linux-kernel - [RFC] kernel/pid.c pid allocation wierdness

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Date:	Wed, 14 Mar 2007 10:30:59 +0300
From:	Pavel Emelianov <xemul@...ru>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Sukadev Bhattiprolu <sukadev@...ibm.com>,
	Serge Hallyn <serue@...ibm.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: [RFC] kernel/pid.c pid allocation wierdness

Hi.

I'm looking at how alloc_pid() works and can't understand
one (simple/stupid) thing.

It first kmem_cache_alloc()-s a strct pid, then calls
alloc_pidmap() and at the end it taks a global pidmap_lock()
to add new pid to hash.

The question is - why does alloc_pidmap() use at least
two atomic ops and potentially loop to find a zero bit
in pidmap? Why not call alloc_pidmap() under pidmap_lock
and find zero pid in pidmap w/o any loops and atomics?

The same is for free_pid(). Do I miss something?

Thank,
Pavel
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