Re: [BUG] long freezes on thinkpad t60

[Davide Libenzi <davidel@xmailserver.org>]

On Wed, 27 Jun 2007, Linus Torvalds wrote:

> > IOW shouldn't an mfence always be there? Not only loads could leak up 
> > into the wait phase, but stores too, if they have no dependency with the 
> > "head" and "tail" loads.
> 
> Stores never "leak up". They only ever leak down (ie past subsequent loads 
> or stores), so you don't need to worry about them. That's actually already 
> documented (although not in those terms), and if it wasn't true, then we 
> couldn't do the spin unlock with just a regular store anyway.

Yes, Intel has never done that. They'll probably never do it since it'll 
break a lot of system software (unless they use a new mode-bit that 
allows system software to enable lose-ordering). Although I clearly 
remember to have read in one of their P4 optimization manuals to not 
assume this in the future.



- Davide


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