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

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Message-ID: <alpine.LFD.2.00.0810151058540.3288@nehalem.linux-foundation.org>
Date:	Wed, 15 Oct 2008 11:03:08 -0700 (PDT)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Nick Piggin <nickpiggin@...oo.com.au>
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 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.

I don't think either of those assumptions are at all relevant or 
interesting. Quite the reverse - I'd expect them to be in a very small 
minority.

Now, obviously, on pretty much all machines out there (ie x86[-64] and UP 
ARM), smp_wmb() is a no-op, so in that sense we could certainly say that 
"sure, this is a total special case, but we can add a smp_wmb() anyway 
since it won't cost us anything".

On the other hand, on the machines where it doesn't cost us anything, it 
obviously doesn't _do_ anything either, so that argument is pretty 
dubious. 

And on machines where the memory ordering _can_ matter, it's going to add 
cost to the wrong point.

		Linus
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