linux-kernel - [rfc] SLOB memory ordering issue

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Message-Id: <200810160334.13082.nickpiggin@yahoo.com.au>
Date:	Thu, 16 Oct 2008 03:34:12 +1100
From:	Nick Piggin <nickpiggin@...oo.com.au>
To:	torvalds@...ux-foundation.org,
	Pekka Enberg <penberg@...helsinki.fi>,
	Matt Mackall <mpm@...enic.com>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: [rfc] SLOB memory ordering issue

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.

Anyone care or think it is a problem? Should we just document
that ->ctor doesn't imply any barriers? Better ideas?

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