[PATCH] rcu: Make rcu_state.n_online_cpus updates atomic

[Neeraj Upadhyay <quic_neeraju@quicinc.com>]

To support onlining multiple CPUs concurrently,
change rcu_state.n_online_cpus updates to be atomic.
Note, it's ok for rcu_blocking_is_gp() to do a
atomic_read(&rcu_state.n_online_cpus), as the
value of .n_online_cpus switches from 1->2, in
rcutree_prepare_cpu(), which runs before the new
CPU comes online. Similarly 2->1 transition happens
from rcutree_dead_cpu(), which executes after the
CPU is offlined, and runs on the last online CPU.

Signed-off-by: Neeraj Upadhyay <quic_neeraju@quicinc.com>
---
 kernel/rcu/tree.c | 6 +++---
 kernel/rcu/tree.h | 2 +-
 2 files changed, 4 insertions(+), 4 deletions(-)

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 3da7826865f7..c1db01c4ea39 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -2460,7 +2460,7 @@ int rcutree_dead_cpu(unsigned int cpu)
 	if (!IS_ENABLED(CONFIG_HOTPLUG_CPU))
 		return 0;
 
-	WRITE_ONCE(rcu_state.n_online_cpus, rcu_state.n_online_cpus - 1);
+	atomic_dec(&rcu_state.n_online_cpus);
 	/* Adjust any no-longer-needed kthreads. */
 	rcu_boost_kthread_setaffinity(rnp, -1);
 	// Stop-machine done, so allow nohz_full to disable tick.
@@ -3740,7 +3740,7 @@ static int rcu_blocking_is_gp(void)
 	 * in the code, without the need for additional memory barriers.
 	 * Those memory barriers are provided by CPU-hotplug code.
 	 */
-	ret = READ_ONCE(rcu_state.n_online_cpus) <= 1;
+	ret = atomic_read(&rcu_state.n_online_cpus) <= 1;
 	preempt_enable();
 	return ret;
 }
@@ -4199,7 +4199,7 @@ int rcutree_prepare_cpu(unsigned int cpu)
 	raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
 	rcu_spawn_one_boost_kthread(rnp);
 	rcu_spawn_cpu_nocb_kthread(cpu);
-	WRITE_ONCE(rcu_state.n_online_cpus, rcu_state.n_online_cpus + 1);
+	atomic_inc(&rcu_state.n_online_cpus);
 
 	return 0;
 }
diff --git a/kernel/rcu/tree.h b/kernel/rcu/tree.h
index 4b889081f4f4..f1017e7e1e9e 100644
--- a/kernel/rcu/tree.h
+++ b/kernel/rcu/tree.h
@@ -299,7 +299,7 @@ struct rcu_state {
 						/* Hierarchy levels (+1 to */
 						/*  shut bogus gcc warning) */
 	int ncpus;				/* # CPUs seen so far. */
-	int n_online_cpus;			/* # CPUs online for RCU. */
+	atomic_t n_online_cpus;			/* # CPUs online for RCU. */
 
 	/* The following fields are guarded by the root rcu_node's lock. */
 
-- 
2.17.1