[PATCHv2 1/3] rcu: Keep qsmaskinitnext fresh when rcutree_online_cpu()

[Pingfan Liu <kernelfans@xxxxxxxxx>]

rcutree_online_cpu() is concurrent, so there is the following race
scene:

        CPU 1                               CPU2
mask_old = rcu_rnp_online_cpus(rnp);
...

                                   mask_new = rcu_rnp_online_cpus(rnp);
                                   ...
                                   set_cpus_allowed_ptr(t, cm);

set_cpus_allowed_ptr(t, cm);

Consequently, the old mask will overwrite the new mask in the task's cpus_ptr.

Since there is a mutex ->boost_kthread_mutex, using it to build an
order, then the latest ->qsmaskinitnext will be fetched for updating cpus_ptr.

But for concurrent offlining, ->qsmaskinitnext is not reilable when
rcutree_offline_cpu(). That is another story and comes in the following
patch.

Signed-off-by: Pingfan Liu <kernelfans@xxxxxxxxx>
Cc: "Paul E. McKenney" <paulmck@xxxxxxxxxx>
Cc: David Woodhouse <dwmw@xxxxxxxxxxxx>
Cc: Frederic Weisbecker <frederic@xxxxxxxxxx>
Cc: Neeraj Upadhyay <quic_neeraju@xxxxxxxxxxx>
Cc: Josh Triplett <josh@xxxxxxxxxxxxxxxx>
Cc: Steven Rostedt <rostedt@xxxxxxxxxxx>
Cc: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx>
Cc: Lai Jiangshan <jiangshanlai@xxxxxxxxx>
Cc: Joel Fernandes <joel@xxxxxxxxxxxxxxxxx>
Cc: "Jason A. Donenfeld" <Jason@xxxxxxxxx>
To: rcu@xxxxxxxxxxxxxxx
---
 kernel/rcu/tree_plugin.h | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index 438ecae6bd7e..ef6d3ae239b9 100644
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -1224,7 +1224,7 @@ static void rcu_spawn_one_boost_kthread(struct rcu_node *rnp)
 static void rcu_boost_kthread_setaffinity(struct rcu_node *rnp, int outgoingcpu)
 {
 	struct task_struct *t = rnp->boost_kthread_task;
-	unsigned long mask = rcu_rnp_online_cpus(rnp);
+	unsigned long mask;
 	cpumask_var_t cm;
 	int cpu;
 
@@ -1233,6 +1233,11 @@ static void rcu_boost_kthread_setaffinity(struct rcu_node *rnp, int outgoingcpu)
 	if (!zalloc_cpumask_var(&cm, GFP_KERNEL))
 		return;
 	mutex_lock(&rnp->boost_kthread_mutex);
+	/*
+	 * Relying on the lock to serialize, so when onlining, the latest
+	 * qsmaskinitnext is for cpus_ptr.
+	 */
+	mask = rcu_rnp_online_cpus(rnp);
 	for_each_leaf_node_possible_cpu(rnp, cpu)
 		if ((mask & leaf_node_cpu_bit(rnp, cpu)) &&
 		    cpu != outgoingcpu)
-- 
2.31.1