Thomas writes:
Hi Brian,
The next patches where less straightforward. When running java I was getting a ton of messages like: Thread 832744400 has exited with leftover thread-specific data after 4 destructor iterations After doing a lot of digging and debugging on Linux, I found the code path for Linux was identical for Freebsd and the cleanup destructor was being executed 4 times just like Freebsd, the difference being that Freebsd would print out this benign warning while Linux would just ignore it. The problem is that all threads that are created and initialize TLS current thread data, must clean them up by explicitly setting the TLS current thread to null. I’ve come up with two approaches to accomplish this.
clean up TLS current thread at end of ::run functions similar to how it's done in openjdk8.
http://brian.timestudybuddy.com/webrev/hotspot__clear_thread_current/webrev/ clear current thread before exiting java_start to avoid warnings from leftover pthread_setspecific data
http://brian.timestudybuddy.com/webrev/hotspot__clear_thread_current_alt/web...
I do not think this is a real leak. From what I remember of how the glibc implements TLS, setting the TLS slot value to NULL would not in itself delete anything. In VM, this slot keeps the pointer to the current Thread*, which is correctly deleted at the end of the thread (void JavaThread::thread_main_inner()).
Digging further, I found the pthread key destructor "restore_thread_pointer(void* p)" in threadLocalStorage_posix.cpp:
// Restore the thread pointer if the destructor is called. This is in case // someone from JNI code sets up a destructor with pthread_key_create to run // detachCurrentThread on thread death. Unless we restore the thread pointer we // will hang or crash. When detachCurrentThread is called the key will be set // to null and we will not be called again. If detachCurrentThread is never // called we could loop forever depending on the pthread implementation. extern "C" void restore_thread_pointer(void* p) { ThreadLocalStorage::set_thread((Thread*) p); }
So, it seems we even reset deliberately the thread pointer to a non-NULL value. The comment claims that we reset the Thread* value in case there is another user-provided destructor which runs afterwards and which does detachCurrentThread () which would require Thread::current() to work. But there a details I do not understand:
- At this point, should the Thread* object not already be deallocated, so this would be a dangling pointer anyway?
- Also, according to Posix, this is unspecified. Doc on pthread_setspecific() states: "Calling pthread_setspecific() from a thread-specific data destructor routine may result either in lost storage (after at least PTHREAD_DESTRUCTOR_ITERATIONS attempts at destruction) or in an infinite loop."
- In jdk8, we did reset the slot value to NULL before Thread exit. So, in this case detachCurrentThread() from a pthread_key destructor should not have worked at all.
Could someone from Oracle maybe shed light on this?
Please see the following discussion and bug report: http://mail.openjdk.java.net/pipermail/hotspot-runtime-dev/2014-February/010... Note I don't follow this list so please include me directly in any follow-ups if needed. Thanks, David