Hi Yasumasa,
Thanks for tackling this. I've had an initial look at it and have a few
concerns.
On 24/06/2020 4:50 pm, Yasumasa Suenaga wrote:
Hi all,
Please review this change:
JBS: https://bugs.openjdk.java.net/browse/JDK-8242428
webrev: http://cr.openjdk.java.net/~ysuenaga/JDK-8242428/webrev.00/
Some typos:
invaliant -> invariant
directry -> directly
This change replace following VM operations to direct handshake.
- VM_GetFrameCount (GetFrameCount())
- VM_GetFrameLocation (GetFrameLocation())
- VM_GetThreadListStackTraces (GetThreadListStackTrace())
- VM_GetCurrentLocation
It would have been better to split these out into separate changes. I am
finding it very hard to track through the webrev and try to compare the
old safepoint based operation with the new direct handshake approach, to
check they are functionally equivalent.
You are not checking the return value of Handshake::execute_direct and
so are missing the possibility that the target thread has terminated
before you got to do the operation on it. It isn't clear to me under
what other circumstances execute_direct can also return false.
You don't seem to have these checks anymore in some places:
&& !_java_thread->is_exiting() && _java_thread->threadObj() != NULL)
why not?
It is not clear that all the code that previously could execute at a
safepoint, due to being called from a VM_Operation, is still executable
at a safepoint e.g. JvmtiThreadState::count_frames()
GetThreadListStackTrace() uses direct handshake if thread count == 1. In
other case (thread count > 1), it would be performed as VM operation
(VM_GetThreadListStackTraces).
This introduces a large chunk of duplicated code for the frame fill in
and final allocation. Can you not reuse the existing logic that does
this - and in the process do away with the the use of
_needs_thread_state? I really wanted to see simpler code after this
conversion.
I'm also wondering whether we can hide all this logic in the closure, as
was done with the VM_Operation i.e.
*stack_info_ptr = op.stack_info();
Caller of VM_GetCurrentLocation
(JvmtiEnvThreadState::reset_current_location()) might be called at
safepoint. So I added safepoint check in its caller.
I could not figure out what you were referring to here.
This change has been tested in serviceability/jvmti serviceability/jdwp
vmTestbase/nsk/jvmti vmTestbase/nsk/jdi vmTestbase/ns
k/jdwp.
Just a general comment on testing for these conversions to direct
handshakes. We have no control over whether the handshake gets executed
in the original thread or the target thread, so for all we know all our
testing could be executing only one of the cases. This concerns me but I
am not yet sure what to do about it.
Thanks,
David
-----
Also I tested it on submit repo, then it has execution error
(mach5-one-ysuenaga-JDK-8242428-20200624-0054-12034717) due to
dependency error. So I think it does not occur by this change.
Thanks,
Yasumasa
