kmem_cache_destroy() badness with SLUB

[Benjamin Herrenschmidt <benh@kernel.crashing.org>]

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.