Re: [v3.13][v3.14][Regression] kthread: make kthread_create() killable

[Oleg Nesterov]

On 03/17, Andrew Morton wrote:
>
> On Fri, 14 Mar 2014 16:46:26 -0400 Joseph Salisbury <joseph.salisbury@xxxxxxxxxxxxx> wrote:
>
> > Hi Tetsuo,
> >
> > A kernel bug report was opened against Ubuntu[0].  We performed a kernel
> > bisect, and found that reverting the following commit resolved this bug:
> >
> >
> > commit 786235eeba0e1e85e5cbbb9f97d1087ad03dfa21
> > Author: Tetsuo Handa <penguin-kernel@xxxxxxxxxxxxxxxxxxx>
> > Date:   Tue Nov 12 15:06:45 2013 -0800
> >
> >     kthread: make kthread_create() killable
> >
> > The regression was introduced as of v3.13-rc1.
> >
> > The bug indicates an issue with the SAS controller during
> > initialization, which prevents the system from booting.  Additional
> > details are available in the bug report or on request.
> >
> > I was hoping to get your feedback, since you are the patch author.  Do
> > you think gathering any additional data will help diagnose this issue,
> > or would it be best to submit a revert request?
> >
> > [0] http://pad.lv/1276705
>
> What process is running here?  Presumably modprobe.
>
> A possible explanation is that modprobe has genuinely received a
> SIGKILL.  Can you identify anything in this setup which might send a
> SIGKILL to the modprobe process?

See also other discussion in this thread, I thinks the code in drivers/
is buggy anyway.

> kthread_create_on_node() thinks that SIGKILL came from the oom-killer
> and it cheerfully returns -ENOMEM, which is incorrect if that signal
> came from userspace.

Yes, I think it should return -EINTR.

> And I don't _think_ we prevent
> userspace-originated signals from unblocking
> wait_for_completion_killable()?

And we should not.

> Root cause time: it's wrong for the oom-killer to use SIGKILL.

Not sure... what else?

> In fact
> it's basically always wrong to send signals from in-kernel.

Well, SIGSEGV, SIGIO...

> Signals
> are a userspace IPC mechanism and using them in-kernel a) makes it hard
> (or impossible) to distinguish them from userspace-originated signals
> and b) permits userspace to produce surprising results in the kernel,
> which I suspect is what we're seeing here.

Well, I think in this case it doesn't matter who/why sends a signal.
The task is killed, it should react and exit asap. And kthread_create()
can fail in any case, at least the kernel should not crash.

Oleg.

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