linux-kernel - Re: [PATCH] kthread: Enhance kthread_stop to abort interruptible sleeps

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Date:	Tue, 24 Apr 2007 04:30:22 -0600
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Oleg Nesterov <oleg@...sign.ru>,
	Davide Libenzi <davidel@...ilserver.org>,
	Ingo Molnar <mingo@...e.hu>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	"Rafael J. Wysocki" <rjw@...k.pl>,
	Roland McGrath <roland@...hat.com>,
	Rusty Russell <rusty@...tcorp.com.au>,
	linux-kernel@...r.kernel.org, <linux-arch@...r.kernel.org>
Subject: Re: [PATCH] kthread: Enhance kthread_stop to abort interruptible sleeps

Andrew Morton <akpm@...ux-foundation.org> writes:

> On Fri, 13 Apr 2007 21:13:13 -0600 ebiederm@...ssion.com (Eric W. Biederman)
> wrote:
>
>> This patch reworks kthread_stop so it is more flexible and it causes
>> the target kthread to abort interruptible sleeps.  Allowing a larger
>> class of kernel threads to use to the kthread API.
>> 
>> The changes start by defining TIF_KTHREAD_STOP on all architectures.
>> TIF_KTHREAD_STOP is a per process flag that I can set from another
>> process to indicate that a kernel thread should stop.
>> 
>> wake_up_process in kthread_stop has been replaced by signal_wake_up
>> ensuring that the kernel thread if sleeping is woken up in a timely
>> manner and with TIF_SIGNAL_PENDING set, which causes us to break out
>> of interruptible sleeps.
>> 
>> recalc_signal_pending was modified to keep TIF_SIGNAL_PENDING set for
>> as long as TIF_KTHREAD_STOP is set.
>> 
>> Arbitrary paths to do_exit are now allowed.  I have placed a
>> completion on the thread stack and pointed vfork_done at it, when the
>> mm_release is called from do_exit the completion will be called.
>> Since the completion is stored on the stack it is important that
>> kthread() now calls do_exit ensuring the stack frame that holds the
>> completion is never released, and so that our exit_code is certain to
>> make it unchanged all the way to do_exit.
>> 
>> To allow kthread_stop to read the process exit code when exit_mm wakes
>> it up  I have moved the setting of exit_code to the beginning of
>> do_exit. 
>
> This patch causes this oops: http://userweb.kernel.org/~akpm/s5000508.jpg
> with this config: http://userweb.kernel.org/~akpm/config-x.txt

Thanks.  If I am reading the oops properly this happened during bootup and
vfork_done was set to NULL?

The NULL vfork_done is really weird as exec is the only thing that sets
vfork_done to NULL.

Either I've got a stupid bug in there somewhere or we have just found
the weirdest memory stomp.  I will take a look and see if I can reproduce
this shortly.

Eric
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