On Thu, 21 Sep 2023 05:15:26 GMT, Patricio Chilano Mateo <pchilanom...@openjdk.org> wrote:
>> `JVM_MoreStackWalk` has a bug that always assumes that the Java frame >> stream is currently at the frame decoded in the last patch and so always >> advances to the next frame before filling in the new batch of stack frame. >> However `JVM_MoreStackWalk` may return 0. The library will set >> the continuation to its parent. It then call `JVM_MoreStackWalk` to continue >> the stack walking but the last decoded frame has already been advanced. >> The Java frame stream is already at the top frame of the parent >> continuation. . >> The current implementation skips "Continuation::yield0" mistakenly. This >> only happens with `-XX:+ShowHiddenFrames` or >> `StackWalker.Option.SHOW_HIDDEN_FRAMES`. >> >> The fix is to pass the number of frames decoded in the last batch to >> `JVM_MoreStackWalk` >> so that the VM will determine if the current frame should be skipped or not. >> >> `test/jdk/jdk/internal/vm/Continuation/Scoped.java` test now correctly checks >> the expected result where "yield0" exists between "enter" and "run" frames. > > src/java.base/share/classes/java/lang/StackStreamFactory.java line 443: > >> 441: >> 442: // If the last batch didn't fetch any frames, keep the >> current batch size. >> 443: int lastBatchFrameCount = frameBuffer.numFrames(); > > I run some tests to understand the issue and I got the same failure if I now > set MIN_BATCH_SIZE to 7. This just forces the same situation where > Continuation::enter is the last frame in the buffer, otherwise since the > patch also changes the batch sizes we don't fall into that issue anymore. The > problem is with this numFrames() method which still returns a number > 0 > after the fetch attempt that returns with no frames. Maybe there is a reset > missing for origin and fence when fetching the next batch? Thanks for catching this. The problem is that `fetchStackFrames(int batchSize)` is supposed to call `setBatch` to set origin and fence for the new batch. But if no frame is fetched, it skips not calling that. I have a fix for it. ------------- PR Review Comment: https://git.openjdk.org/jdk/pull/15804#discussion_r1333433443
