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From: Peter Zijlstra <peterz@infradead.org>
To: Valentin Schneider <valentin.schneider@arm.com>
Cc: syzbot <syzbot+9362b31a2e0cad8b749d@syzkaller.appspotmail.com>,
	bp@alien8.de, dwmw@amazon.co.uk, hpa@zytor.com,
	linux-kernel@vger.kernel.org, luto@kernel.org, mingo@redhat.com,
	syzkaller-bugs@googlegroups.com, tglx@linutronix.de,
	x86@kernel.org
Subject: Re: [syzbot] WARNING in kthread_is_per_cpu
Date: Tue, 20 Apr 2021 12:11:54 +0200	[thread overview]
Message-ID: <YH6o6gd4oqqs6sHr@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <87lf9duw8g.mognet@arm.com>

On Tue, Apr 20, 2021 at 10:43:43AM +0100, Valentin Schneider wrote:
> On 20/04/21 10:51, Peter Zijlstra wrote:

> > I think free_kthread_struct() is ok, because a task at that point in its
> > lifetime cannot be also doing exec().
> >
> 
> What if it's one of those kthreads created by directly invoking
> kernel_thread()? AFAICT right now it's only umh, and that one does execve()
> so it ends up stripped of PF_KTHREAD. It could however go through an error
> path, i.e. not call exec, and exit, giving us:
> 
>   put_task_struct(p)
>   `\
>     free_task(p)
>     `\
>       if (tsk->flags & PF_KTHREAD)
>           free_kthread_struct(tsk);
>           `\
>             to_kthread(p)

I'm not following, at the point we hit free_task() it had better be dead
and p->flags had better be stable. Either it will, or will not, have
PF_KTHREAD.



  reply	other threads:[~2021-04-20 10:12 UTC|newest]

Thread overview: 10+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-04-19 10:36 [syzbot] WARNING in kthread_is_per_cpu syzbot
2021-04-19 11:30 ` Thomas Gleixner
2021-04-19 11:31 ` Valentin Schneider
2021-04-19 18:45   ` Peter Zijlstra
2021-04-19 19:58     ` Valentin Schneider
2021-04-20  8:51       ` Peter Zijlstra
2021-04-20  9:43         ` Valentin Schneider
2021-04-20 10:11           ` Peter Zijlstra [this message]
2021-04-20 10:17             ` Valentin Schneider
2021-04-22  7:36         ` [tip: sched/core] kthread: Fix PF_KTHREAD vs to_kthread() race tip-bot2 for Peter Zijlstra

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