RFR: 8339285: test fails with assert(depth < max_critical_stack_depth) failed: can't have more than 10 critical frames

[Alan Bateman]

On Wed, 4 Sep 2024 15:23:51 GMT, Maurizio Cimadamore <mcimadamore at openjdk.org> wrote:

> Scoped methods are critical methods in the FFM API where memory is accessed in a potentially unsafe way. When closing shared arenas, we look at threads in the middle of a scoped operation involving that arena, and if we find one, we make it fail (by installing an async handshake on that thread).
> 
> To find whether a thread is in a scoped method or not, we need a stack walk. For performance reasons, it is preferrable to have the stack walk to be bounded in size.
> 
> A test started picking up a JVM assertion where the stack of a scoped method (namely `ScopedMemoryAccess::isLoaded`) is too big. This is caused by the scoped method stack walk finding the thread using the scoped method in the middle of some JNI lookup (which is required as `isLoaded` eventually ends up in a native method). This condition seems to have been made easier by the fact that these [changes](https://git.openjdk.org/jdk/pull/19213).
> 
> This PR reverts the stack trace associated with JNI lookup to what it was before, by eliminating the extra frame with a bit of refactoring/cleanup. But this is not enough: the stress test introduced in this PR still fails, even when the stack associated with `ClassLoader::findNative` is restored.
> 
> To address this problem in full, I have resorted to `registerNatives` - that is, the native `isLoaded0`, `load0`, `unload0` and `force0` are pre-registered, when the static initializer of `MappedMemoryUtils` is ran. This means that we no longer need to run a JNI lookup in the middle of a scoped method call. This brings the stack back under control, and passes the stress test.
> 
> Of course there's more to do in this area - we should have a more direct test to check the stack associated with scoped methods (for instance, vector load/store operations are also potential suspects), in order to catch "suspicious refactoring" earlier in the process. For this reason I also filed a follow up i[ssue](https://bugs.openjdk.org/browse/JDK-8339551).

src/java.base/aix/native/libnio/MappedMemoryUtils.c line 229:

> 227:     {"unload0",   "(JJ)V",              (void *)&MMU_unload0},
> 228:     {"force0",    "(" FD "JJ)V",        (void *)&MMU_force0},
> 229: };

I did a pass over this and I think it looks okay.

Do the MMU_* functions still need to be jni-exported? In any case, probably better to rename them to MemoryMemoryUtils_* to avoid anything looking at symbols and thinking they are something to do with the memory management unit :-)

It looks most of the signatures for this functions are misaligned with edits.

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PR Review Comment: https://git.openjdk.org/jdk/pull/20854#discussion_r1745168389