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

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Date:	Thu, 16 Oct 2008 03:46:58 +1100
From:	Nick Piggin <nickpiggin@...oo.com.au>
To:	torvalds@...ux-foundation.org
Cc:	Pekka Enberg <penberg@...helsinki.fi>,
	Matt Mackall <mpm@...enic.com>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [rfc] SLOB memory ordering issue

On Thursday 16 October 2008 03:34, Nick Piggin wrote:
> I think I see a possible memory ordering problem with SLOB:
> In slab caches with constructors, the constructor is run
> before returning the object to caller, with no memory barrier
> afterwards.
>
> Now there is nothing that indicates the _exact_ behaviour
> required here. Is it at all reasonable to expect ->ctor() to
> be visible to all CPUs and not just the allocating CPU?
>
> SLAB and SLUB don't appear to have this problem. Of course,
> they have per-CPU fastpath queues, so _can_ have effectively
> exactly the same ordering issue if the object was brought
> back into the "initialized" state before being freed, rather
> than by ->ctor(). However in that case, it is at least
> kind of visible to the caller.

Although I guess it's just as much of a SLAB implementation
detail as the lack of ->ctor() barrier... And I really doubt
_any_ of the callers would have ever thought about either
possible problem.

I'd really hate to add a branch to the slab fastpath for this
though. Maybe we just have to document it, assume there are
no problems, and maybe take a look at some of the core users
of this.
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