Re: 5.8-rc*: kernel BUG at kernel/signal.c:1917

[Peter Zijlstra <peterz@infradead.org>]

On Mon, Jul 20, 2020 at 04:02:24PM +0200, Oleg Nesterov wrote:
> I have to admit, I do not understand the usage of prev_state in schedule(),
> it looks really, really subtle...

Right, so commit dbfb089d360 solved a problem where schedule() re-read
prev->state vs prev->on_rq = 0. That is, schedule()'s dequeue and
ttwu()'s enqueue disagreed over sched_contributes_to_load. and as a
result load-accounting went wobbly.

Now, looking at that commit again, I might've solved the problem twice
:-P

So on the one hand, I provider ordering:

	LOAD p->state		LOAD-ACQUIRE p->on_rq == 0
	MB
	STORE p->on_rq, 0	STORE p->state, TASK_WAKING

such that ttwu() will only change p->state, after on_rq==0, which is
after loading p->state in schedule().

At the same time, I also had schedule() set
p->sched_contributes_to_load once, and then consistently used that value
throughout, without ever looking at p->state again, which too makes it
much harder to mess load-avg up.


Now, the ordering in schedule(), relies on doing the p->state load
before:

	spin_lock(rq->lock)
	smp_mb__after_spinlock();

and doing a re-load check after, with the assumption that if the reload
is different, it will not block.

That said, in a crossed email, I just proposed we could simplify all
this like so.. but now I need to go ask people to re-validate that
loadavg muck again :-/


diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index a2a244af9a53..437fc3b241f2 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -4193,9 +4193,6 @@ 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)
@@ -4223,7 +4220,8 @@ static void __sched notrace __schedule(bool preempt)
 	 * We must re-load prev->state in case ttwu_remote() changed it
 	 * before we acquired rq->lock.
 	 */
-	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 {
@@ -4237,10 +4235,12 @@ 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;
+			 *   if (prev_state)		  if (p->on_rq && ...)
+			 *     p->on_rq = 0;		    goto out;
+			 *				  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.
 			 */