linux-kernel - Re: slab quirks in DEBUG, ctor, and initialization

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Message-ID: <Pine.LNX.4.64.0712172343370.22511@sbz-30.cs.Helsinki.FI>
Date:	Mon, 17 Dec 2007 23:55:17 +0200 (EET)
From:	Pekka J Enberg <penberg@...helsinki.fi>
To:	John Reiser <jreiser@...Wagon.com>
cc:	linux-kernel@...r.kernel.org
Subject: Re: slab quirks in DEBUG, ctor, and initialization

Hi John,

On Mon, 17 Dec 2007, John Reiser wrote:
> idr_pre_get calls kmem_cache_alloc, which constructs 'struct idr_layer'
> via the cachep->ctor() call from cache_alloc_debugcheck_after to
> idr_cache_ctor, and not via cache_init_objs.  So if DEBUG is off,
> then idr_cache_ctor does not get its chance to call memset,
> which could leave the struct idr_layer potentially undefined.

No, init_cache_objs() will call the constructor, if the cache has one when 
DEBUG is not set so the struct idr_layer can never be undefined. However, 
struct idr_layer can contain non-zero elements if someone does a 
kmem_cache_free() on an object that hasn't been zeroed. If that matters 
here, idr_pre_get should call kmem_cache_zalloc() and drop the 
constructor.

				Pekka
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