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

[Paul Gortmaker <paul.gortmaker@windriver.com>]

[Re: 5.8-rc*: kernel BUG at kernel/signal.c:1917] On 20/07/2020 (Mon 16:21) Peter Zijlstra wrote:

> 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

[...]

> 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 :-/

After a two hour "quick" sanity test I then gave it a full 7h run (which
always seemed to break before dbfb089d360) and I didn't see any stuck
load average with master from today + this change.

Paul.

root@t5610:/home/paul/git/linux-head#
[1]+  Done                    nohup tools/testing/selftests/rcutorture/bin/kvm.sh --cpus 24 --duration 420 --configs TREE03 --trust-make > /tmp/kvm.sh.out 2>&1
root@t5610:/home/paul/git/linux-head# cat /proc/version
Linux version 5.8.0-rc6-00001-g5714ee50bb43-dirty (paul@t5610) (gcc (Ubuntu 9.3.0-10ubuntu2) 9.3.0, GNU ld (GNU Binutils for Ubuntu) 2.34) #3 SMP Mon Jul 20 12:30:33 EDT 2020
root@t5610:/home/paul/git/linux-head# uptime
 00:49:18 up  7:41,  2 users,  load average: 0.01, 0.00, 0.63
root@t5610:/home/paul/git/linux-head# 

--

> 
> 
> 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.
>  			 */