Re: [RFC][PATCH 0/5] arch: atomic rework

[Paul E. McKenney]

On Wed, Feb 12, 2014 at 10:19:07AM +0100, Peter Zijlstra wrote:
> > I don't know the specifics of your example, but from how I understand
> > it, I don't see a problem if the compiler can prove that the store will
> > always happen.
> > 
> > To be more specific, if the compiler can prove that the store will
> > happen anyway, and the region of code can be assumed to always run
> > atomically (e.g., there's no loop or such in there), then it is known
> > that we have one atomic region of code that will always perform the
> > store, so we might as well do the stuff in the region in some order.
> > 
> > Now, if any of the memory accesses are atomic, then the whole region of
> > code containing those accesses is often not atomic because other threads
> > might observe intermediate results in a data-race-free way.
> > 
> > (I know that this isn't a very precise formulation, but I hope it brings
> > my line of reasoning across.)
> 
> So given something like:
> 
> 	if (x)
> 		y = 3;
> 
> assuming both x and y are atomic (so don't gimme crap for now knowing
> the C11 atomic incantations); and you can prove x is always true; you
> don't see a problem with not emitting the conditional?

You need volatile semantics to force the compiler to ignore any proofs
it might otherwise attempt to construct.  Hence all the ACCESS_ONCE()
calls in my email to Torvald.  (Hopefully I translated your example
reasonably.)

							Thanx, Paul

> Avoiding the conditional changes the result; see that control dependency
> email from earlier. In the above example the load of X and the store to
> Y are strictly ordered, due to control dependencies. Not emitting the
> condition and maybe not even emitting the load completely wrecks this.
> 
> Its therefore an invalid optimization to take out the conditional or
> speculate the store, since it takes out the dependency.
> 

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