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

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Message-Id: <20070424034220.ca8d5d96.akpm@linux-foundation.org>
Date:	Tue, 24 Apr 2007 03:42:20 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	ebiederm@...ssion.com (Eric W. Biederman)
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

On Tue, 24 Apr 2007 04:30:22 -0600 ebiederm@...ssion.com (Eric W. Biederman) wrote:

> 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 wake_up_process in kthread_stop has been replaced by signal_wake_up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?

Yes, it was fairly early in boot.  I didn't check what we're oopsing on.

> 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.

That was on a Fedora Core 3 machine.  One of those older distros I keep
around to trip people up.

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