[10] RFR: 8184603: Create ObjectStreamField signature lazily when possible

[Claes Redestad]

On 2017-07-14 15:46, Aleksey Shipilev wrote:
> On 07/14/2017 03:40 PM, Claes Redestad wrote:
>> signature is already made volatile in this patch, deliberately to ensure that
>> for the private constructors we keep correctness:
>>
>> in these private constructors, the signature field is initialized in the
>> constructor (we can assert that it's non-null) to a value that would not
>> match getClassSignature(type), but since the field is volatile it will never
>> be observed as null by a call to getSignature() on an object created with
>> these constructors (unless I'm mistaken, and we *do* need an added
>> synchonization point here).
> Thing is, "volatile" is not as strong as "final" for racy initializations:
>
> https://shipilev.net/blog/2016/close-encounters-of-jmm-kind/#wishful-volatiles-are-finals
>
> If that is not a concern -- e.g. if private constructor is never used to make
> shared OSFs -- then "volatile" is superfluous. If it is a concern, then
> "volatile" is not enough, and you have to keep "final".

Gosh!

Seeing how it's possible to lookup ObjectStreamClass and get at
ObjectStreamField's that might have been created from the internal
constructors, I think it *is* a concern.

Thus the only viable option that allows us to be correct *and* lazy 
might be to
add a new field that is lazily initialized only when using the public 
constructors:

http://cr.openjdk.java.net/~redestad/8184603/jdk.02/

WDYT? Adding a field is of course a possible footprint increase, but
ObjectStreamField is a rare object in most applications so footprint 
risk is
minimal to begin with, and since the signature String will now be 
created lazily
this is likely still a net footprint win for many (most?) applications.

Thanks!

/Claes

>
> The hair-pull scenario under the race would be like this: OSF gets constructed
> with Ljava/lol/ConcurrencyIsHard, published, another thread calls getSignature()
> on it, eads null from $signature, regenerates it from Object $type, returns
> Ljava/lang/Object as signature. Something fails not expecting j/l/Object in
> signature, you go debug it, and see getSignature() returns
> Ljava/lol/ConcurrencyIsHard, as if nothing had happened.
>
> Thanks,
> -Aleksey
>