Re: Async resume patch (was: Re: [GIT PULL] PM updates for 2.6.33)

[Alan Stern]

On Tue, 8 Dec 2009, Linus Torvalds wrote:

> > No, that's not needed.  Unlike reads, writes can't move in front of
> > data or control dependencies.  Or so I've been lead to believe...
> 
> Sure they can. Control dependencies are trivial - it's called "branch 
> prediction", and everybody does it, and data dependencies don't exist on 
> many CPU architectures (even to the point of reading through a pointer 
> that you loaded).

Wait a second.  Are you saying that with code like this:

	if (x == 1)
		y = 5;

the CPU may write to y before it has finished reading the value of x?  
And this write is visible to other CPUs, so that if x was initially 0
and a second CPU sets x to 1, the second CPU may see y == 5 before it
executes the write to x (whatever that may mean)?

Alan Stern

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