From:   Neeraj Upadhyay <quic_neeraju@quicinc.com>
To:     <paulmck@kernel.org>, <frederic@kernel.org>, <dwmw@amazon.co.uk>,
        <josh@joshtriplett.org>, <rostedt@goodmis.org>,
        <mathieu.desnoyers@efficios.com>, <jiangshanlai@gmail.com>,
        <joel@joelfernandes.org>
CC:     <rcu@vger.kernel.org>, <linux-kernel@vger.kernel.org>,
        <urezki@gmail.com>, <boqun.feng@gmail.com>,
        Neeraj Upadhyay <quic_neeraju@quicinc.com>
Subject: [PATCH v2] rcu/nocb: Handle concurrent nocb kthreads creation
Date:   Sat, 11 Dec 2021 22:31:39 +0530
Message-ID: <20211211170139.27711-1-quic_neeraju@quicinc.com>
MIME-Version: 1.0
Content-Type: text/plain
Precedence: bulk

When multiple CPUs in the same nocb gp/cb group concurrently
come online, they might try to concurrently create the same
rcuog kthread. Fix this by using nocb gp CPU's spawn mutex to
provide mutual exclusion for the rcuog kthread creation code.

Signed-off-by: Neeraj Upadhyay <quic_neeraju@quicinc.com>
---
Change in v2:
 Fix missing mutex_unlock in nocb gp kthread creation err path.

 kernel/rcu/tree.h      | 2 ++
 kernel/rcu/tree_nocb.h | 7 ++++++-
 2 files changed, 8 insertions(+), 1 deletion(-)

diff --git a/kernel/rcu/tree.h b/kernel/rcu/tree.h
index 4b889081f4f4..9815b7844e58 100644
--- a/kernel/rcu/tree.h
+++ b/kernel/rcu/tree.h
@@ -204,6 +204,8 @@ struct rcu_data {
 	int nocb_defer_wakeup;		/* Defer wakeup of nocb_kthread. */
 	struct timer_list nocb_timer;	/* Enforce finite deferral. */
 	unsigned long nocb_gp_adv_time;	/* Last call_rcu() CB adv (jiffies). */
+	struct mutex nocb_gp_kthread_mutex; /* Exclusion for nocb gp kthread */
+					    /* spawning */
 
 	/* The following fields are used by call_rcu, hence own cacheline. */
 	raw_spinlock_t nocb_bypass_lock ____cacheline_internodealigned_in_smp;
diff --git a/kernel/rcu/tree_nocb.h b/kernel/rcu/tree_nocb.h
index eeafb546a7a0..dd4753219dca 100644
--- a/kernel/rcu/tree_nocb.h
+++ b/kernel/rcu/tree_nocb.h
@@ -1226,6 +1226,7 @@ static void __init rcu_boot_init_nocb_percpu_data(struct rcu_data *rdp)
 	raw_spin_lock_init(&rdp->nocb_gp_lock);
 	timer_setup(&rdp->nocb_timer, do_nocb_deferred_wakeup_timer, 0);
 	rcu_cblist_init(&rdp->nocb_bypass);
+	mutex_init(&rdp->nocb_gp_kthread_mutex);
 }
 
 /*
@@ -1248,13 +1249,17 @@ static void rcu_spawn_cpu_nocb_kthread(int cpu)
 
 	/* If we didn't spawn the GP kthread first, reorganize! */
 	rdp_gp = rdp->nocb_gp_rdp;
+	mutex_lock(&rdp_gp->nocb_gp_kthread_mutex);
 	if (!rdp_gp->nocb_gp_kthread) {
 		t = kthread_run(rcu_nocb_gp_kthread, rdp_gp,
 				"rcuog/%d", rdp_gp->cpu);
-		if (WARN_ONCE(IS_ERR(t), "%s: Could not start rcuo GP kthread, OOM is now expected behavior\n", __func__))
+		if (WARN_ONCE(IS_ERR(t), "%s: Could not start rcuo GP kthread, OOM is now expected behavior\n", __func__)) {
+			mutex_unlock(&rdp_gp->nocb_gp_kthread_mutex);
 			return;
+		}
 		WRITE_ONCE(rdp_gp->nocb_gp_kthread, t);
 	}
+	 mutex_unlock(&rdp_gp->nocb_gp_kthread_mutex);
 
 	/* Spawn the kthread for this CPU. */
 	t = kthread_run(rcu_nocb_cb_kthread, rdp,
-- 
2.17.1