On Mon, Jul 28, 2014 at 03:56:12PM -0700, Paul E. McKenney wrote:
> +/* RCU-tasks kthread that detects grace periods and invokes callbacks. */
> +static int __noreturn rcu_tasks_kthread(void *arg)
> +{
> + unsigned long flags;
> + struct task_struct *g, *t;
> + struct rcu_head *list;
> + struct rcu_head *next;
> +
> + /* FIXME: Add housekeeping affinity. */
> +
> + /*
> + * Each pass through the following loop makes one check for
> + * newly arrived callbacks, and, if there are some, waits for
> + * one RCU-tasks grace period and then invokes the callbacks.
> + * This loop is terminated by the system going down. ;-)
> + */
> + for (;;) {
> +
> + /* Pick up any new callbacks. */
> + raw_spin_lock_irqsave(&rcu_tasks_cbs_lock, flags);
> + smp_mb__after_unlock_lock(); /* Enforce GP memory ordering. */
> + list = rcu_tasks_cbs_head;
> + rcu_tasks_cbs_head = NULL;
> + rcu_tasks_cbs_tail = &rcu_tasks_cbs_head;
> + raw_spin_unlock_irqrestore(&rcu_tasks_cbs_lock, flags);
> +
> + /* If there were none, wait a bit and start over. */
> + if (!list) {
> + schedule_timeout_interruptible(HZ);
> + flush_signals(current);
> + continue;
> + }
> +
> + /*
> + * There were callbacks, so we need to wait for an
> + * RCU-tasks grace period. Start off by scanning
> + * the task list for tasks that are not already
> + * voluntarily blocked. Mark these tasks and make
> + * a list of them in rcu_tasks_holdouts.
> + */
> + rcu_read_lock();
> + do_each_thread(g, t) {
> + if (t != current && ACCESS_ONCE(t->on_rq) &&
> + !is_idle_task(t)) {
> + t->rcu_tasks_holdout = 1;
> + list_add(&t->rcu_tasks_holdout_list,
> + &rcu_tasks_holdouts);
> + }
> + } while_each_thread(g, t);
> + rcu_read_unlock();
I think you need for_each_process_thread() to be RCU-safe.
while_each_thread() has shown to be racy against exec even with rcu_read_lock().
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