rcu: Create call_rcu_tasks() kthread at boot time

Currently the call_rcu_tasks() kthread is created upon first
invocation of call_rcu_tasks().  This has the advantage of avoiding
creation if there are never any invocations of call_rcu_tasks() and of
synchronize_rcu_tasks(), but it requires an unreliable heuristic to
determine when it is safe to create the kthread.  For example, it is
not safe to create the kthread when call_rcu_tasks() is invoked with
a spinlock held, but there is no good way to detect this in !PREEMPT
kernels.

This commit therefore creates this kthread unconditionally at
core_initcall() time.  If you don't want this kthread created, then
build with CONFIG_TASKS_RCU=n.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

diff --git a/kernel/rcu/update.c b/kernel/rcu/update.c
index 5033b66..e9bbedb 100644 (file)
--- a/kernel/rcu/update.c
+++ b/kernel/rcu/update.c
@@ -575,7 +575,6 @@ DEFINE_STATIC_SRCU(tasks_rcu_exit_srcu);
 static int rcu_task_stall_timeout __read_mostly = RCU_TASK_STALL_TIMEOUT;
 module_param(rcu_task_stall_timeout, int, 0644);
 
-static void rcu_spawn_tasks_kthread(void);
 static struct task_struct *rcu_tasks_kthread_ptr;
 
 /**
@@ -600,7 +599,6 @@ void call_rcu_tasks(struct rcu_head *rhp, rcu_callback_t func)
 {
        unsigned long flags;
        bool needwake;
-       bool havetask = READ_ONCE(rcu_tasks_kthread_ptr);
 
        rhp->next = NULL;
        rhp->func = func;
@@ -610,11 +608,8 @@ void call_rcu_tasks(struct rcu_head *rhp, rcu_callback_t func)
        rcu_tasks_cbs_tail = &rhp->next;
        raw_spin_unlock_irqrestore(&rcu_tasks_cbs_lock, flags);
        /* We can't create the thread unless interrupts are enabled. */
-       if ((needwake && havetask) ||
-           (!havetask && !irqs_disabled_flags(flags))) {
-               rcu_spawn_tasks_kthread();
+       if (needwake && READ_ONCE(rcu_tasks_kthread_ptr))
                wake_up(&rcu_tasks_cbs_wq);
-       }
 }
 EXPORT_SYMBOL_GPL(call_rcu_tasks);
 
@@ -853,27 +848,18 @@ static int __noreturn rcu_tasks_kthread(void *arg)
        }
 }
 
-/* Spawn rcu_tasks_kthread() at first call to call_rcu_tasks(). */
-static void rcu_spawn_tasks_kthread(void)
+/* Spawn rcu_tasks_kthread() at core_initcall() time. */
+static int __init rcu_spawn_tasks_kthread(void)
 {
-       static DEFINE_MUTEX(rcu_tasks_kthread_mutex);
        struct task_struct *t;
 
-       if (READ_ONCE(rcu_tasks_kthread_ptr)) {
-               smp_mb(); /* Ensure caller sees full kthread. */
-               return;
-       }
-       mutex_lock(&rcu_tasks_kthread_mutex);
-       if (rcu_tasks_kthread_ptr) {
-               mutex_unlock(&rcu_tasks_kthread_mutex);
-               return;
-       }
        t = kthread_run(rcu_tasks_kthread, NULL, "rcu_tasks_kthread");
        BUG_ON(IS_ERR(t));
        smp_mb(); /* Ensure others see full kthread. */
        WRITE_ONCE(rcu_tasks_kthread_ptr, t);
-       mutex_unlock(&rcu_tasks_kthread_mutex);
+       return 0;
 }
+core_initcall(rcu_spawn_tasks_kthread);
 
 /* Do the srcu_read_lock() for the above synchronize_srcu().  */
 void exit_tasks_rcu_start(void)