Re: [RFC][PATCH 6/8] mm: handle_speculative_fault()

[Linus Torvalds]

On Tue, 5 Jan 2010, Peter Zijlstra wrote:
>
> On Tue, 2010-01-05 at 10:25 -0800, Linus Torvalds wrote:
> > The readers are all hitting the 
> > lock (and you can try to solve the O(n*2) issue with back-off, but quite 
> > frankly, anybody who does that has basically already lost 
> 
> /me sneaks in a reference to local spinning spinlocks just to have them
> mentioned.

Were you in on the discussion with (I think) Andy Glew about that? I don't 
think he ever came up with an actual workable solution, but he tried. He 
worked for Intel at the time (maybe still does), and was looking at 
architectural improvements. I think you need a "compare-and-exchange-2- 
separate-words" instruction to make it work (not "cmpxchg8/16b" - 
literally two _different_ words).

I think m68k can do it. Even then it's a lot more expensive than a regular 
lock for any reasonable common case.

		Linus
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