sched: Consider task_struct::saved_state in wait_task_inactive()

With the introduction of task_struct::saved_state in commit
5f220be ("sched/wakeup: Prepare for RT sleeping spin/rwlocks")
matching the task state has gotten more complicated. That same commit
changed try_to_wake_up() to consider both states, but
wait_task_inactive() has been neglected.

Sebastian noted that the wait_task_inactive() usage in
ptrace_check_attach() can misbehave when ptrace_stop() is blocked on
the tasklist_lock after it sets TASK_TRACED.

Therefore extract a common helper from ttwu_state_match() and use that
to teach wait_task_inactive() about the PREEMPT_RT locks.

Originally-by: Sebastian Andrzej Siewior <[email protected]>
Signed-off-by: Peter Zijlstra (Intel) <[email protected]>
Tested-by: Sebastian Andrzej Siewior <[email protected]>
Link: https://lkml.kernel.org/r/[email protected]

Author: Peter Zijlstra

kernel/sched/core.c

@@ -2213,6 +2213,39 @@ void check_preempt_curr(struct rq *rq, struct task_struct *p, int flags)
 		rq_clock_skip_update(rq);
 }
 
+static __always_inline
+int __task_state_match(struct task_struct *p, unsigned int state)
+{
+	if (READ_ONCE(p->__state) & state)
+		return 1;
+
+#ifdef CONFIG_PREEMPT_RT
+	if (READ_ONCE(p->saved_state) & state)
+		return -1;
+#endif
+	return 0;
+}
+
+static __always_inline
+int task_state_match(struct task_struct *p, unsigned int state)
+{
+#ifdef CONFIG_PREEMPT_RT
+	int match;
+
+	/*
+	 * Serialize against current_save_and_set_rtlock_wait_state() and
+	 * current_restore_rtlock_saved_state().
+	 */
+	raw_spin_lock_irq(&p->pi_lock);
+	match = __task_state_match(p, state);
+	raw_spin_unlock_irq(&p->pi_lock);
+
+	return match;
+#else
+	return __task_state_match(p, state);
+#endif
+}
+
 /*
  * wait_task_inactive - wait for a thread to unschedule.
  *
@@ -2231,7 +2264,7 @@ void check_preempt_curr(struct rq *rq, struct task_struct *p, int flags)
  */
 unsigned long wait_task_inactive(struct task_struct *p, unsigned int match_state)
 {
-	int running, queued;
+	int running, queued, match;
 	struct rq_flags rf;
 	unsigned long ncsw;
 	struct rq *rq;
@@ -2257,7 +2290,7 @@ unsigned long wait_task_inactive(struct task_struct *p, unsigned int match_state
 		 * is actually now running somewhere else!
 		 */
 		while (task_on_cpu(rq, p)) {
-			if (!(READ_ONCE(p->__state) & match_state))
+			if (!task_state_match(p, match_state))
 				return 0;
 			cpu_relax();
 		}
@@ -2272,8 +2305,15 @@ unsigned long wait_task_inactive(struct task_struct *p, unsigned int match_state
 		running = task_on_cpu(rq, p);
 		queued = task_on_rq_queued(p);
 		ncsw = 0;
-		if (READ_ONCE(p->__state) & match_state)
+		if ((match = __task_state_match(p, match_state))) {
+			/*
+			 * When matching on p->saved_state, consider this task
+			 * still queued so it will wait.
+			 */
+			if (match < 0)
+				queued = 1;
 			ncsw = p->nvcsw | LONG_MIN; /* sets MSB */
+		}
 		task_rq_unlock(rq, p, &rf);
 
 		/*
@@ -4003,15 +4043,14 @@ static void ttwu_queue(struct task_struct *p, int cpu, int wake_flags)
 static __always_inline
 bool ttwu_state_match(struct task_struct *p, unsigned int state, int *success)
 {
+	int match;
+
 	if (IS_ENABLED(CONFIG_DEBUG_PREEMPT)) {
 		WARN_ON_ONCE((state & TASK_RTLOCK_WAIT) &&
 			     state != TASK_RTLOCK_WAIT);
 	}
 
-	if (READ_ONCE(p->__state) & state) {
-		*success = 1;
-		return true;
-	}
+	*success = !!(match = __task_state_match(p, state));
 
 #ifdef CONFIG_PREEMPT_RT
 	/*
@@ -4027,12 +4066,10 @@ bool ttwu_state_match(struct task_struct *p, unsigned int state, int *success)
 	 * p::saved_state to TASK_RUNNING so any further tests will
 	 * not result in false positives vs. @success
 	 */
-	if (p->saved_state & state) {
+	if (match < 0)
 		p->saved_state = TASK_RUNNING;
-		*success = 1;
-	}
 #endif
-	return false;
+	return match > 0;
 }
 
 /*