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

[Paul E. McKenney]

On Mon, Feb 10, 2014 at 01:27:51AM +0100, Torvald Riegel wrote:
> On Fri, 2014-02-07 at 10:02 -0800, Paul E. McKenney wrote:
> > On Fri, Feb 07, 2014 at 04:55:48PM +0000, Will Deacon wrote:

[ . . . ]

> > And then it is a short and uncontroversial step to the following:
> > 
> > Initial state: x == y == 0
> > 
> > T1:	atomic_store_explicit(42, y, memory_order_relaxed);
> > 	r1 = atomic_load_explicit(x, memory_order_relaxed);
> > 	if (r1 != 42)
> > 		atomic_store_explicit(r1, y, memory_order_relaxed);
> > 
> > T2:	r2 = atomic_load_explicit(y, memory_order_relaxed);
> > 	atomic_store_explicit(r2, x, memory_order_relaxed);
> > 
> > This can of course result in r1 == r2 == 42, even though the constant
> > 42 never appeared in the original code.  This is one way to generate
> > an out-of-thin-air value.
> > 
> > As near as I can tell, compiler writers hate the idea of prohibiting
> > speculative-store optimizations because it requires them to introduce
> > both control and data dependency tracking into their compilers.
> 
> I wouldn't characterize the situation like this (although I can't speak
> for others, obviously).  IMHO, it's perfectly fine on sequential /
> non-synchronizing code, because we know the difference isn't observable
> by a correct program.  For synchronizing code, compilers just shouldn't
> do it, or they would have to truly prove that speculation is harmless.
> That will be hard, so I think it should just be avoided.
> 
> Synchronization code will likely have been tuned anyway (especially if
> it uses relaxed MO), so I don't see a large need for trying to optimize
> using speculative atomic stores.
> 
> Thus, I think there's an easy and practical solution.

I like this approach, but there has been resistance to it in the past.
Definitely worth a good try, though!

> > Many of
> > them seem to hate dependency tracking with a purple passion.  At least,
> > such a hatred would go a long way towards explaining the incomplete
> > and high-overhead implementations of memory_order_consume, the long
> > and successful use of idioms based on the memory_order_consume pattern
> > notwithstanding [*].  ;-)
> 
> I still think that's different because it blurs the difference between
> sequential code and synchronizing code (ie, atomic accesses).  With
> consume MO, the simple solution above doesn't work anymore, because
> suddenly synchronizing code does affect optimizations in sequential
> code, even if that wouldn't reorder across the synchronizing code (which
> would be clearly "visible" to the implementation of the optimization).

I understand that memory_order_consume is a bit harder on compiler
writers than the other memory orders, but it is also pretty valuable.

							Thanx, Paul

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