From: David Woodhouse <dwmw@amazon.co.uk>
Date: Wed, 8 Dec 2021 23:41:53 +0000 (+0000)
Subject: rcu: Add mutex for rcu boost kthread spawning and affinity setting
X-Git-Url: https://www.infradead.org/git/?a=commitdiff_plain;h=a87edff9f3ba07238ba6f2b5ca29e294f2ec7307;p=users%2Fdwmw2%2Flinux.git

rcu: Add mutex for rcu boost kthread spawning and affinity setting

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@amazon.co.uk>
Acked-by: Paul E. McKenney <paulmck@kernel.org>
---

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index f690cd1e2b259..9284abbcf8b29 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -4532,6 +4532,7 @@ 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 aff4cc9303fb4..055e30b3e5e0d 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 5199559fbbf0c..3b4ee0933710b 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);
 }