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

[Frederic Weisbecker]

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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