[foreign] RFR 8215553: Remove SystemABI::freeUpcallStub

[Maurizio Cimadamore]

Right - I will simplify.

Thanks for the comments

Maurizio

On 18/12/2018 16:05, Henry Jen wrote:
> Just curious, why use a Supplier in registerUpcallStub? Simply because to get the return type?
>
> Cheers,
> Henry
>
>> On Dec 18, 2018, at 5:52 AM, Sundararajan Athijegannathan <sundararajan.athijegannathan at oracle.com> wrote:
>>
>> File: src/java.base/share/classes/jdk/internal/foreign/abi/x64/sysv/SysVx64ABI.java
>> Line: + cleaner.register(up, () -> freeUpcallStub(ptr));
>>
>> This could be
>>
>>       cleaner.register(up, () -> freeUpcallStub(ptr.addr()));
>>
>> which will avoid capturing "this" object in the cleaner lambda - because freeUpcallStub(long) is a static method whereas freeUpcallStub(Pointer) is an instance method. Or you could make freeUpcallStub(Pointer) method as static.
>>
>> -Sundar
>>
>> On 18/12/18, 6:44 PM, Maurizio Cimadamore wrote:
>>> Forgot to add: I tested the patch by running all tests multiple times (esp. the heavy TestUpcall :-)) and adding println statements when the cleaner was called, to make sure that the logic was indeed running when executing long-lived tests.
>>>
>>> Maurizio
>>>
>>> On 18/12/2018 13:12, Maurizio Cimadamore wrote:
>>>> Hi,
>>>> few months ago we moved to a new model for managing lifecycle of callbacks, where callbacks were explicitly allocated through a Scope; when the scope is closed, all callbacks registered onto that scope are freed.
>>>>
>>>> Back then, I failed to realize that we don't really need an API point to free the callback - we can just rely on a Cleaner to do its job when the callback is no longer referenced from Java code. Since a callback will always be stored in some scope (when active), this will effectively prevent it from being collected.
>>>>
>>>> Note: this model is very different from what we had before, where callbacks were NOT allocated using a Scope, but only implicitly by passing lambdas to functions. In that case, using a Cleaner was bad, as it  sometimes led to GC happening _in the middle_ of a function call. This can no longer happen, as a Scope will now hold tight on the callbacks used at any given point in time.
>>>>
>>>> Of course, users of the low-level SystemABI, must pay extra care: as soon as an upcall symbol goes out of scope, it is ready for collection; that means that if a client of SystemABI wants an upcall symbol to remain alive, it has to stash it somewhere. Given the audience of this SystemABI API, I think this is an acceptable compromise (and one I've seen elsewhere, e.g. JNR). On the plus side, the API got tighter, as we no longer need a 'free' method.
>>>>
>>>> Webrev:
>>>> http://cr.openjdk.java.net/~mcimadamore/panama/8215553/
>>>>
>>>> Cheers
>>>> Maurizio
>>>>