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Date:   Wed, 16 Jan 2019 18:32:30 +0100
From:   Oleg Nesterov <oleg@redhat.com>
To:     "Kohli, Gaurav" <gkohli@codeaurora.org>
Cc:     Qian Cai <cai@lca.pw>, Peter Zijlstra <peterz@infradead.org>,
        Ingo Molnar <mingo@redhat.com>,
        linux kernel <linux-kernel@vger.kernel.org>
Subject: Re: kernel BUG at kernel/sched/core.c:3490!
Message-ID: <20190116173229.GA20060@redhat.com>
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On 01/12, Kohli, Gaurav wrote:
>
> HI Peter, Oleg,
>
> as per flag and state this seems to be possible only from below code:

Not sure I understand you,

> XXX: 0 1 0x40844c
> PF_NOFREEZE
> PF_RANDOMIZE
> PF_SIGNALED
> PF_FORKNOEXEC
> PF_EXITING
> PF_EXITPIDONE
>
> above state shows do_exit runs properely and  if somehow after parked stated
> , TASK_WAKEKILL got set and signal_pending_state returns 1 in below case:
>
>  switch_count = &prev->nivcsw;
>         if (!preempt && prev->state) {
>                 if (unlikely(signal_pending_state(prev->state, prev))) {
>                         prev->state = TASK_RUNNING;
>                 } else {
>                         deactivate_task(rq, prev, DEQUEUE_SLEEP |
> DEQUEUE_NOCLOCK);

or task->state was TASK_RUNNING when __schedule() was called, or the deactivated
dead task was woken up later...

The only problem is that every case looks "obviously impossible" ;) I have no
idea whats going on, I can only suggest more stupid debugging patches which might
narrow the problem.

Oleg.