linux-kernel - Re: [PATCH] kthread: Make kthread

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Message-ID: <20130915163919.GC13344@redhat.com>
Date:	Sun, 15 Sep 2013 18:39:19 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
Cc:	security@...nel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] kthread: Make kthread_create() killable.

On 09/15, Tetsuo Handa wrote:
>
> Oleg Nesterov wrote:
> > I am wondering if this can be simplified...
> >
> > At least you can move create->done from kthread_create_info to the
> > stack, and turn create->owner into the pointer to that completion.
>
> Use of DECLARE_COMPLETION_ONSTACK() looks harmful to me because current thread
> needs to be able to terminate as soon as possible if SIGKILLed

Sure.

> If we move something from kmalloc()ed zone to stack,
> current thread cannot be terminated until that something is guaranteed to no
> longer be used.

Please look at call_usermodehelper_exec() which does this trick. The
logic is the same, just you need to xchg(create->completion) instead
of create->owner.

In any case "void *owner" looks strange. It is in fact boolean (in
your patch). You can even use, say, test_and_set_bit() instead of
xchg.

Oleg.

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