RFR(xs): 8184339: Thread::current_or_null() should not assert if Posix TLS is not yet initialized

[Thomas Stüfe]

Hi all,

may I please have reviews + a sponsor for the following fix:

bug: https://bugs.openjdk.java.net/browse/JDK-8184339
webrev:
http://cr.openjdk.java.net/~stuefe/webrevs/8184339-Thread-current-or-null-shall-not-assert/webrev.00/webrev/

The problem is caused by the fact that Posix TLS cannot be used before it
is initialized. It is initialized in os::init(). If we use Posix TLS (eg
via Thread::current()) before, we assert. It is used now (since JDK-8183925
<https://bugs.openjdk.java.net/browse/JDK-8183925>) before os::init() (see
callstack in bug description).

There are two functions, Thread::current() and Thread::current_or_null().
The latter should not assert if Posix TLS is not yet available but return
NULL instead. This is what callers expect: this function is safe to call,
but it might not return a valid Thread*.

Note that this issue currently only comes up at AIX, because AIX is the
only platform using Posix TLS for Thread::current() - all other platforms
use Compiler-based TLS (__thread variables).

However, we want to keep the Posix TLS code path alive, so this may also
affect other platforms. There have been compiler bugs in the past (e.g. gcc
+ glibc) leading to errors when using compiler-based TLS, so it is good to
have a reliable fallback.

Thanks,

Thomas