->expmask),
 			".T"[!!data_race(rnp_root->exp_tasks)]);
 		if (ndetected) {

kernel/rcu/tree_stall.h

@@ -8,6 +8,7 @@
  */
 
 #include <linux/kvm_para.h>
+#include <linux/rcu_notifier.h>
 
 //////////////////////////////////////////////////////////////////////////////
 //
@@ -770,6 +771,7 @@ static void check_cpu_stall(struct rcu_data *rdp)
 		if (kvm_check_and_clear_guest_paused())
 			return;
 
+		rcu_stall_notifier_call_chain(RCU_STALL_NOTIFY_NORM, (void *)j - gps);
 		if (self_detected) {
 			/* We haven't checked in, so go dump stack. */
 			print_cpu_stall(gps);
@@ -790,7 +792,7 @@ static void check_cpu_stall(struct rcu_data *rdp)
 
 //////////////////////////////////////////////////////////////////////////////
 //
-// RCU forward-progress mechanisms, including of callback invocation.
+// RCU forward-progress mechanisms, including for callback invocation.
 
 
 /*
@@ -1042,3 +1044,58 @@ static int __init rcu_sysrq_init(void)
 	return 0;
 }
 early_initcall(rcu_sysrq_init);
+
+
+//////////////////////////////////////////////////////////////////////////////
+//
+// RCU CPU stall-warning notifiers
+
+static ATOMIC_NOTIFIER_HEAD(rcu_cpu_stall_notifier_list);
+
+/**
+ * rcu_stall_chain_notifier_register - Add an RCU CPU stall notifier
+ * @n: Entry to add.
+ *
+ * Adds an RCU CPU stall notifier to an atomic notifier chain.
+ * The @action passed to a notifier will be @RCU_STALL_NOTIFY_NORM or
+ * friends.  The @data will be the duration of the stalled grace period,
+ * in jiffies, coerced to a void* pointer.
+ *
+ * Returns 0 on success, %-EEXIST on error.
+ */
+int rcu_stall_chain_notifier_register(struct notifier_block *n)
+{
+	return atomic_notifier_chain_register(&rcu_cpu_stall_notifier_list, n);
+}
+EXPORT_SYMBOL_GPL(rcu_stall_chain_notifier_register);
+
+/**
+ * rcu_stall_chain_notifier_unregister - Remove an RCU CPU stall notifier
+ * @n: Entry to add.
+ *
+ * Removes an RCU CPU stall notifier from an atomic notifier chain.
+ *
+ * Returns zero on success, %-ENOENT on failure.
+ */
+int rcu_stall_chain_notifier_unregister(struct notifier_block *n)
+{
+	return atomic_notifier_chain_unregister(&rcu_cpu_stall_notifier_list, n);
+}
+EXPORT_SYMBOL_GPL(rcu_stall_chain_notifier_unregister);
+
+/*
+ * rcu_stall_notifier_call_chain - Call functions in an RCU CPU stall notifier chain
+ * @val: Value passed unmodified to notifier function
+ * @v: Pointer passed unmodified to notifier function
+ *
+ * Calls each function in the RCU CPU stall notifier chain in turn, which
+ * is an atomic call chain.  See atomic_notifier_call_chain() for more
+ * information.
+ *
+ * This is for use within RCU, hence the omission of the extra asterisk
+ * to indicate a non-kerneldoc format header comment.
+ */
+int rcu_stall_notifier_call_chain(unsigned long val, void *v)
+{
+	return atomic_notifier_call_chain(&rcu_cpu_stall_notifier_list, val, v);
+}