hrtimer: clean up cpu->base locking tricks

In order to more easily allow for the scheduler to use timers, clean up
the locking a bit.

Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

kernel/hrtimer.c

@@ -1063,7 +1063,9 @@ void hrtimer_interrupt(struct clock_event_device *dev)
 		basenow = ktime_add(now, base->offset);
 
 		while ((node = base->first)) {
+			enum hrtimer_restart (*fn)(struct hrtimer *);
 			struct hrtimer *timer;
+			int restart;
 
 			timer = rb_entry(node, struct hrtimer, node);
 
@@ -1091,13 +1093,29 @@ void hrtimer_interrupt(struct clock_event_device *dev)
 					 HRTIMER_STATE_CALLBACK, 0);
 			timer_stats_account_hrtimer(timer);
 
+			fn = timer->function;
+			if (timer->cb_mode == HRTIMER_CB_IRQSAFE_NO_SOFTIRQ) {
+				/*
+				 * Used for scheduler timers, avoid lock
+				 * inversion with rq->lock and tasklist_lock.
+				 *
+				 * These timers are required to deal with
+				 * enqueue expiry themselves and are not
+				 * allowed to migrate.
+				 */
+				spin_unlock(&cpu_base->lock);
+				restart = fn(timer);
+				spin_lock(&cpu_base->lock);
+			} else
+				restart = fn(timer);
+
 			/*
 			 * Note: We clear the CALLBACK bit after
 			 * enqueue_hrtimer to avoid reprogramming of
 			 * the event hardware. This happens at the end
 			 * of this function anyway.
 			 */
-			if (timer->function(timer) != HRTIMER_NORESTART) {
+			if (restart != HRTIMER_NORESTART) {
 				BUG_ON(timer->state != HRTIMER_STATE_CALLBACK);
 				enqueue_hrtimer(timer, base, 0);
 			}

kernel/time/tick-sched.c

@@ -514,7 +514,6 @@ static enum hrtimer_restart tick_sched_timer(struct hrtimer *timer)
 {
 	struct tick_sched *ts =
 		container_of(timer, struct tick_sched, sched_timer);
-	struct hrtimer_cpu_base *base = timer->base->cpu_base;
 	struct pt_regs *regs = get_irq_regs();
 	ktime_t now = ktime_get();
 	int cpu = smp_processor_id();
@@ -552,15 +551,8 @@ static enum hrtimer_restart tick_sched_timer(struct hrtimer *timer)
 			touch_softlockup_watchdog();
 			ts->idle_jiffies++;
 		}
-		/*
-		 * update_process_times() might take tasklist_lock, hence
-		 * drop the base lock. sched-tick hrtimers are per-CPU and
-		 * never accessible by userspace APIs, so this is safe to do.
-		 */
-		spin_unlock(&base->lock);
 		update_process_times(user_mode(regs));
 		profile_tick(CPU_PROFILING);
-		spin_lock(&base->lock);
 	}
 
 	/* Do not restart, when we are in the idle loop */