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

Christian Borntraeger
2009-12-08T21:52:51Z

> > 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)?

No, the write really depends on x being 1 at any time before the comparison.
On the other hand x being != 0 during the comparison does not prevent the
write without proper locking or barriers.

Have a look at

http://www.linuxjournal.com/article/8211
http://www.linuxjournal.com/article/8212

especially at the alpha part what can happen when dealing with pointer accesses.

Christian