[jdk17] RFR: 8269240: java/foreign/stackwalk/TestAsyncStackWalk.java test failed with concurrent GC [v2]

Jorn Vernee jvernee at openjdk.java.net
Thu Jul 15 12:29:26 UTC 2021

On Wed, 14 Jul 2021 00:47:47 GMT, David Holmes <dholmes at openjdk.org> wrote:

>> Jorn Vernee has updated the pull request incrementally with one additional commit since the last revision:
>>   Assert frame is correct type in frame_data_for_frame
> src/hotspot/share/prims/universalUpcallHandler.cpp line 76:
>> 74: 
>> 75: // modelled after JavaCallWrapper::JavaCallWrapper
>> 76: Thread* ProgrammableUpcallHandler::on_entry(OptimizedEntryBlob::FrameData* context) {
> This should return JavaThread not Thread.


I've been careful here to return a `Thread*` since the result is stored in `r15_thread` and I thought converting between sub and super types could potentially result in different pointers due to the way super types are laid out within a subtype. I thought it worked like this:

| {Subclass vtable pointer}
| +--- (base class Super)
| | {Super vtable pointer}
| +---

So, I thought plainly using a `JavaThread*` in generated machine code where a `Thread*` was expected could cause trouble, since the pointer needs to be offset for the type conversion.

But now that I'm looking at some cases with compiler explorer, the pointer offset only seems to be needed when using multiple inheritance, for instance:

class SuperA {
    virtual void foo();

class SuperB {
    virtual void bar();

class Sub : public SuperA, public SuperB {
    virtual void baz();

Results in:

class Sub	size(16):
 0	| +--- (base class SuperA)
 0	| | {vfptr}
	| +---
 8	| +--- (base class SuperB)
 8	| | {vfptr}
	| +---

Sub::$vftable at SuperA@:
	| &Sub_meta
	|  0
 0	| &SuperA::foo 
 1	| &Sub::baz 

Sub::$vftable at SuperB@:
	| -8
 0	| &SuperB::bar 

Sub::baz this adjustor: 0


It seems that the sub type just reuses the vtable pointer of the first super type (probably to avoid having to do this pointer offsetting). Though, converting between `SuperB*` and `Sub*` would require offsetting the pointer. I'm still not sure this is guaranteed to work like this with all compilers though (the example is with MSVC, which has options to dump class layouts).

The result of `on_entry` is stored in `r15_thread`, so I guess I'm wondering if it's safe to store a `JavaThread*` instead of a `Thread*` in `r15`, and other code, which may expect `r15` to hold a `Thread*`, is guaranteed to keep working? (FWIW, after changing the return type to `JavaThread*` the tests that exercise this code still pass on Windows with MSVC, and on WSL Linux with GCC).


PR: https://git.openjdk.java.net/jdk17/pull/149

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