linux-kernel - Re: [PATCH] change kernel threads to ignore signals instead of blocking them

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Message-Id: <20070413140802.3798a6bc.akpm@linux-foundation.org>
Date:	Fri, 13 Apr 2007 14:08:02 -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>,
	Roland McGrath <roland@...hat.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] change kernel threads to ignore signals instead of
 blocking them

On Fri, 13 Apr 2007 08:13:32 -0600
ebiederm@...ssion.com (Eric W. Biederman) wrote:

> Oleg Nesterov <oleg@...sign.ru> writes:
> 
> > On top of Eric's
> >
> > 	kthread-dont-depend-on-work-queues-take-2.patch
> >
> > Currently kernel threads use sigprocmask(SIG_BLOCK) to protect against signals.
> > This doesn't prevent the signal delivery, this only blocks signal_wake_up().
> > Every "killall -33 kthreadd" means a "struct siginfo" leak.
> >
> > Change kthreadd_setup() to set all handlers to SIG_IGN instead of blocking them
> > (make a new helper ignore_signals() for that). If the kernel thread needs some
> > signal, it should use allow_signal() anyway, and in that case it should not use
> > CLONE_SIGHAND.
> >
> > Note that we can't change daemonize() (should die!) in the same way, because
> > it can be used along with CLONE_SIGHAND. This means that allow_signal() still
> > should unblock the signal to work correctly with daemonize()ed threads.
> >
> > However, disallow_signal() doesn't block the signal any longer but ignores it.
> >
> > NOTE: with or without this patch the kernel threads are not protected from
> > handle_stop_signal(), this seems harmless, but not good.
> 
> Hmm.  I like it all except for disallow_signal.
> 
> disallow_signal currently only has one user, jffs2.  While jffs2
> currently doesn't care, given the way jffs2 is using disallow_signal I
> would expect it would prefer to have the signal blocked.
> 
> Thinking about this some more if jffs2 or anyone else wants blocked
> signal behavior they can go ahead and block the signal.  Keeping
> disallow_signal in sync with allow_signal seems to make sense.

jffs2 actually wants its head examined.  W. T. F. does it think it's
doing in there?

Sigh.
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