linux-kernel - Re: [rfc] SLOB memory ordering issue

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Date:	Thu, 16 Oct 2008 06:19:53 +1100
From:	Nick Piggin <nickpiggin@...oo.com.au>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Matt Mackall <mpm@...enic.com>,
	Pekka Enberg <penberg@...helsinki.fi>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [rfc] SLOB memory ordering issue

On Thursday 16 October 2008 05:43, Linus Torvalds wrote:
> On Thu, 16 Oct 2008, Nick Piggin wrote:
> > Actually, there are surprisingly huge number of them. What I would be
> > most comfortable doing, if I was making a kernel to run my life support
> > system on an SMP powerpc box, would be to spend zero time on all the
> > drivers and whacky things with ctors and just add smp_wmb() after them
> > if they are not _totally_ obvious.
>
> WHY?

I guess I wouldn't bother with your kernel. I was being hypothetical.
Can you _prove_ no code has a bug due specifically to this issue?

> THIS HAS NOTHING TO DO WITH CONSTRUCTORS!
>
> If the driver is using locking, there is no memory ordering issues
> what-so-ever.
>
> And if the driver isn't using locking, IT IS BROKEN.

Did you read the anon_vma example? It's broken if it assumes the objects
coming out of its slab are always "stable".

> It's that simple. Why do you keep bringing up non-issues?

Why are you being antagonistic and assuming I'm wrong instead of
considering you mistunderstand me, maybe I'm not a retard? I am bad
at explaining myself, but I'll try once more.

> What matters is not constructors. Never has been. Constructors are
> actually very rare, it's much more common to do
>
> 	ptr = kmalloc(..)
> 	.. initialize it by hand ..
>
> and why do you think constructors are somehow different? They're not.

I think they might be interpreted or viewed by the caller as giving
a "stable" object. It is rather more obvious to a caller that it has
previous unordered stores if it is doing this
 	ptr = kmalloc(..)
 	.. initialize it by hand ..

I haven't dealt much with constructors myself so I haven't really
had to think about it. But I'm sure I could have missed it and been
fooled.

If you still don't agree, then fine; if I find a bug I'll send a patch.
I don't want to keep arguing.

> What matter is how you look things up on the other CPU's. If you don't use
> locking, you use some lockless thing, and then you need to be careful
> about memory ordering.
>
> And quite frankly, if you're a driver, and you're trying to do lockless
> algorithms, you're just being crazy. You're going to have much worse bugs,
> and again, whether you use constructors or pink elephants is going to be
> totally irrelevant.
>
> So why do you bring up these totally pointless things? Why do you bring up
> drivers? Why do you bring up constructors? Why, why, why?

I'll try to keep them to myself in future.

Thanks,
Nick
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