[foreign-memaccess+abi] RFR: 8317514: Ensure MemorySegment is initialized before touching NativeMemorySegmentImpl

[Maurizio Cimadamore]

On Mon, 9 Oct 2023 18:15:01 GMT, Jorn Vernee <jvernee at openjdk.org> wrote:

> Please review this fix for a class initializer deadlock.
> 
> The issue is that `MemorySegment` references `NativeMemorySegmentImpl` in its initializer, through the `NULL` constant. `NativeMemorySegmentImpl` also depends on `MemorySegment` to be initialized, since it's a super interface with default methods. The VM does not drop the initialization monitor of `NativeMemorySegmentImpl` while trying to initialize super classes/interfaces (as mandated by the JVM spec).
> 
> When 2 threads try to intialize `MemorySegment` (e.g. through `ofAddress`) and `NativeMemorySegmentImpl` (e.g. through Arena::allocateFrom), at the same time, a deadlock can occur.
> 
> This PR fixes this issue by ensuring `MemorySegment` is initialized in all cases where we directly access `NativeMemorySegmentImpl` or `MappedMemorySegmentImpl`.
> 
> We might consider a fix that removes the cycle in the future as well.

src/java.base/share/classes/jdk/internal/foreign/Utils.java line 59:

> 57:     public static final boolean IS_WINDOWS = privilegedGetProperty("os.name").startsWith("Windows");
> 58: 
> 59:     static {

Another possibility here is to make sure that the static methods here (and also in `NativeMemorySegmentImpl` go via `reinterpret`, so that there's nothing additional to do.

test/jdk/java/foreign/TestDeadlock.java line 51:

> 49:         });
> 50: 
> 51:         Thread t2 = Thread.ofPlatform().start(() -> {

We do not seem to test:
* mapped segments
* symbol lookup segments
* linker-generated segments

Is that ok?

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PR Review Comment: https://git.openjdk.org/panama-foreign/pull/902#discussion_r1350656362
PR Review Comment: https://git.openjdk.org/panama-foreign/pull/902#discussion_r1350656854