[PATCH 1/3] rcu: Use static initializer for krc.lock

[Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx>]

The per-CPU variable is initialized at runtime in
kfree_rcu_batch_init(). This function is invoked before
`rcu_scheduler_active' is set to `RCU_SCHEDULER_RUNNING'. After the
initialisation, `->initialized' is to true.

The spin_lock is only acquired if `->initialized' is set to true. The
worqueue item is only used if `rcu_scheduler_active' set to
RCU_SCHEDULER_RUNNING' which happens after initialisation.

Use a static initializer for krc.lock and remove the runtime
initialisation of the lock. Since the lock can now be always acquired,
remove the `->initialized' check.
The local_irq_save() is removed and raw_cpu_ptr() + spin_lock_irqsave()
is used. The worst case scenario is that after raw_cpu_ptr() the code
has been moved to another CPU. This is "okay" because the data strucure
itself is protected with a lock.
Add a warning in kfree_rcu_batch_init() to ensure that this function is
invoked before `RCU_SCHEDULER_RUNNING' to ensure that the worker is not
used earlier.

Reported-by: Mike Galbraith <efault@xxxxxx>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx>
---
 kernel/rcu/tree.c | 22 +++++++---------------
 1 file changed, 7 insertions(+), 15 deletions(-)

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index f288477ee1c26..5b0b63dd04b02 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -2893,7 +2893,6 @@ struct kfree_rcu_cpu_work {
  * @lock: Synchronize access to this structure
  * @monitor_work: Promote @head to @head_free after KFREE_DRAIN_JIFFIES
  * @monitor_todo: Tracks whether a @monitor_work delayed work is pending
- * @initialized: The @lock and @rcu_work fields have been initialized
  *
  * This is a per-CPU structure.  The reason that it is not included in
  * the rcu_data structure is to permit this code to be extracted from
@@ -2908,12 +2907,13 @@ struct kfree_rcu_cpu {
 	spinlock_t lock;
 	struct delayed_work monitor_work;
 	bool monitor_todo;
-	bool initialized;
 	// Number of objects for which GP not started
 	int count;
 };
 
-static DEFINE_PER_CPU(struct kfree_rcu_cpu, krc);
+static DEFINE_PER_CPU(struct kfree_rcu_cpu, krc) = {
+	.lock	= __SPIN_LOCK_UNLOCKED(krc.lock),
+};
 
 static __always_inline void
 debug_rcu_head_unqueue_bulk(struct rcu_head *head)
@@ -3080,9 +3080,6 @@ kfree_call_rcu_add_ptr_to_bulk(struct kfree_rcu_cpu *krcp,
 {
 	struct kfree_rcu_bulk_data *bnode;
 
-	if (unlikely(!krcp->initialized))
-		return false;
-
 	lockdep_assert_held(&krcp->lock);
 
 	/* Check if a new block is required. */
@@ -3139,10 +3136,8 @@ void kfree_call_rcu(struct rcu_head *head, rcu_callback_t func)
 	unsigned long flags;
 	struct kfree_rcu_cpu *krcp;
 
-	local_irq_save(flags);	// For safely calling this_cpu_ptr().
-	krcp = this_cpu_ptr(&krc);
-	if (krcp->initialized)
-		spin_lock(&krcp->lock);
+	krcp = raw_cpu_ptr(&krc);
+	spin_lock_irqsave(&krcp->lock, flags);
 
 	// Queue the object but don't yet schedule the batch.
 	if (debug_rcu_head_queue(head)) {
@@ -3172,9 +3167,7 @@ void kfree_call_rcu(struct rcu_head *head, rcu_callback_t func)
 	}
 
 unlock_return:
-	if (krcp->initialized)
-		spin_unlock(&krcp->lock);
-	local_irq_restore(flags);
+	spin_unlock_irqrestore(&krcp->lock, flags);
 }
 EXPORT_SYMBOL_GPL(kfree_call_rcu);
 
@@ -4137,17 +4130,16 @@ static void __init kfree_rcu_batch_init(void)
 	int cpu;
 	int i;
 
+	WARN_ON(rcu_scheduler_active == RCU_SCHEDULER_RUNNING);
 	for_each_possible_cpu(cpu) {
 		struct kfree_rcu_cpu *krcp = per_cpu_ptr(&krc, cpu);
 
-		spin_lock_init(&krcp->lock);
 		for (i = 0; i < KFREE_N_BATCHES; i++) {
 			INIT_RCU_WORK(&krcp->krw_arr[i].rcu_work, kfree_rcu_work);
 			krcp->krw_arr[i].krcp = krcp;
 		}
 
 		INIT_DELAYED_WORK(&krcp->monitor_work, kfree_rcu_monitor);
-		krcp->initialized = true;
 	}
 	if (register_shrinker(&kfree_rcu_shrinker))
 		pr_err("Failed to register kfree_rcu() shrinker!\n");
-- 
2.26.0