Subject: Re: [RESEND][RFC PATCH v2] waitfd
From: Scott James Remnant <scott@canonical.com>
To: Roland McGrath <roland@redhat.com>
Cc: Ingo Molnar <mingo@elte.hu>, Casey Dahlin <cdahlin@redhat.com>,
       Linux Kernel <linux-kernel@vger.kernel.org>,
       Randy Dunlap <randy.dunlap@oracle.com>, Oleg Nesterov <oleg@redhat.com>,
       Davide Libenzi <davidel@xmailserver.org>,
       Peter Zijlstra <a.p.zijlstra@chello.nl>
In-Reply-To: <20090107205322.5F8C7FC3E0@magilla.sf.frob.com>
References: <49639EB8.40204@redhat.com> <4963ABF0.6070400@redhat.com>
	 <20090107123457.GB16268@elte.hu>
	 <20090107205322.5F8C7FC3E0@magilla.sf.frob.com>
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Date: Sat, 10 Jan 2009 14:45:14 +0000
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On Wed, 2009-01-07 at 12:53 -0800, Roland McGrath wrote:

> New syscall should have gone to linux-api, I think.
>=20
> Do we really need another one for this?  How about using signalfd plus
> setting the child's exit_signal to a queuing (SIGRTMIN+n) signal instead =
of
> SIGCHLD?  It's slightly more magical for the userland process to know to =
do
> that (fork -> clone SIGRTMIN).  But compared to adding a syscall we don't
> really have to add, maybe better.
>=20
This wouldn't help the init daemon case:

- the exit_signal is set on the child, not on the parent.

  While the init daemon could clone() every new process and set
  exit_signal, this would not be set for processes reparented to init.

  Even if we had a new syscall to change the exit_signal of a given
  process, *and* had the init reparent notification patch, this still
  wouldn't be sufficient; you'd have a race condition between the time
  you were notified of the reparent, and the time you set exit_signal,
  in which the child could die.

  Since exit_signal is always reset to SIGCHLD before reparenting, this
  could be done by resetting it to a different signal; but at this point
  we're getting into a rather twisty method full of traps.


- exit_signal is reset to SIGCHLD on exec().

  Pretty much a plan-killer ;) =20

Scott
--=20
Scott James Remnant
scott@canonical.com

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