linux-kernel - Re: [PATCH] signals: do_tkill: don't use tasklist

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Date:	Sat, 8 Mar 2008 00:32:17 +0300
From:	Oleg Nesterov <oleg@...sign.ru>
To:	Roland McGrath <roland@...hat.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Ingo Molnar <mingo@...e.hu>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] signals: do_tkill: don't use tasklist_lock

On 03/07, Roland McGrath wrote:
>
> > Btw, a question: we are buggy or just "not perfect" ? After all, the
> > main thread actually exits although this is just linux's implementation
> > detail.
> 
> I think it's buggy.  The SIGKILL should kill the whole process.

OK.

> > Suppose that the main thread is already dead (dequeued SIGKILL), but
> > not yet released. This window is not that small. In that window (before
> > de_thread() switches pids) any private signal (even SIGKILL) sent to the
> > main thread will be silently lost.
> 
> This is the big problem with exec that I've cited before.  It can even
> happen with group-wide signals that should be fatal, but avoided the
> __group_complete_signal special fatal case.  (e.g. the thread racing with
> the exec thread just now unblocked the signal and dequeued it.)  IIRC it
> was the biggest reason we wanted to revisit the whole MT exec plan.

Oh. Could you clarify? Afaics, currently exec() can't miss the fatal group
signal?

> > We can change __group_complete_signal/zap_other_threads so that they won't
> > do sigaddset(), just signal_wake_up(). But in that case dequeue_signal()
> > and recalc_signal() should take signal_group_exit into account...
> 
> I'd like to revisit the use of "fake" SIGKILL for group exits.  That goes
> well with a rethink of MT exec.  But let's not get into all of that right now.

Yes.

Oleg.

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