[RFC 06/10] rcu/hotplug: Make rcutree_dead_cpu() parallel

[Pingfan Liu]

In order to support parallel, rcu_state.n_online_cpus should be
atomic_dec()

Signed-off-by: Pingfan Liu <kernelfans@xxxxxxxxx>
Cc: "Paul E. McKenney" <paulmck@xxxxxxxxxx>
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: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Steven Price <steven.price@xxxxxxx>
Cc: "Peter Zijlstra
Cc: Mark Rutland <mark.rutland@xxxxxxx>
Cc: Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@xxxxxxxxxxxxxxx>
Cc: "Jason A. Donenfeld" <Jason@xxxxxxxxx>
To: linux-kernel@xxxxxxxxxxxxxxx
To: rcu@xxxxxxxxxxxxxxx
---
 kernel/cpu.c      | 1 +
 kernel/rcu/tree.c | 3 ++-
 2 files changed, 3 insertions(+), 1 deletion(-)

diff --git a/kernel/cpu.c b/kernel/cpu.c
index 1261c3f3be51..90debbe28e85 100644
--- a/kernel/cpu.c
+++ b/kernel/cpu.c
@@ -1872,6 +1872,7 @@ static struct cpuhp_step cpuhp_hp_states[] = {
 		.name			= "RCU/tree:prepare",
 		.startup.single		= rcutree_prepare_cpu,
 		.teardown.single	= rcutree_dead_cpu,
+		.support_kexec_parallel	= true,
 	},
 	/*
 	 * On the tear-down path, timers_dead_cpu() must be invoked
diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 79aea7df4345..07d31e16c65e 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -2168,7 +2168,8 @@ 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);
+	/* Hot remove path allows parallel, while Hot add races against remove on lock */
+	atomic_dec((atomic_t *)&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.
-- 
2.31.1