Re: RFR: 8274196: Crashes in VM_HeapDumper::work after JDK-8252842 [v4]
On Mon, 27 Sep 2021 13:06:57 GMT, Per Liden wrote:
>> Lin Zang has updated the pull request incrementally with one additional
>> commit since the last revision:
>>
>> remove redundant empty line
>
> src/hotspot/share/services/heapDumper.cpp line 1601:
>
>> 1599: void JNILocalsDumper::do_oop(oop* obj_p) {
>> 1600: // ignore null handles
>> 1601: oop o = NativeAccess::oop_load(obj_p);
>
> The JNI Local roots do not need a load barrier, only JNI Global roots need
> that. The JNI Local roots are processed on safepoint entry as part of the
> "thread head" (via
> `ZStackWatermark::ZStackWatermark::start_processing_impl()` ->
> `Thread::oops_do_no_frames()`), so once you are in
> `VM_HeapDumper::do_thread()` the JNI Local roots have already passed a load
> barrier.
Get it, Thanks!
-
PR: https://git.openjdk.java.net/jdk/pull/5681
Re: RFR: 8274196: Crashes in VM_HeapDumper::work after JDK-8252842 [v4]
On Mon, 27 Sep 2021 12:02:53 GMT, Lin Zang wrote:
>> The root cause for crash in ZGC is that the JNIHandles are processed before
>> object iteration. And ZGC would update the JNIHandles at object iteration
>> with read barrier. So the crash is cause by accessing the invalid address
>> which can be dummy info after zgc, and hence crash.
>>
>> The lock rank issue can be fixed because the related mutexes are acquired in
>> safepoint. so the safepoint_check_required could be safepoint_check_always.
>>
>> The Epsilon issue is caused by wrong _num_dumper_thread calculated when the
>> gang==NULL.
>
> Lin Zang has updated the pull request incrementally with one additional
> commit since the last revision:
>
> remove redundant empty line
src/hotspot/share/services/heapDumper.cpp line 1601:
> 1599: void JNILocalsDumper::do_oop(oop* obj_p) {
> 1600: // ignore null handles
> 1601: oop o = NativeAccess::oop_load(obj_p);
The JNI Local roots do not need a load barrier, only JNI Global roots need
that. The JNI Local roots are processed on safepoint entry as part of the
"thread head" (via `ZStackWatermark::ZStackWatermark::start_processing_impl()`
-> `Thread::oops_do_no_frames()`), so once you are in
`VM_HeapDumper::do_thread()` the JNI Local roots have already passed a load
barrier.
-
PR: https://git.openjdk.java.net/jdk/pull/5681
Re: RFR: 8274196: Crashes in VM_HeapDumper::work after JDK-8252842 [v4]
> The root cause for crash in ZGC is that the JNIHandles are processed before > object iteration. And ZGC would update the JNIHandles at object iteration > with read barrier. So the crash is cause by accessing the invalid address > which can be dummy info after zgc, and hence crash. > > The lock rank issue can be fixed because the related mutexes are acquired in > safepoint. so the safepoint_check_required could be safepoint_check_always. > > The Epsilon issue is caused by wrong _num_dumper_thread calculated when the > gang==NULL. Lin Zang has updated the pull request incrementally with one additional commit since the last revision: remove redundant empty line - Changes: - all: https://git.openjdk.java.net/jdk/pull/5681/files - new: https://git.openjdk.java.net/jdk/pull/5681/files/c1658e19..f6cb2123 Webrevs: - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=5681&range=03 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=5681&range=02-03 Stats: 2 lines in 1 file changed: 0 ins; 2 del; 0 mod Patch: https://git.openjdk.java.net/jdk/pull/5681.diff Fetch: git fetch https://git.openjdk.java.net/jdk pull/5681/head:pull/5681 PR: https://git.openjdk.java.net/jdk/pull/5681
