[PATCH tip/core/rcu 4/6] srcu: Initialize SRCU after timers

["Paul E. McKenney" <paulmck@kernel.org>]

From: Frederic Weisbecker <frederic@kernel.org>

Once srcu_init() is called, the SRCU core will make use of delayed
workqueues, which rely on timers.  However init_timers() is called
several steps after rcu_init().  This means that a call_srcu() after
rcu_init() but before init_timers() would find itself within a dangerously
uninitialized timer core.

This commit therefore creates a separate call to srcu_init() after
init_timer() completes, which ensures that we stay in early SRCU mode
until timers are safe(r).

Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Cc: Uladzislau Rezki <urezki@gmail.com>
Cc: Boqun Feng <boqun.feng@gmail.com>
Cc: Lai Jiangshan <jiangshanlai@gmail.com>
Cc: Neeraj Upadhyay <neeraju@codeaurora.org>
Cc: Josh Triplett <josh@joshtriplett.org>
Cc: Joel Fernandes <joel@joelfernandes.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
---
 include/linux/srcu.h  | 6 ++++++
 init/main.c           | 2 ++
 kernel/rcu/rcu.h      | 6 ------
 kernel/rcu/srcutree.c | 5 +++++
 kernel/rcu/tiny.c     | 1 -
 kernel/rcu/tree.c     | 1 -
 6 files changed, 13 insertions(+), 8 deletions(-)

diff --git a/include/linux/srcu.h b/include/linux/srcu.h
index a0895bbf71ce..e6011a9975af 100644
--- a/include/linux/srcu.h
+++ b/include/linux/srcu.h
@@ -64,6 +64,12 @@ unsigned long get_state_synchronize_srcu(struct srcu_struct *ssp);
 unsigned long start_poll_synchronize_srcu(struct srcu_struct *ssp);
 bool poll_state_synchronize_srcu(struct srcu_struct *ssp, unsigned long cookie);
 
+#ifdef CONFIG_SRCU
+void srcu_init(void);
+#else /* #ifdef CONFIG_SRCU */
+static inline void srcu_init(void) { }
+#endif /* #else #ifdef CONFIG_SRCU */
+
 #ifdef CONFIG_DEBUG_LOCK_ALLOC
 
 /**
diff --git a/init/main.c b/init/main.c
index eb01e121d2f1..7b6f49c4d388 100644
--- a/init/main.c
+++ b/init/main.c
@@ -42,6 +42,7 @@
 #include <linux/profile.h>
 #include <linux/kfence.h>
 #include <linux/rcupdate.h>
+#include <linux/srcu.h>
 #include <linux/moduleparam.h>
 #include <linux/kallsyms.h>
 #include <linux/writeback.h>
@@ -979,6 +980,7 @@ asmlinkage __visible void __init __no_sanitize_address start_kernel(void)
 	tick_init();
 	rcu_init_nohz();
 	init_timers();
+	srcu_init();
 	hrtimers_init();
 	softirq_init();
 	timekeeping_init();
diff --git a/kernel/rcu/rcu.h b/kernel/rcu/rcu.h
index bf0827d4b659..ca3f2af32bf8 100644
--- a/kernel/rcu/rcu.h
+++ b/kernel/rcu/rcu.h
@@ -422,12 +422,6 @@ do {									\
 
 #endif /* #if defined(CONFIG_SRCU) || !defined(CONFIG_TINY_RCU) */
 
-#ifdef CONFIG_SRCU
-void srcu_init(void);
-#else /* #ifdef CONFIG_SRCU */
-static inline void srcu_init(void) { }
-#endif /* #else #ifdef CONFIG_SRCU */
-
 #ifdef CONFIG_TINY_RCU
 /* Tiny RCU doesn't expedite, as its purpose in life is instead to be tiny. */
 static inline bool rcu_gp_is_normal(void) { return true; }
diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c
index f4f0cbf7a02b..9ec35c158740 100644
--- a/kernel/rcu/srcutree.c
+++ b/kernel/rcu/srcutree.c
@@ -1384,6 +1384,11 @@ void __init srcu_init(void)
 {
 	struct srcu_struct *ssp;
 
+	/*
+	 * Once that is set, call_srcu() can follow the normal path and
+	 * queue delayed work. This must follow RCU workqueues creation
+	 * and timers initialization.
+	 */
 	srcu_init_done = true;
 	while (!list_empty(&srcu_boot_list)) {
 		ssp = list_first_entry(&srcu_boot_list, struct srcu_struct,
diff --git a/kernel/rcu/tiny.c b/kernel/rcu/tiny.c
index c8a029fbb114..340b3f8b090d 100644
--- a/kernel/rcu/tiny.c
+++ b/kernel/rcu/tiny.c
@@ -221,5 +221,4 @@ void __init rcu_init(void)
 {
 	open_softirq(RCU_SOFTIRQ, rcu_process_callbacks);
 	rcu_early_boot_tests();
-	srcu_init();
 }
diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 8e78b2430c16..c35b15229cff 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -4735,7 +4735,6 @@ void __init rcu_init(void)
 	WARN_ON(!rcu_gp_wq);
 	rcu_par_gp_wq = alloc_workqueue("rcu_par_gp", WQ_MEM_RECLAIM, 0);
 	WARN_ON(!rcu_par_gp_wq);
-	srcu_init();
 
 	/* Fill in default value for rcutree.qovld boot parameter. */
 	/* -After- the rcu_node ->lock fields are initialized! */
-- 
2.31.1.189.g2e36527f23