sched/core: Fix preempt warning in ttwu

John reported a DEBUG_PREEMPT warning caused by commit:

  aacedf26fb76 ("sched/core: Optimize try_to_wake_up() for local wakeups")

I overlooked that ttwu_stat() requires preemption disabled.

Reported-by: John Stultz <john.stultz@linaro.org>
Tested-by: John Stultz <john.stultz@linaro.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Fixes: aacedf26fb76 ("sched/core: Optimize try_to_wake_up() for local wakeups")
Link: https://lkml.kernel.org/r/20190710105736.GK3402@hirez.programming.kicks-ass.net
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Change-Id: I385c5e0796c36e35761cc4658edd3c00ac908bfb
Signed-off-by: Alexander Winkowski <dereference23@outlook.com>

kernel/sched/core.c

@@ -2099,6 +2099,7 @@ try_to_wake_up(struct task_struct *p, unsigned int state, int wake_flags,
 	unsigned long flags;
 	int cpu, success = 0;
 
+	preempt_disable();
 	if (p == current) {
 		/*
 		 * We're waking current, this means 'p->on_rq' and 'task_cpu(p)
@@ -2112,7 +2113,7 @@ try_to_wake_up(struct task_struct *p, unsigned int state, int wake_flags,
 		 *    it disabling IRQs (this allows not taking ->pi_lock).
 		 */
 		if (!(p->state & state))
-			return false;
+			goto out;
 
 		success = 1;
 		cpu = task_cpu(p);
@@ -2228,6 +2229,7 @@ unlock:
 out:
 	if (success)
 		ttwu_stat(p, cpu, wake_flags);
+	preempt_enable();
 
 #ifdef CONFIG_SCHED_WALT
 	if (success && sched_predl) {