URGENT (XS) RFR: 8149697: Fix for 8145725 is broken

[Kim Barrett]

> On Feb 11, 2016, at 3:37 PM, David Holmes <david.holmes at oracle.com> wrote:
> 
> bug: https://bugs.openjdk.java.net/browse/JDK-8149697
> 
> webrev: http://cr.openjdk.java.net/~dholmes/8149697/webrev/
> 
> In the refactoring for 8145725 a check of _cur_index was moved outside the mutex block, but this was wrong as the value could be changed by a parking thread. As per the existing comment we needed to check the saved 'index' instead:
> 
> --- old/src/os/linux/vm/os_linux.cpp	2016-02-11 15:23:59.696802390 -0500
> +++ new/src/os/linux/vm/os_linux.cpp	2016-02-11 15:23:57.440675175 -0500
> @@ -5756,7 +5756,7 @@
>   int index = _cur_index;
>   status = pthread_mutex_unlock(_mutex);
>   assert_status(status == 0, status, "invariant");
> -  if (s < 1 && _cur_index != -1) {
> +  if (s < 1 && index != -1) {
>     // thread is definitely parked
>     status = pthread_cond_signal(&_cond[index]);
>     assert_status(status == 0, status, "invariant");
> 
> This could introduce spurious wakeups for a parking thread.
> 
> Thanks,
> David

Looks good.