linux-kernel - Re: kmem_cache_destroy() badness with SLUB

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Date:	Mon, 28 Jun 2010 02:03:16 -0700 (PDT)
From:	David Rientjes <rientjes@...gle.com>
To:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
cc:	linux-mm@...ck.org, Christoph Lameter <cl@...ux-foundation.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: kmem_cache_destroy() badness with SLUB

On Mon, 28 Jun 2010, Benjamin Herrenschmidt wrote:

> Hi folks !
> 
> Internally, I'm hitting a little "nit"...
> 
> sysfs_slab_add() has this check:
> 
> 	if (slab_state < SYSFS)
> 		/* Defer until later */
> 		return 0;
> 
> But sysfs_slab_remove() doesn't.
> 
> So if the slab is created -and- destroyed at, for example, arch_initcall
> time, then we hit a WARN in the kobject code, trying to dispose of a
> non-existing kobject.
> 

Indeed, but shouldn't we be appropriately handling the return value of 
sysfs_slab_add() so that it fails cache creation?  We wouldn't be calling 
sysfs_slab_remove() on a cache that was never created.

> Now, at first sight, just adding the same test to sysfs_slab_remove()
> would do the job... but it all seems very racy to me.
> 
> I don't understand in fact how this slab_state deals with races at all. 
> 

All modifiers of slab_state are intended to be run only on the boot cpu so 
the only concern is the ordering.  We need slab_state to indicate how far 
slab has been initialized since we can't otherwise enforce how code uses 
slab in between things like kmem_cache_init(), kmem_cache_init_late(), and 
initcalls on the boot cpu.
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