[RFC][PATCH 04/11] sched: Rework TASK_DEAD preemption exception

[Peter Zijlstra]

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

This leads to a violation of our new scheduling invariant which states
that the preempt count should be 2. Move the check for TASK_DEAD out
of the debug check and use it to decrement the preempt count (from 2
to 1).

Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
---
 kernel/sched/core.c |   19 +++++++++++++------
 1 file changed, 13 insertions(+), 6 deletions(-)

--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -2955,12 +2955,8 @@ static inline void schedule_debug(struct
 #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);
 		preempt_count_set(PREEMPT_DISABLED);
 	}
@@ -3061,6 +3057,17 @@ 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 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))


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