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On 26/06/2020 11:31 am, Yasumasa Suenaga wrote:
On 2020/06/25 21:48, David Holmes wrote:
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.
I will add it. According to Handshake::execute_direct() and
HandshakeOperation::do_handshake(), it seems to return false if the
target thread has terminated as you said.
Yes, but also if the handshake is not executed - but I don't know
under what conditions that can occur.
You don't seem to have these checks anymore in some places:
&& !_java_thread->is_exiting() && _java_thread->threadObj() != NULL)
why not?
I thought the thread which enters handshake is always alive and it
has threadObj.
As far as I can see we can still engage in a handshake with a thread
after it has marked itself as exiting.
I think the handshake should not be run if its state is exiting because
we can deem it as "dead".
What do you think?
The thread is marked as exiting fairly early in its termination path and
can still interact with oops after that point so we must continue to
obey all safety protocols in relation to handshakes and safepoints. I
think it is up to the handshake operation to check that the target
thread is in a suitable state for processing.
The threadObj() can only be null while a thread is attaching, which
means it would have to checked in the general case, but for these JVM
TI operations if we already have a jthread reference to the target
thread then it must be beyond that point. Mind you that same logic
applies to the existing code so ...
I will recover their conditions.
(I also should recover them for GetOwnedMonitorInfoClosure and
GetCurrentContendedMonitorClosure - I removed them in JDK-8242425)
I think so - and we need to check the return value of execute_direct
to determine when to report JVMTI_ERROR_THREAD_NOT_ALIVE.
I will file it to JBS.
We can get the result from result() in their Closures.
JVMTI_ERROR_THREAD_NOT_ALIVE is set by default in
GetCurrentContendedMonitorClosure, so we can get this error if the
handshake is not completed.
Should I check result of execute_direct() even if that?
(Of course, we should fix GetOwnedMonitorInfoClosure and threadObj() check)
I think it is a little bit too subtle to rely on a default setting for
the result (and begs the question why GetOwnedMonitorInfoClosure doesn't
also set result to JVMTI_ERROR_THREAD_NOT_ALIVE?). I think we should be
establishing a common pattern for writing these Handshake closures and
the related operation, in a clear, correct way.
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();
I will try to refactor this change.
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.
I guess following callpath is available:
VM_GetCurrentLocation
JvmtiEnvThreadState::reset_current_location()
JvmtiEventControllerPrivate::recompute_env_thread_enabled()
JvmtiEventControllerPrivate::recompute_thread_enabled()
JvmtiEventControllerPrivate::set_frame_pop()
JvmtiEventController::set_frame_pop()
JvmtiEnvThreadState::set_frame_pop()
VM_SetFramePop::doit()
However, VM_SetFramePop seems not to allow nested VM operations.
It is the outer operation that has to allow nesting but
VM_GetCurrentLocation doesn't allow it either. So if this path is
possible then something is broken.
I'm not sure this path would be happen. However following comments are
left in the code:
```
// The java thread stack may not be walkable for a running thread
// so get current location at safepoint.
VM_GetCurrentLocation op(_thread);
```
I'm not sure what point is being made with all this. It's not safe to
ask a running thread for the current location, so it must be done via a
safepoint VM operation, or now a direct handshake operation with the thread.
Thanks,
David
-----
Thanks,
Yasumasa
