[PATCH 03/11] rcu: Add mutex for rcu boost kthread spawning and affinity setting

[David Woodhouse <dwmw2@xxxxxxxxxxxxx>]

From: David Woodhouse <dwmw@xxxxxxxxxxxx>

As we handle parallel CPU bringup, we will need to take care to avoid
spawning multiple boost threads, or race conditions when setting their
affinity. Spotted by Paul McKenney.

Signed-off-by: David Woodhouse <dwmw@xxxxxxxxxxxx>
---
 kernel/rcu/tree.c        |  1 +
 kernel/rcu/tree.h        |  3 +++
 kernel/rcu/tree_plugin.h | 10 ++++++++--
 3 files changed, 12 insertions(+), 2 deletions(-)

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index a1bb0b1229ed..809855474b39 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -4527,6 +4527,7 @@ static void __init rcu_init_one(void)
 			init_waitqueue_head(&rnp->exp_wq[2]);
 			init_waitqueue_head(&rnp->exp_wq[3]);
 			spin_lock_init(&rnp->exp_lock);
+			mutex_init(&rnp->boost_kthread_mutex);
 		}
 	}
 
diff --git a/kernel/rcu/tree.h b/kernel/rcu/tree.h
index aff4cc9303fb..055e30b3e5e0 100644
--- a/kernel/rcu/tree.h
+++ b/kernel/rcu/tree.h
@@ -108,6 +108,9 @@ struct rcu_node {
 				/*  side effect, not as a lock. */
 	unsigned long boost_time;
 				/* When to start boosting (jiffies). */
+	struct mutex boost_kthread_mutex;
+				/* Exclusion for thread spawning and affinity */
+				/*  manipulation. */
 	struct task_struct *boost_kthread_task;
 				/* kthread that takes care of priority */
 				/*  boosting for this rcu_node structure. */
diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index 5199559fbbf0..3b4ee0933710 100644
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -1162,15 +1162,16 @@ static void rcu_spawn_one_boost_kthread(struct rcu_node *rnp)
 	struct sched_param sp;
 	struct task_struct *t;
 
+	mutex_lock(&rnp->boost_kthread_mutex);
 	if (rnp->boost_kthread_task || !rcu_scheduler_fully_active)
-		return;
+		goto out;
 
 	rcu_state.boost = 1;
 
 	t = kthread_create(rcu_boost_kthread, (void *)rnp,
 			   "rcub/%d", rnp_index);
 	if (WARN_ON_ONCE(IS_ERR(t)))
-		return;
+		goto out;
 
 	raw_spin_lock_irqsave_rcu_node(rnp, flags);
 	rnp->boost_kthread_task = t;
@@ -1178,6 +1179,9 @@ static void rcu_spawn_one_boost_kthread(struct rcu_node *rnp)
 	sp.sched_priority = kthread_prio;
 	sched_setscheduler_nocheck(t, SCHED_FIFO, &sp);
 	wake_up_process(t); /* get to TASK_INTERRUPTIBLE quickly. */
+
+ out:
+	mutex_unlock(&rnp->boost_kthread_mutex);
 }
 
 /*
@@ -1200,6 +1204,7 @@ static void rcu_boost_kthread_setaffinity(struct rcu_node *rnp, int outgoingcpu)
 		return;
 	if (!zalloc_cpumask_var(&cm, GFP_KERNEL))
 		return;
+	mutex_lock(&rnp->boost_kthread_mutex);
 	for_each_leaf_node_possible_cpu(rnp, cpu)
 		if ((mask & leaf_node_cpu_bit(rnp, cpu)) &&
 		    cpu != outgoingcpu)
@@ -1207,6 +1212,7 @@ static void rcu_boost_kthread_setaffinity(struct rcu_node *rnp, int outgoingcpu)
 	if (cpumask_weight(cm) == 0)
 		cpumask_setall(cm);
 	set_cpus_allowed_ptr(t, cm);
+	mutex_unlock(&rnp->boost_kthread_mutex);
 	free_cpumask_var(cm);
 }
 
-- 
2.31.1