linux-kernel - Re: [patch 09/10] SLUB: Do our own locking via slab_lock and slab

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Message-ID: <Pine.LNX.4.64.0710301124520.11531@schroedinger.engr.sgi.com>
Date:	Tue, 30 Oct 2007 11:32:39 -0700 (PDT)
From:	Christoph Lameter <clameter@....com>
To:	Nick Piggin <nickpiggin@...oo.com.au>
cc:	Matthew Wilcox <matthew@....cx>, akpm@...ux-foundation.org,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	Pekka Enberg <penberg@...helsinki.fi>
Subject: Re: [patch 09/10] SLUB: Do our own locking via slab_lock and
 slab_unlock.

On Tue, 30 Oct 2007, Nick Piggin wrote:

> Is this actually a speedup on any architecture to roll your own locking
> rather than using bit spinlock?

It avoids one load from memory when allocating and the release is simply 
writing the page->flags back. Less instructions.

> I am not exactly convinced that smp_wmb() is a good idea to have in your
> unlock, rather than the normally required smp_mb() that every other open
> coded lock in the kernel is using today. If you comment every code path
> where a load leaking out of the critical section would not be a problem,
> then OK it may be correct, but I still don't think it is worth the
> maintenance overhead.

I thought you agreed that release semantics only require a write barrier? 
The issue here is that other processors see the updates before the 
updates to page-flags.

A load leaking out of a critical section would require that the result of 
the load is not used to update other information before the slab_unlock 
and that the source of the load is not overwritten in the critical 
section. That does not happen in sluib.

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