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 */
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.
This appears to be an optimisation for the assumed common case where
threads are first suspended and then the monitors are queried.
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
