sched/core: Rework TASK_DEAD preemption exception

TASK_DEAD is special in that the final schedule call from do_exit()
must be done with preemption disabled.

This means we end up scheduling with a preempt_count() higher than
usual (3 instead of the 'expected' 2).

Since future patches will want to rely on an invariant
preempt_count() value during schedule, fix this up.
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Frederic Weisbecker <fweisbec@gmail.com>
Reviewed-by: Steven Rostedt <rostedt@goodmis.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>

kernel/sched/core.c

@@ -2949,12 +2949,8 @@ static inline void schedule_debug(struct task_struct *prev)
 #ifdef CONFIG_SCHED_STACK_END_CHECK
 	BUG_ON(unlikely(task_stack_end_corrupted(prev)));
 #endif
-	/*
-	 * Test if we are atomic. Since do_exit() needs to call into
-	 * schedule() atomically, we ignore that path. Otherwise whine
-	 * if we are scheduling when we should not.
-	 */
-	if (unlikely(in_atomic_preempt_off() && prev->state != TASK_DEAD))
+
+	if (unlikely(in_atomic_preempt_off()))
 		__schedule_bug(prev);
 	rcu_sleep_check();
 
@@ -3053,6 +3049,17 @@ static void __sched __schedule(void)
 	rcu_note_context_switch();
 	prev = rq->curr;
 
+	/*
+	 * do_exit() calls schedule() with preemption disabled as an exception;
+	 * however we must fix that up, otherwise the next task will see an
+	 * inconsistent (higher) preempt count.
+	 *
+	 * It also avoids the below schedule_debug() test from complaining
+	 * about this.
+	 */
+	if (unlikely(prev->state == TASK_DEAD))
+		preempt_enable_no_resched_notrace();
+
 	schedule_debug(prev);
 
 	if (sched_feat(HRTICK))