On Mon, 21 Nov 2022 13:02:48 GMT, Per Minborg <pminb...@openjdk.org> wrote:
>> This PR proposes the introduction of **guarding** of the use of
>> `DirectBuffer::address` within the JDK. With the introduction of the Foreign
>> Function and Memory access, it is possible to derive Buffer instances that
>> are backed by native memory that, in turn, can be closed asynchronously by
>> the user (and not only via a `Cleaner` when there is no other reference to
>> the `Buffer` object). If another thread is invoking `MemorySession::close`
>> while a thread is doing work using raw addresses, the outcome is undefined.
>> This means the JVM might crash or even worse, silent modification of
>> unrelated memory might occur.
>>
>> Design choices in this PR:
>>
>> There is already a method `MemorySession::whileAlive` that takes a runnable
>> and that will perform the provided action while acquiring the underlying`
>> MemorySession` (if any). However, using this method entailed relatively
>> large changes while converting larger/nested code segments into lambdas.
>> This would also risk introducing lambda capturing. So, instead, a
>> try-with-resources friendly access method was added. This made is more easy
>> to add guarding and did not risk lambda capturing. Also, introducing lambdas
>> in certain fundamental JDK classes might incur bootstrap problems.
>>
>> The aforementioned TwR is using a "session acquisition" that is not used
>> explicitly in the try block itself. This raises a warning that is suppressed
>> using `@SuppressWarnings("try")`. In the future, we might be able to remove
>> these suppressions by using the reserved variable name `_`. Another
>> alternative was evaluated where a non-autocloseable resource was used.
>> However, it became relatively complicated to guarantee that the session was
>> always released and, at the same time, the existing 'final` block was always
>> executed properly (as they both might throw exceptions). In the end, the
>> proposed solution was much simpler and robust despite requiring a
>> non-referenced TwR variable.
>>
>> In some cases, where is is guaranteed that the backing memory session is
>> non-closeable, we do not have to guard the use of `DirectBuffer::address`.
>> ~~These cases have been documented in the code.~~
>>
>> On some occasions, a plurality of acquisitions are made. This would never
>> lead to deadlocks as acquisitions are fundamentally concurrent counters and
>> not resources that only one thread can "own".
>>
>> I have added comments (and not javadocs) before the declaration of the
>> non-public-api `DirectBuffer::address` method, that the use of the returned
>> address needs to be guarded. It can be discussed if this is preferable or
>> not.
>>
>> This PR spawns several areas of responsibility and so, I expect more than
>> one reviewer before promoting the PR.
>
> Per Minborg has updated the pull request incrementally with one additional
> commit since the last revision:
>
> Rename methods
src/java.base/share/classes/java/util/zip/Adler32.java line 105:
> 103: adler = updateByteBuffer(adler,
> ((DirectBuffer)buffer).address(), pos, rem);
> 104: } finally {
> 105: Reference.reachabilityFence(buffer);
The updated naming is a bit better but there are it still feels that that there
are too many things going on at the use sites ("guard", "session",
"reachability fence"). Ideally the acquire would return something with an
accessor for the direct buffer address but that may not be possible. I wonder
if changing NOP_CLOSE.close to do reachabilityFence(this) would work as
expected and improve many of these use sites?
-------------
PR: https://git.openjdk.org/jdk/pull/11260
