Subject: Re: OT: fork(): parent or child should run first?
From: Bernd Petrovitsch <bernd@firmix.at>
To: Ian Campbell <ijc@hellion.org.uk>
Cc: linux-kernel@vger.kernel.org, Arjan van de Ven <arjan@infradead.org>,
       lgb@lgb.hu
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Organization: Firmix Software GmbH
Date: Wed, 11 Jan 2006 14:55:43 +0100
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On Wed, 2006-01-11 at 13:49 +0000, Ian Campbell wrote:
> On Wed, 2006-01-11 at 14:25 +0100, Bernd Petrovitsch wrote:
> > Then this leaves the race if an old pid is reused in a newly created
> > process before the last instances of that pid is cleaned up.
> 
> The PID won't be available to be re-used until the signal handler has
> called waitpid() on it?

Yes.
But ATM the signal handler calls waitpid() and stores the pid in a
to-be-cleaned-pids array (at time X).
The main loop at some time in the future (say at time X+N) walks through
the to-be-cleaned-pids array and cleans them from the active-childs
array.
If a new process is started between X and X+N with a pid in the
to-be-cleaned-pids it may happen (depends on the implementation - if the
active-childs array is "sorted" as childs are created (i.e. just append
the new pid at the end), holes of terminated childs are closed with
moving the rest of array and *never* reordered, than it may even work
then) that the wrong one (or both) are cleaned up.
But IMHO a too fragile solution in the log run as this doesn't scale and
people are inclined to tune it with sorting, hashing, etc.

	Bernd
-- 
Firmix Software GmbH                   http://www.firmix.at/
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