rcu-tasks: Pull sampling of ->percpu_dequeue_lim out of loop

The rcu_tasks_need_gpcb() samples ->percpu_dequeue_lim as part of the
condition clause of a "for" loop, which is a bit confusing.  This commit
therefore hoists this sampling out of the loop, using the result loaded
in the condition clause.

So why does this work in the face of a concurrent switch from single-CPU
queueing to per-CPU queueing?

o	The call_rcu_tasks_generic() that makes the change has already
	enqueued its callback, which means that all of the other CPU's
	callback queues are empty.

o	For the call_rcu_tasks_generic() that first notices
	the switch to per-CPU queues, the smp_store_release()
	used to update ->percpu_enqueue_lim pairs with the
	raw_spin_trylock_rcu_node()'s full barrier that is
	between the READ_ONCE(rtp->percpu_enqueue_shift) and the
	rcu_segcblist_enqueue() that enqueues the callback.

o	Because this CPU's queue is empty (unless it happens to
	be the original single queue, in which case there is no
	need for synchronization), this call_rcu_tasks_generic()
	will do an irq_work_queue() to schedule a handler for the
	needed rcuwait_wake_up() call.	This call will be ordered
	after the first call_rcu_tasks_generic() function's change to
	->percpu_dequeue_lim.

o	This rcuwait_wake_up() will either happen before or after the
	set_current_state() in rcuwait_wait_event().  If it happens
	before, the "condition" argument's call to rcu_tasks_need_gpcb()
	will be ordered after the original change, and all callbacks on
	all CPUs will be visible.  Otherwise, if it happens after, then
	the grace-period kthread's state will be set back to running,
	which will result in a later call to rcuwait_wait_event() and
	thus to rcu_tasks_need_gpcb(), which will again see the change.

So it all works out.

Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Frederic Weisbecker <frederic@kernel.org>

Author: paulmckrcu

kernel/rcu/tasks.h

@@ -432,14 +432,16 @@ static void rcu_barrier_tasks_generic(struct rcu_tasks *rtp)
 static int rcu_tasks_need_gpcb(struct rcu_tasks *rtp)
 {
 	int cpu;
+	int dequeue_limit;
 	unsigned long flags;
 	bool gpdone = poll_state_synchronize_rcu(rtp->percpu_dequeue_gpseq);
 	long n;
 	long ncbs = 0;
 	long ncbsnz = 0;
 	int needgpcb = 0;
 
-	for (cpu = 0; cpu < smp_load_acquire(&rtp->percpu_dequeue_lim); cpu++) {
+	dequeue_limit = smp_load_acquire(&rtp->percpu_dequeue_lim);
+	for (cpu = 0; cpu < dequeue_limit; cpu++) {
 		struct rcu_tasks_percpu *rtpcp = per_cpu_ptr(rtp->rtpcpu, cpu);
 
 		/* Advance and accelerate any new callbacks. */