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

[Paul E. McKenney]

On Thu, Feb 20, 2014 at 11:45:29AM -0800, Linus Torvalds wrote:
> On Thu, Feb 20, 2014 at 10:56 AM, Paul E. McKenney
> <paulmck@xxxxxxxxxxxxxxxxxx> wrote:
> >
> > The example gcc breakage was something like this:
> >
> >         i = atomic_load(idx, memory_order_consume);
> >         x = array[0 + i - i];
> >
> > Then gcc optimized this to:
> >
> >         i = atomic_load(idx, memory_order_consume);
> >         x = array[0];
> >
> > This same issue would hit control dependencies.  You are free to argue
> > that this is the fault of ARM and PowerPC memory ordering, but the fact
> > remains that your suggested change has -exactly- the same vulnerability
> > as memory_order_consume currently has.
> 
> No it does not, for two reasons, first the legalistic (and bad) reason:
> 
> As I actually described it, the "consume" becomes an "acquire" by
> default. If it's not used as an address to the dependent load, then
> it's an acquire. The use "going away" in no way makes the acquire go
> away in my simplistic model.
> 
> So the compiler would actually translate that to a load-with-acquire,
> not be able to remove the acquire, and we have end of story. The
> actual code generation would be that "ld + sync + ld" on powerpc, or
> "ld.acq" on ARM.
> 
> Now, the reason I claim that reason was "legalistic and bad" is that
> it's actually a cop-out, and if you had made the example be something
> like this:
> 
>    p = atomic_load(&ptr, memory_order_consume);
>    x = array[0 + p - p];
>    y = p->val;
> 
> then yes, I actually think that the order of loads of 'x' and 'p' are
> not enforced by the "consume". The only case that is clear is the
> order of 'y' and 'p', because that is the only one that really *USES*
> the value.
> 
> The "use" of "+p-p" is syntactic bullshit. It's very obvious to even a
> slightly developmentally challenged hedgehog that "+p-p" doesn't have
> any actual *semantic* meaning, it's purely syntactic.
> 
> And the syntactic meaning is meaningless and doesn't matter. Because I
> would just get rid of the whole "dependency chain" language
> ALTOGETHER.
> 
> So in fact, in my world, I would consider your example to be a
> non-issue. In my world, there _is_ no "dependency chain" at a
> syntactic level. In my SANE world, none of that insane crap language
> exists. That language is made-up and tied to syntax exactly because it
> *cannot* be tied to semantics.
> 
> In my sane world, "consume" has a much simpler meaning, and has no
> legalistic syntactic meaning: only real use matters. If the value can
> be optimized away, the so can the barrier, and so can the whole load.
> The value isn't "consumed", so it has no meaning.
> 
> So if you write
> 
>    i = atomic_load(idx, memory_order_consume);
>    x = array[0+i-i];
> 
> then in my world that "+i-i" is meaningless. It's semantic fluff, and
> while my naive explanation would have left it as an acquire (because
> it cannot be peep-holed away), I actually do believe that the compiler
> should be obviously allowed to optimize the load away entirely since
> it's meaningless, and if no use of 'i' remains, then it has no
> consumer, and so there is no dependency.
> 
> Put another way: "consume" is not about getting a lock, it's about
> getting a *value*. Only the dependency on the *value* matters, and if
> the value is optimized away, there is no dependency.
> 
> And the value itself does not have any semantics. There's nothing
> "volatile" about the use of the value that would mean that the
> compiler cannot re-order it or remove it entirely. There's no barrier
> "carried around" by the value per se. The barrier is between the load
> and use. That's the *point* of "consume" after all.
> 
> The whole "chain of dependency" language is pointless. It's wrong.
> It's complicated, it is illogical, and it causes subtle problems
> exactly because it got tied to the language *syntax* rather than to
> any logical use.
> 
> Don't try to re-introduce the whole issue. It was a mistake for the C
> standard to talk about dependencies in the first place, exactly
> because it results in these idiotic legalistic practices.
> 
> You do realize that that whole "*(q+flag-flag)" example in the
> bugzilla comes from the fact that the programmer tried to *fight* the
> fact that the C standard got the control dependency wrong?
> 
> In other words, the *deepest* reason for that bugzilla is that the
> programmer tried to force the logical dependency by rewriting it as a
> (fake, and easily optimizable) data dependency.
> 
> In *my* world, the stupid data-vs-control dependency thing goes away,
> the test of the value itself is a use of it, and "*p ? *q :0" just
> does the right thing, there's no reason to do that "q+flag-flag" thing
> in the first place, and if you do, the compiler *should* just ignore
> your little games.

Linus, given that you are calling me out for pushing "legalistic and bad"
things, "syntactic bullshit", and playing "little games", I am forced
to conclude that you have never attended any sort of standards-committee
meeting.  ;-)

That said, I am fine with pushing control/data dependencies with this
general approach.  There will be complications, but there always are
and they can be dealt with as they come up.

FWIW, the last time I tried excluding things like "f-f", "x%1", "y*0" and
so on, I got a lot of pushback.  The reason I didn't argue too much back
(2007 or some such) then was that my view at the time was that I figured
the kernel code wouldn't do things like that anyway, so it didn't matter.
However, that was more than five years ago, so worth another try.

							Thanx, Paul

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