Question about thread initialization (original) (raw)

David Holmes david.holmes at oracle.com
Mon Oct 15 05:32:03 UTC 2018

On 13/10/2018 5:49 PM, Thomas Stüfe wrote:

On Sat, Oct 13, 2018 at 8:36 AM Thomas Stüfe <thomas.stuefe at gmail.com> wrote:

Hi David, On Sat, Oct 13, 2018 at 1:42 AM David Holmes <david.holmes at oracle.com> wrote:

Hi Thomas, On 13/10/2018 3:56 AM, Thomas Stüfe wrote: Hi David,

thank you for your brain cycles. On Fri, Oct 12, 2018 at 12:39 PM David Holmes <david.holmes at oracle.com> wrote:

Hi Thomas, On 12/10/2018 6:18 PM, Thomas Stüfe wrote: Hi all,

a small question. JVMStartThread calls new JavaThread() JavaThread::JavaThread() calls os::createthread() os::createthread() starts the new thread and waits for the handshake, then returns Back in JVMStartThread, we call JavaThread::prepare(), which adds the new thread to the Threads list. By that time, the new thread is already running, but how far it has gotten is unknown. Right - though the new thread can't enter run() until after it has been prepared and "started". There are two parts to the handshake: Parent thread New Thread start new Thread wait for new thread signal parent wait for parent prepare new thread "start" new thread signal new thread run()

Ah, I see. The new thread is taken off the leash only at the end of JVMStartThread(), when Thread::start() is called and the final part of the handshake is completed. The new thread's stack dimensions are set from within Thread::run() (for some reason, every child class does this on its own?) by calling Thread::recordstackbaseandsize(). So, after the handshake with its parent thread. Why? Good question. Undoubtedly historical. :) This means we have a little race: in the Threads list there may be threads which have been just created and Thread::run() did not yet get around to set the stack size. In tests I stumbled over this, very rarely, when iterating Threads to check the stack sizes. Hmmm. Threads that are still threadnew should really be invisible until more fully initialized. How exactly were you iterating the threads? This might be an oversight in the related APIs. This was because of a little unrelated test I wrote to accumulate the combined thread stack sizes. They did not add up and I was confused. I am now less confused: In JVMStartThread(): { grab thread list lock new JavaThread() - calls pthreadcreate, new thread starts and waits for handshake. stack base, size still NULL,0. JavaThread::prepare() - thread gets added to Threads list a } // relinquish threads lock ... ... Threads::start() - signal new thread to run b - new thread completes handshake, calls Thread::run(), and all implementations pretty much immediately set the stackbase/size. Between (a) and (b) another thread could grab the Threads lock, iterate the threads and would see the new thread with uninitialized base/size. To prove this I did a little test right there, in JVMStartThread, after (a): + { + MutexLocker mu(Threadslock); + MyThreadClosure tc; + Threads::threadsdo(&tc); + } the thread closure just prints the stack dimensions of the thrad. Sure enough, the new thread still has NULL/0 for stack base/size. In fact, Thread::stackbase() asserts because of this. So, could not another thread happen to do the same in this little time interval? There should be some rules about how thread closures gather their target threads based on the thread state. Part of that should (now that I think about it) filter out threads that are still "new" - but whether they do or not is a different matter. This may well just be an oversight. I think so too. Is there any reason why we could not just call recordstackbaseandsize() before calling Thread::run(), right at the start of the native entry function (threadnativeentry in the case of Linux)? Have you tried it? :) I can't immediately see why this can't happen in the threadnativeentry. It's possible there was once a dependency with something in thread preparation etc, but that's just speculation on my part. Funny, I wanted to test it and then it occurred to me that we do this all along already on AIX: At the entrace of threadnativeentry I set stack base and size (that was even my own code from the initial AIX port). Unfortunately I have lost the history for that change and do not know anymore why I did this. Since we never had problems on AIX I guess this is okay for other platforms too - as long os::currentstackbase() / currentstacksize() have no side effects and do not rely on anything but OS functionality. Okay so are you going to propose making such a change? Yes. I'll prepare a webrev. Yikes Thread::recordstackbaseandsize() is complex. That thing's name is a blatant lie, it does way more than that, especially on Solaris.

To be fair the problem is in the naming of os::initialize_thread() and the fact that on Solaris it has also been used for additional thread initialization when only intended for fixing up issues with the primordial thread's stack. And it seems only an issue with Solaris as everywhere else it is a empty method (and I'm unclear how it ended up as an _.cpp function!)

I would think you can move record_stack_base_and_size() in thread_native_entry; delete initialize_thread(Thread t) completely. Put the code that was in the Solaris initialize_thread into the Solaris naitve_thread_entry after record_stack_base_and_size().

I feel unsure about moving this up to the start of the native entry function, since it is not aware of its surrounding Thread class being only half initialized.

On AIX, we just call "setstackbase" and "setstacksize" directly, and later, in shared code, run thru Thread::recordstackbaseandsize() like everyone else. So, we just pre-initialized base and size. I do not want to do this for all platforms, since this is ugly. I wonder whether a better solution would be to change Thread::recordstackbaseandsize(): 1 move the calls to setstackbase() and setstacksize() to the OS specific threadnativeentry(). We can call this right away in the newly born thread, since it relies on nothing else. 2 leave the rest in place and rename the function to something like "Thread::finishinitialization()" 3 move the call to Thread::finishinitialization() up out of the ::run() functions to threadnativeentry() - just before it calls Thread::run(). 4 Alternative: to (3), make Thread::run() a non-virtual public function - a place for os-generic and thread-class-generic common stuff to-be-run-before-run(). Add a second, protected method, virtual Thread::dorun() = 0, which is the real function child classes should override. BTW I remember now why I did move the stack base/size initialization on AIX up to the start of threadnativeentry. We had crashes in newly spawned child threads and no usable hs-err files since to print a stack trace you need to know the stack dimensions. Another reason to change this.

Well ... a thread should be doing very little interesting in terms of crashes prior to calling run() and setting up the stack information, and there will always be somewhere it can crash before this is set up, so perhaps the error reporter also needs to be bit more cautious about assuming things about the thread initialization state and the ability to use the Thread API.

Cheers, David

..Thomas

Cheers, Thomas

Cheers, David

Cheers, Thomas Am I missing something here? I guess we will find out :) Cheers, David Thanks,

Thomas

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