On 15/06/2020 4:02 pm, Yasumasa Suenaga wrote:
Hi David,
On 2020/06/15 14:15, David Holmes wrote:
Hi Yasumasa,
On 15/06/2020 2:49 pm, Yasumasa Suenaga wrote:
Hi all,
I wonder why JvmtiEnvBase::get_object_monitor_usage() (implementation
of GetObjectMonitorUsage()) does not perform at safepoint.
GetObjectMonitorUsage will use a safepoint if the target is not
suspended:
jvmtiError
JvmtiEnv::GetObjectMonitorUsage(jobject object, jvmtiMonitorUsage*
info_ptr) {
JavaThread* calling_thread = JavaThread::current();
jvmtiError err = get_object_monitor_usage(calling_thread, object,
info_ptr);
if (err == JVMTI_ERROR_THREAD_NOT_SUSPENDED) {
// Some of the critical threads were not suspended. go to a
safepoint and try again
VM_GetObjectMonitorUsage op(this, calling_thread, object, info_ptr);
VMThread::execute(&op);
err = op.result();
}
return err;
} /* end GetObject */
I saw this code, so I guess there are some cases when
JVMTI_ERROR_THREAD_NOT_SUSPENDED is not returned from
get_object_monitor_usage().
Monitor owner would be acquired from monitor object at first [1], but
it would perform concurrently.
If owner thread is not suspended, the owner might be changed to
others in subsequent code.
For example, the owner might release the monitor before [2].
The expectation is that when we find an owner thread it is either
suspended or not. If it is suspended then it cannot release the
monitor. If it is not suspended we detect that and redo the whole
query at a safepoint.
I think the owner thread might resume unfortunately after suspending check.
Yes you are right. I was thinking resuming also required a safepoint but
it only requires the Threads_lock. So yes the code is wrong.
JavaThread::is_ext_suspend_completed() is used to check thread state, it
returns `true` when the thread is sleeping [3], or when it performs in
native [4].
Sure but if the thread is actually suspended it can't continue execution
in the VM or in Java code.
This appears to be an optimisation for the assumed common case where
threads are first suspended and then the monitors are queried.
I agree with this, but I could find out it from JVMTI spec - it just
says "Get information about the object's monitor."
Yes it was just an implementation optimisation, nothing to do with the spec.
GetObjectMonitorUsage() might return incorrect information in some case.
It starts with finding owner thread, but the owner might be just before
wakeup.
So I think it is more safe if GetObjectMonitorUsage() is called at
safepoint in any case.
Except we're moving away from safepoints to using Handshakes, so this
particular operation will require that the apparent owner is
Handshake-safe (by entering a handshake with it) before querying the
monitor. This would still be preferable I think to always using a
safepoint for the entire operation.
Cheers,
David
-----
Thanks,
Yasumasa
[3]
http://hg.openjdk.java.net/jdk/jdk/file/76a17c8143d8/src/hotspot/share/runtime/thread.cpp#l671
[4]
http://hg.openjdk.java.net/jdk/jdk/file/76a17c8143d8/src/hotspot/share/runtime/thread.cpp#l684
However there is still a potential bug as the thread reported as the
owner may not be suspended at the time we first see it, and may
release the monitor, but then it may get suspended before we call:
owning_thread =
Threads::owning_thread_from_monitor_owner(tlh.list(), owner);
and so we think it is still the monitor owner and proceed to query the
monitor information in a racy way. This can't happen when suspension
itself requires a safepoint as the current thread won't go to that
safepoint during this code. However, if suspension is implemented via
a direct handshake with the target thread then we have a problem.
David
-----
Thanks,
Yasumasa
[1]
http://hg.openjdk.java.net/jdk/jdk/file/76a17c8143d8/src/hotspot/share/prims/jvmtiEnvBase.cpp#l973
[2]
http://hg.openjdk.java.net/jdk/jdk/file/76a17c8143d8/src/hotspot/share/prims/jvmtiEnvBase.cpp#l996
