On Thu, 23 Nov 2023 11:14:29 GMT, Erik Ă–sterlund <[email protected]> wrote:
> The current logic for closing memory in panama today is susceptible to live
> lock if we have a closing thread that wants to close the memory in a loop
> that keeps failing, and a bunch of accessing threads that want to perform
> accesses as long as the memory is alive. They can both create impediments for
> the other.
>
> By using asynchronous handshakes to install an exception onto threads that
> are in @Scoped memory accesses, we can have close always succeed, and the
> accessing threads bail out. The idea is that we perform a synchronous
> handshake first to find threads that are in scoped methods. They might
> however be in the middle of throwing an exception or something wild like
> there, where an exception can't be delivered. We install an async handshake
> that will roll us forward to the first place where we can indeed install
> exceptions, then we reevaluate if we still need to do that, or if we have
> unwound out from the scoped method. If we are still inside of it, we ensure
> an exception is installed so we don't continue executing bytecodes that might
> access the memory that we have freed.
>
> Tested tier 1-5 as well as running test/jdk/java/foreign/TestHandshake.java
> hundreds of times, which tests this API pretty well.
src/hotspot/share/prims/scopedMemoryAccess.cpp line 151:
> 149: ResourceMark rm;
> 150: if (_session != nullptr && last_frame.is_compiled_frame() &&
> last_frame.can_be_deoptimized()) {
> 151: CloseScopedMemoryFindOopClosure cl(_session);
Pre-existing, but this value (and class) is unused since we do an unconditional
deopt. If you feel like it, you could remove the
`CloseScopedMemoryFindOopClosure`. We can get it back from the git history
later when that bug is fixed (https://bugs.openjdk.org/browse/JDK-8290892)
src/java.base/share/classes/jdk/internal/foreign/SharedSession.java line 86:
> 84: throw alreadyAcquired(prevState);
> 85: }
> 86: SCOPED_MEMORY_ACCESS.closeScope(this);
đŸ¥³
src/java.base/share/classes/jdk/internal/misc/X-ScopedMemoryAccess.java.template
line 87:
> 85:
> 86: public void closeScope(MemorySessionImpl session) {
> 87: closeScope0(session, MemorySessionImpl.ALREADY_CLOSED);
I suggest passing in the `ALREADY_CLOSED` instance as an argument to this
method instead. Then we can avoid making the field in `MemorySessionImpl`
public.
test/jdk/java/foreign/TestHandshake.java line 107:
> 105: if (!failed.get()) {
> 106: // ignore - this means segment was alive, but
> was closed while we were accessing it
> 107: // next isAlive test should fail
If we see the exception, we should be able to test that the scope is not alive
here as well
Suggestion:
// next isAlive test should fail
assertFalse(segment.scope().isAlive());
-------------
PR Review Comment: https://git.openjdk.org/jdk/pull/16792#discussion_r1403266514
PR Review Comment: https://git.openjdk.org/jdk/pull/16792#discussion_r1403255404
PR Review Comment: https://git.openjdk.org/jdk/pull/16792#discussion_r1403257610
PR Review Comment: https://git.openjdk.org/jdk/pull/16792#discussion_r1403330411
