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

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Date:	Wed, 15 Oct 2008 13:19:13 -0500
From:	Matt Mackall <mpm@...enic.com>
To:	Nick Piggin <nickpiggin@...oo.com.au>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Pekka Enberg <penberg@...helsinki.fi>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [rfc] SLOB memory ordering issue

On Thu, 2008-10-16 at 05:12 +1100, Nick Piggin wrote:
> On Thursday 16 October 2008 05:03, Linus Torvalds wrote:
> > On Thu, 16 Oct 2008, Nick Piggin wrote:
> > > What do you mean by the allocation is stable?
> >
> > "all writes done to it before it's exposed".
> >
> > > 2. I think it could be easy to assume that the allocated object that was
> > > initialised with a ctor for us already will have its initializing stores
> > > ordered when we get it from slab.
> >
> > You make tons of assumptions.
> >
> > You assume that
> >  (a) unlocked accesses are the normal case and should be something the
> >      allocator should prioritize/care about.
> >  (b) that if you have a ctor, it's the only thing the allocator will do.
> 
> Yes, as I said, I do not want to add a branch and/or barrier to the
> allocator for this. I just want to flag the issue and discuss whether
> there is anything that can be done about it.

Well the alternative is to have someone really smart investigate all the
lockless users of ctors and add appropriate barriers. I suspect that's a
fairly small set and that you're already familiar with most of them.

But yes, I think you may be on to a real problem. It might also be worth
devoting a few neurons to thinking about zeroed allocations.

-- 
Mathematics is the supreme nostalgia of our time.

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