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    SubjectRe: [PATCH RFC tip/core/rcu 1/9] rcu: Add call_rcu_tasks()
    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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