Merge tag 'sched-urgent-2020-07-25' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip into master

Pull scheduler fixes from Ingo Molnar:
 "Fix a race introduced by the recent loadavg race fix, plus add a debug
  check for a hard to debug case of bogus wakeup function flags"

* tag 'sched-urgent-2020-07-25' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip:
  sched: Warn if garbage is passed to default_wake_function()
  sched: Fix race against ptrace_freeze_trace()

Author: torvalds

kernel/sched/core.c

@@ -4119,9 +4119,6 @@ static void __sched notrace __schedule(bool preempt)
 	local_irq_disable();
 	rcu_note_context_switch(preempt);
 
-	/* See deactivate_task() below. */
-	prev_state = prev->state;
-
 	/*
 	 * Make sure that signal_pending_state()->signal_pending() below
 	 * can't be reordered with __set_current_state(TASK_INTERRUPTIBLE)
@@ -4145,11 +4142,16 @@ static void __sched notrace __schedule(bool preempt)
 	update_rq_clock(rq);
 
 	switch_count = &prev->nivcsw;
+
 	/*
-	 * We must re-load prev->state in case ttwu_remote() changed it
-	 * before we acquired rq->lock.
+	 * We must load prev->state once (task_struct::state is volatile), such
+	 * that:
+	 *
+	 *  - we form a control dependency vs deactivate_task() below.
+	 *  - ptrace_{,un}freeze_traced() can change ->state underneath us.
 	 */
-	if (!preempt && prev_state && prev_state == prev->state) {
+	prev_state = prev->state;
+	if (!preempt && prev_state) {
 		if (signal_pending_state(prev_state, prev)) {
 			prev->state = TASK_RUNNING;
 		} else {
@@ -4163,10 +4165,12 @@ static void __sched notrace __schedule(bool preempt)
 
 			/*
 			 * __schedule()			ttwu()
-			 *   prev_state = prev->state;	  if (READ_ONCE(p->on_rq) && ...)
-			 *   LOCK rq->lock		    goto out;
-			 *   smp_mb__after_spinlock();	  smp_acquire__after_ctrl_dep();
-			 *   p->on_rq = 0;		  p->state = TASK_WAKING;
+			 *   prev_state = prev->state;    if (p->on_rq && ...)
+			 *   if (prev_state)		    goto out;
+			 *     p->on_rq = 0;		  smp_acquire__after_ctrl_dep();
+			 *				  p->state = TASK_WAKING
+			 *
+			 * Where __schedule() and ttwu() have matching control dependencies.
 			 *
 			 * After this, schedule() must not care about p->state any more.
 			 */
@@ -4481,6 +4485,7 @@ asmlinkage __visible void __sched preempt_schedule_irq(void)
 int default_wake_function(wait_queue_entry_t *curr, unsigned mode, int wake_flags,
 			  void *key)
 {
+	WARN_ON_ONCE(IS_ENABLED(CONFIG_SCHED_DEBUG) && wake_flags & ~WF_SYNC);
 	return try_to_wake_up(curr->private, mode, wake_flags);
 }
 EXPORT_SYMBOL(default_wake_function);