Re: kmem_cache_destroy() badness with SLUB

From: David Rientjes
Date: Mon Jun 28 2010 - 05:03:26 EST

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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