Re: kmem_cache_destroy() badness with SLUB

From: Benjamin Herrenschmidt
Date: Mon Jun 28 2010 - 17:45:23 EST

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 ?


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