8203341: Add a safepoint-aware Semaphore

[Erik Österlund]

Hi Stefan,

I wonder if it would make sense to let Semaphore::wait have a bool 
safepoint_check, similar to Mutex (possibly with a default value), 
instead of introducing a new class with the same API, that only differs 
in this property.

Thanks,
/Erik

On 2018-05-17 09:46, Stefan Karlsson wrote:
> Hi all,
>
> Please review this patch to add a safepoint-aware Semaphore (and 
> refactor the semaphore usage in thread-local handshakes).
>
> http://cr.openjdk.java.net/~stefank/8203341/webrev.01/
> https://bugs.openjdk.java.net/browse/JDK-8203341
>
> Both ZGC and the thread-local handshakes need to move the JavaThread 
> to a blocked state before waiting on a Semaphore. I propose that we 
> introduce a new SemaphoreSafepointAware wrapper around Semaphore.
>
> There are two things to consider with this patch:
>
> 1) In ZGC, where this patch originates from, we set the JavaThread's 
> osthread state with osthread->set_state(CONDVAR_WAIT) (using 
> OSThreadWaitState). This means that we get the following printed when 
> the threads are dumped: "waiting on condition ". I've kept this 
> behavior for the SemaphoreSafepointAware class. This means that we now 
> change the osthread state for JavaThreads blocking in 
> HandshakeState::process_self_inner.
>
> 2) The thread-local handshakes uses trywait before entering wait:
>    if (!_semaphore.trywait()) {
> -    ThreadBlockInVM tbivm(thread);
>      _semaphore.wait();
>    }
>
> At the time when SemaphoreSafepointAware was written, we didn't have 
> trywait on all platforms, now we do. Maybe we should always do the 
> trywait dance inside SemaphoreSafepointAware::wait() ?
>
> inline void SemaphoreSafepointAware::wait() {
>   if (Thread::current()->is_Java_thread()) {
>     if (!_semaphore.trywait) {
>       JavaThread* thread = JavaThread::current();
>
>       // Prepare to block and allow safepoints while blocked
>       ThreadBlockInVM tbivm(thread);
>       OSThreadWaitState osts(thread->osthread(), false /* not in 
> Object.wait() */);
>
>       // Wait for value
>      _semaphore.wait();
>    }
>
> Thanks,
> StefanK