linux-kernel - Re: lib/idr.c: initialize struct idr

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Message-ID: <84144f020801271335v4f2491a2n18b12de2ef245499@mail.gmail.com>
Date:	Sun, 27 Jan 2008 23:35:49 +0200
From:	"Pekka Enberg" <penberg@...helsinki.fi>
To:	"Vegard Nossum" <vegard.nossum@...il.com>
Cc:	"Jim Houston" <jim.houston@...r.com>,
	"Ingo Molnar" <mingo@...e.hu>, "Tejun Heo" <htejun@...il.com>,
	"Linux Kernel Mailing List" <linux-kernel@...r.kernel.org>
Subject: Re: lib/idr.c: initialize struct idr_layer

Hi Vegard,

On Jan 27, 2008 11:30 PM, Vegard Nossum <vegard.nossum@...il.com> wrote:
> > Depends on how you track object initialization. An object returned by
> > kmem_cache_alloc() is always initialized if the cache it comes from has a
> > constructor.
>
> I think there's a pretty big chance I'm wrong (or misunderstanding
> something) here, so I'll just ask:
> setup_object() from mm/slub.c is what calls the ctor. Shouldn't this
> be called from slab_alloc() as well? (I'm marking the data
> "uninitialized" there before returning the object.) Otherwise you
> might get back an object that is initialized with the previous owner's
> data. Or is this intentional?

It's intentional. The caller of kmem_cache_free() is expected to put
the object in such a state that it can be recycled immediately when
kmem_cache_alloc() for that cache is called. You can find the design
rationale for that in Bonwick's original paper on slab:
http://citeseer.ist.psu.edu/bonwick94slab.html

                        Pekka
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