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

[Torvald Riegel]

On Thu, 2014-02-20 at 09:01 -0800, Linus Torvalds wrote:
> On Thu, Feb 20, 2014 at 12:30 AM, Paul E. McKenney
> <paulmck@xxxxxxxxxxxxxxxxxx> wrote:
> >>
> >> So lets make this really simple: if you have a consume->cmp->read, is
> >> the ordering of the two reads guaranteed?
> >
> > Not as far as I know.  Also, as far as I know, there is no difference
> > between consume and relaxed in the consume->cmp->read case.
> 
> Ok, quite frankly, I think that means that "consume" is misdesigned.
> 
> > The above example can have a return value of 0 if translated
> > straightforwardly into either ARM or Power, right?
> 
> Correct. And I think that is too subtle. It's dangerous, it makes code
> that *looks* correct work incorrectly, and it actually happens to work
> on x86 since x86 doesn't have crap-for-brains memory ordering
> semantics.
> 
> > So, if you make one of two changes to your example, then I will agree
> > with you.
> 
> No. We're not playing games here. I'm fed up with complex examples
> that make no sense.

Note that Paul's second suggestion for a change was to just use
mo_acquire;  that's a simple change, and the easiest option, so it
should be just fine.

> Nobody sane writes code that does that pointer comparison, and it is
> entirely immaterial what the compiler can do behind our backs. The C
> standard semantics need to make sense to the *user* (ie programmer),
> not to a CPU and not to a compiler. The CPU and compiler are "tools".
> They don't matter. Their only job is to make the code *work*, dammit.
> 
> So no idiotic made-up examples that involve code that nobody will ever
> write and that have subtle issues.
> 
> So the starting point is that (same example as before, but with even
> clearer naming):
> 
> Initialization state:
>   initialized = 0;
>   value = 0;
> 
> Consumer:
> 
>     return atomic_read(&initialized, consume) ? value : -1;
> 
> Writer:
>     value = 42;
>     atomic_write(&initialized, 1, release);
> 
> and because the C memory ordering standard is written in such a way
> that this is subtly buggy (and can return 0, which is *not* logically
> a valid value), then I think the C memory ordering standard is broken.
> 
> That "consumer" memory ordering is dangerous as hell, and it is
> dangerous FOR NO GOOD REASON.
> 
> The trivial "fix" to the standard would be to get rid of all the
> "carries a dependency" crap, and just say that *anything* that depends
> on it is ordered wrt it.
> 
> That just means that on alpha, "consume" implies an unconditional read
> barrier (well, unless the value is never used and is loaded just
> because it is also volatile), on x86, "consume" is the same as
> "acquire" which is just a plain load with ordering guarantees, and on
> ARM or power you can still avoid the extra synchronization *if* the
> value is used just for computation and for following pointers, but if
> the value is used for a comparison, there needs to be a
> synchronization barrier.
> 
> Notice? Getting rid of the stupid "carries-dependency" crap from the
> standard actually
>  (a) simplifies the standard

Agreed, although it's easy to ignore the parts related to mo_consume, I
think.

>  (b) means that the above obvious example *works*
>  (c) does not in *any* way make for any less efficient code generation
> for the cases that "consume" works correctly for in the current
> mis-designed standard.
>  (d) is actually a hell of a lot easier to explain to a compiler
> writer, and I can guarantee that it is simpler to implement too.

mo_acquire is certainly easier to implement in a compiler.

> Why do I claim (d) "it is simpler to implement" - because on ARM/power
> you can implement it *exactly* as a special "acquire", with just a
> trivial peep-hole special case that follows the use chain of the
> acquire op to the consume, and then just drop the acquire bit if the
> only use is that compute-to-load chain.

That's similar to the way I saw it and described in my reply to your
other email (before getting to this email here).  It seems that this
indeed might be doable transparently in the compiler, without requiring
a special mo_acquire variant visible to programmers.

> In fact, realistically, the *only* thing you need to actually care
> about for the intended use case of "consume" is the question "is the
> consuming load immediately consumed as an address (with offset) of a
> memory operation. So you don't even need to follow any complicated
> computation chain in a compiler - the only case that matters for the
> barrier removal optimization is the "oh, I can see that it is only
> used as an address to a dereference".

To make this mo_acquire optimization apply often, a compiler might have
to try to filter out accesses that don't synchronize (e.g., so that an
access to a non-shared temporary variable doesn't prevent the
optimization).

> Seriously. The current standard is broken.

Please, let's be precise in such statement, so that everyone actually
knows what's meant.  The rest of the memory model can be perfectly fine
even if you think that mo_consume isn't useful at all.  I think your
opinion about mo_consume is clear now (and I have concerns about it too,
FWIW).  If you see issues about all the other parts of the memory model
(or the standard), please state these separately.

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