[lworld] RFR: 8267763: [lworld][lw3] Change "non-tearable" nomenclature to "access atomic" [v2]

Remi Forax forax at univ-mlv.fr
Mon Jun 14 21:42:49 UTC 2021


----- Mail original -----
> De: "John R Rose" <jrose at openjdk.java.net>
> À: "valhalla-dev" <valhalla-dev at openjdk.java.net>
> Envoyé: Lundi 14 Juin 2021 22:59:52
> Objet: Re: [lworld] RFR: 8267763: [lworld][lw3] Change "non-tearable" nomenclature to "access atomic" [v2]

> On Mon, 31 May 2021 09:36:40 GMT, Aleksey Shipilev <shade at openjdk.org> wrote:
> 
>>> Current Valhalla code has the experimental marker interface
>>> `java.lang.NonTearable`, which is actually about access atomicity. It makes
>>> weird claims about word tearing and out-of-thin air values.
>>> 
>>> First, this is not word tearing. Word tearing, as defined by JLS 17.6 is: _"This
>>> problem is sometimes known as word tearing, and on processors that cannot
>>> easily update a single byte in isolation some other approach will be
>>> required"._ That is, word tearing is when we cannot update the _narrow_ member
>>> without doing a _wider_ access, thus necessarily affecting the adjacent
>>> members. In Valhalla case, what we are dealing with is access atomicity: we
>>> sometimes cannot access the _wide_ member without doing a set of _narrower_
>>> accesses. This is why JLS 17.7 says "non-atomic treatment of double and longs",
>>> not "word-tearing of double and longs".
>>> 
>>> Second, the docs for `j.l.NonTearable` mention "out-of-thin-air" (OOTA) values,
>>> which are not related here at all. OOTA are the beasts from the causality
>>> loops: those are values that were never written by normal execution of the
>>> program (i.e. speculative values). In Valhalla case, the writes that produce
>>> the broken hybrid are known and expected writes from the conflicting writers.
>>> 
>>> This nomenclature percolates to Valhalla VM code, so some change is needed there
>>> as well.
>>> 
>>> Additional testing:
>>>  - [x] `runtime/valhalla` tests
>>
>> Aleksey Shipilev has updated the pull request incrementally with one additional
>> commit since the last revision:
>> 
>>   Rename a few other tests
> 
> Thank you for explaining to all of us about word-tearing, but that seems to be
> moot in the context of this PR.  This feature (`NonTearable`) is about
> *structure tearing*, which is a distinct concept.  Hence the different name, as
> you might have noticed.
> 
> There is also a reason for not using the term "atomic".  Indeed it was my
> preference, but Brian (as he can explain himself as well) has at least two good
> reasons for avoiding that term, and going with the new term `NonTearable` (as
> well as the underlying exposition about the *new* phenomenon of structure
> tearing).
> 
> (The term non-tearable is in fact closely synonymous with the term atomic, which
> literally means non-cuttable.)

Another possible term is Indivisible.

Rémi


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