linux-kernel - Re: [PATCH] Make kthread_stop() not oops when passed a bad pointer

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Date:	Wed, 6 Aug 2008 11:22:58 +1000
From:	Rusty Russell <rusty@...tcorp.com.au>
To:	Matthew Wilcox <matthew@....cx>
Cc:	Linus Torvalds <torvalds@...l.org>, Andrew Morton <akpm@...l.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] Make kthread_stop() not oops when passed a bad pointer

On Tuesday 05 August 2008 23:55:59 Matthew Wilcox wrote:
> Make kthread_stop a little more robust against numbskulls
> like me.
>
> Signed-off-by: Matthew Wilcox <willy@...ux.intel.com>

Hi Willy,

I really do sympathize with your problem; but the quest is to figure out how 
to identify it before the code is run, not to put a non-orthogonal bandaid 
here which can hurt other cases.

How about a more ambitious "we've oopsed so break a mutex every 30 seconds of 
waiting" patch?

> +     if (!k || IS_ERR(k))
> +             return -EINVAL;

1) There's no reason that kthread_stop is uniquely difficult to use.  Why pick 
on that one?

2) I know that kfree() handles NULL, but kthread_create/kthread_run never 
return NULL, unlike kmalloc().

3) If we really want to pass a failed kthread_create() through kthread_stop(), 
we should return PTR_ERR(k) here.  But that should only be done if it made it 
harder for the callers to screw up, which I don't think it does.

4) After a successful kthread_run(), kthread_stop() will always return the 
value from the threadfn callback.  ie. kthread_stop() doesn't ever fail.  A 
simple semantic, which this patch breaks.

5) Covering up programmer errors is not good policy.  I dislike WARN_ON() 
because an oops is much harder to miss.  Painful for you, but The System 
Works.

Sorry,
Rusty.
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