rcu/nocb: Handle concurrent nocb kthreads creation

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.

[ paulmck: Whitespace fixes per kernel test robot feedback. ]

Acked-by: David Woodhouse <dwmw@amazon.co.uk>
Signed-off-by: Neeraj Upadhyay <quic_neeraju@quicinc.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

diff --git a/kernel/rcu/tree.h b/kernel/rcu/tree.h
index 486fc901bd085ff1e57a0f7d12988c8bd1c879c2..24dd4b0d805f12dfb11134bc42d02945294b55b5 100644 (file)
--- a/kernel/rcu/tree.h
+++ b/kernel/rcu/tree.h
@@ -203,6 +203,8 @@ 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 eeafb546a7a09f8c790f8f0ec251f374187bbbf7..1e40519d1a05aeaeb7ca4755fc5d1b284610a475 100644 (file)
--- 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,