Hi Dan,
On 15/06/2020 11:38 pm, Daniel D. Daugherty wrote:
On 6/15/20 3:26 AM, David Holmes wrote:
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.
Which code is wrong?
Yes, a rogue resume can happen when the GetObjectMonitorUsage() caller
has started the process of gathering the information while not at a
safepoint. Thus the information returned by GetObjectMonitorUsage()
might be stale, but that's a bug in the agent code.
The code tries to make sure that it either collects data about a monitor
owned by a thread that is suspended, or else it collects that data at a
safepoint. But the owning thread can be resumed just after the code
determined it was suspended. The monitor can then be released and the
information gathered not only stale but potentially completely wrong as
it could now be owned by a different thread and will report that
thread's entry count.
GetObjectMonitorUsage says nothing about thread's being suspended so I
can't see how this could be construed as an agent bug.
Using a handshake on the owner thread will allow this to be fixed in the
future without forcing/using any safepoints.
Cheers,
David
Dan
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
