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

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Message-ID: <aday6dpmfbw.fsf@roland-alpha.cisco.com>
Date:	Mon, 05 Jul 2010 20:58:27 -0700
From:	Roland Dreier <rdreier@...co.com>
To:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
Cc:	linux-mm@...ck.org, Christoph Lameter <cl@...ux-foundation.org>,
	"linux-kernel\@vger.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: kmem_cache_destroy() badness with SLUB

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.

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. 

What prevents us from hitting slab_sysfs_init() at the same time as
another CPU deos sysfs_slab_add() ? How do that deal with collisions
trying to register the same kobject twice ? Similar race with remove...

Shouldn't we have a mutex around those guys ?

Cheers,
Ben.

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Roland Dreier <rolandd@...co.com> || For corporate legal information go to:
http://www.cisco.com/web/about/doing_business/legal/cri/index.html
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