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

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Date:	Tue, 29 Jun 2010 07:44:29 +1000
From:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
To:	David Rientjes <rientjes@...gle.com>
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, 2010-06-28 at 02:03 -0700, David Rientjes wrote:
> 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.

It's eventually created, but yes, we should probably store a state,
unless we have a clean way to know the kobject in there is uninitialized
and test for that.

> > 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.

But initcalls aren't pinned to the boot CPU... IE. I don't see how the
sysfs creation avoids racing with SLAB creation, or am I missing
something ?

Cheers,
Ben.

> 
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