linux-kernel - Re: [sched] 10e7071b2f: BUG:kernel_NULL_pointer

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Message-ID: <20191107144639.GB4114@hirez.programming.kicks-ass.net>
Date:   Thu, 7 Nov 2019 15:46:39 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Valentin Schneider <valentin.schneider@....com>
Cc:     kernel test robot <lkp@...el.com>, Aaron Lu <aaron.lwe@...il.com>,
        Phil Auld <pauld@...hat.com>,
        Julien Desfossez <jdesfossez@...italocean.com>,
        Nishanth Aravamudan <naravamudan@...italocean.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>, lkp@...org
Subject: Re: [sched] 10e7071b2f: BUG:kernel_NULL_pointer_dereference,address

On Thu, Nov 07, 2019 at 01:51:47PM +0000, Valentin Schneider wrote:
> Using that, the fail is on:
> 
> 	if (need_pull_dl_task(rq, prev)) {
> 
> Which is most likely explained by the above call ending up doing a 
> 
>   dl_prio(prev->prio);
> 
> which doesn't play well with 
> 
>   class->pick_next_task(rq, NULL, NULL);
> 
> 
> Now, this is no longer an issue (I think) with the rest of Peter's series,
> since the above deref is gone with
> 
> 67692435c411 ("sched: Rework pick_next_task() slow-path")
> 
> It would be interesting to know whether LKP found this on a mainline kernel
> and bisected it down, or if it stumbled on this while bisecting something
> else.

That seems pausible indeed.