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

Alan Stern
2009-12-08T21:32:47Z

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