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Re: Compilers and RCU readers: Once more unto the breach!


On Wed, May 20, 2015 at 12:47:45PM +0100, Will Deacon wrote:
> Hi Paul,
> 
> On Wed, May 20, 2015 at 03:41:48AM +0100, Paul E. McKenney wrote:
> > On Tue, May 19, 2015 at 07:10:12PM -0700, Linus Torvalds wrote:
> > > On Tue, May 19, 2015 at 6:57 PM, Linus Torvalds
> > > <torvalds@linux-foundation.org> wrote:
> > > So I think you're better off just saying that operations designed to
> > > drop significant bits break the dependency chain, and give things like
> > > "& 1" and "(char *)ptr-(uintptr_t)ptr" as examples of such.
> > > 
> > > Making that just an extension of your existing "& 0" language would
> > > seem to be natural.
> > 
> > Works for me!  I added the following bullet to the list of things
> > that break dependencies:
> > 
> > 	If a pointer is part of a dependency chain, and if the values
> > 	added to or subtracted from that pointer cancel the pointer
> > 	value so as to allow the compiler to precisely determine the
> > 	resulting value, then the resulting value will not be part of
> > 	any dependency chain.  For example, if p is part of a dependency
> > 	chain, then ((char *)p-(uintptr_t)p)+65536 will not be.
> > 
> > Seem reasonable?
> 
> Whilst I understand what you're saying (the ARM architecture makes these
> sorts of distinctions when calling out dependency-based ordering), it
> feels like we're dangerously close to defining the difference between a
> true and a false dependency. If we want to do this in the context of the
> C language specification, you run into issues because you need to evaluate
> the program in order to determine data values in order to determine the
> nature of the dependency.

Indeed, something like this does -not- carry a dependency from the
memory_order_consume load to q:

	char *p, q;

	p = atomic_load_explicit(&gp, memory_order_consume);
	q = gq + (intptr_t)p - (intptr_t)p;

If this was compiled with -O0, ARM and Power might well carry a
dependency, but given any optimization, the assembly language would have
no hint of any such dependency.  So I am not seeing any particular danger.

> You tackle this above by saying "to allow the compiler to precisely
> determine the resulting value", but I can't see how that can be cleanly
> fitted into something like the C language specification.

I am sure that there will be significant rework from where this document
is to language appropriate from the standard.  Which is why I am glad
that Jens is taking an interest in this, as he is particularly good at
producing standards language.

>                                                          Even if it can,
> then we'd need to reword the "?:" treatment that you currently have:
> 
>   "If a pointer is part of a dependency chain, and that pointer appears
>    in the entry of a ?: expression selected by the condition, then the
>    chain extends to the result."
> 
> which I think requires the state of the condition to be known statically
> if we only want to extend the chain from the selected expression. In the
> general case, wouldn't a compiler have to assume that the chain is
> extended from both?

In practice, yes, if the compiler cannot determine which expression is
selected, it must arrange for the dependency to be carried from either,
depending on the run-time value of the condition.  But you would have
to work pretty hard to create code that did not carry the dependencies
as require, not?

> Additionally, what about the following code?
> 
>   char *x = y ? z : z;
> 
> Does that extend a dependency chain from z to x? If so, I can imagine a
> CPU breaking that in practice.

I am not seeing this.  I would expect the compiler to optimize to
something like this:

	char *x = z;

How does this avoid carrying the dependency?  Or are you saying that
ARM loses the dependency via a store to memory and a later reload?
That would be a bit surprising...

> > > Humans will understand, and compiler writers won't care. They will
> > > either depend on hardware semantics anyway (and argue that your
> > > language is tight enough that they don't need to do anything special)
> > > or they will turn the consume into an acquire (on platforms that have
> > > too weak hardware).
> > 
> > Agreed.  Plus Core Working Group will hammer out the exact wording,
> > should this approach meet their approval.
> 
> For the avoidance of doubt, I'm completely behind any attempts to tackle
> this problem, but I anticipate an uphill struggle getting this text into
> the C standard. Is your intention to change the carries-a-dependency
> relation to encompass this change?

I completely agree that this won't be easy, but this is the task at hand.
And yes, the intent is to change carries-a-dependency, given that the
current wording isn't helping anything.  ;-)

							Thanx, Paul


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