linux-kernel - Re: SLUB regression in current Linus

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Message-ID: <BANLkTik=H=1+oqQ9_5ZZaHpFn8_9x5V-nQ@mail.gmail.com>
Date:	Tue, 24 May 2011 16:03:04 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	James Morris <jmorris@...ei.org>
Cc:	Christoph Lameter <cl@...ux.com>,
	Pekka Enberg <penberg@...nel.org>, linux-kernel@...r.kernel.org
Subject: Re: SLUB regression in current Linus

On Tue, May 24, 2011 at 4:52 AM, James Morris <jmorris@...ei.org> wrote:
>
> Reverting the patch appears to fix the hang for me, although I'm not sure
> what the actual problem is.
>
> This is on a quad-core Opteron (1352). Let me know if you need any further
> info.

That whole "deactivate_slab()" + "c->page = NULL" that that patch does
looks bogus.

Look at __slab_alloc: we have:

        page = c->page;
        if (!page)
                goto new_slab;

        slab_lock(page);
        if (unlikely(!node_match(c, node)))
                goto another_slab;

and let's assume we have two users racing on that "c->page". The
"slab_lock()" is going to work for one of them, right?

Ok, so the one it works for will then hit

        if (kmem_cache_debug(s))
                goto debug;

and thus get to the new "deactivate_slab(s,c) + c->page = NULL" and
then unlock the page.

In the meantime, the one that wasn't able to lock the page will now go
forward, but will not have "node_match()" any more, so it does that
"goto another_slab".

Which does "deactivate_slab(s,c)" again, and now c->page is NULL, so
that totally breaks.

What am I missing?

That patch seems to be just broken piece-of-s%^!

Christoph, Pekka, please tell me why I shouldn't immediately revert
it. What am I missing?

                       Linus
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