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

From: Andrew Morton
Date: Mon Mar 17 2014 - 16:02:55 EST

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]

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?

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. And I don't _think_ we prevent
userspace-originated signals from unblocking

Root cause time: it's wrong for the oom-killer to use SIGKILL. In fact
it's basically always wrong to send signals from in-kernel. 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.
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