linux-kernel - Re: [RESEND][RFC PATCH v2] waitfd

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Date:	Sat, 10 Jan 2009 14:45:14 +0000
From:	Scott James Remnant <scott@...onical.com>
To:	Roland McGrath <roland@...hat.com>
Cc:	Ingo Molnar <mingo@...e.hu>, Casey Dahlin <cdahlin@...hat.com>,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	Randy Dunlap <randy.dunlap@...cle.com>,
	Oleg Nesterov <oleg@...hat.com>,
	Davide Libenzi <davidel@...ilserver.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: Re: [RESEND][RFC PATCH v2] waitfd

On Wed, 2009-01-07 at 12:53 -0800, Roland McGrath wrote:

> New syscall should have gone to linux-api, I think.
> 
> 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.
> 
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 ;)  

Scott
-- 
Scott James Remnant
scott@...onical.com

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