Hi David,
Sorry for being late but just wanted to tell you that it looks good to me.
Thank you for catching and taking care about it!
Thanks,
Serguei
On 10/14/18 22:12, David Holmes wrote:
Bug: https://bugs.openjdk.java.net/browse/JDK-8211909
webrev: http://cr.openjdk.java.net/~dholmes/8211909/webrev/
The crash occurs when trying to access a thread that was returned as
part of JVM TI GetThreadGroupChildren. The problem is that the JVM TI
code tries to use the Threads_lock to ensure atomic access to the
ThreadGroup's threads and child groups:
{ // Cannot allow thread or group counts to change.
MutexLocker mu(Threads_lock);
but the Threads_lock does not control concurrency in the Java code
that can cause threads to be added and removed, so we do not get a
stable snapshot of the thread array and its length, and contents. To
get a stable snapshot we have to use the same synchronization
mechanism as used by the Java code: lock the monitor of the
ThreadGroup instance.
Two other pointless acquisitions of the Threads_lock, in GetThreadInfo
and GetThreadGroupInfo, were also removed. These functions could
report an inconsistent snapshot of the Thread or ThreadGroup state, if
the mutable state is mutated concurrently. Again we could acquire the
object's monitor to prevent this but it is unclear that there is any
real benefit in doing so - after all the thread/group information
could change immediately after it has been read anyway. So here I just
delete the Threads_lock usage.
Testing: mach5 tier 1-3
JVM TI tests (serviceability/jvmti vmTestbase/nsk/jvmti/)
A regression test may be possible if we call GetThreadGroupChildren in
a loop, whilst threads add and remove themselves from the group
concurrently. But it is awkward to write.
Thanks,
David
Thanks,
David
