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

[Linus Torvalds]

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.

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
 (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.

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.

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".

Seriously. The current standard is broken. It's broken because it
mis-compiles code that on the face of it looks logical and works, it's
broken because it's overly complex, and it's broken because the
complexity doesn't even *buy* you anything. All this complexity for no
reason. When much simpler wording and implementation actually WORKS
BETTER.

                  Linus
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/