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

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Message-ID: <20070314153341.GA770@tv-sign.ru>
Date:	Wed, 14 Mar 2007 18:33:41 +0300
From:	Oleg Nesterov <oleg@...sign.ru>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
Cc:	Pavel Emelianov <xemul@...ru>,
	Sukadev Bhattiprolu <sukadev@...ibm.com>,
	Serge Hallyn <serue@...ibm.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Linux Containers <containers@...ts.osdl.org>
Subject: Re: [RFC] kernel/pid.c pid allocation wierdness

On 03/14, Eric W. Biederman wrote:
> Pavel Emelianov <xemul@...ru> writes:
> 
> > 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.

We need some global lock. pidmap_lock is already here, and it is
only used to protect pidmap->page allocation. Iow, it is almost
unused. So it was very natural to re-use it while implementing
pidrefs.

> > 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?

Currently we search for zero bit lockless, why do you want
to do it under spin_lock ?

Oleg.

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