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

[Daniel D. Daugherty]

On 7/17/17 1:00 AM, Thomas Stüfe wrote:
> 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/

src/share/vm/runtime/thread.hpp
     No comments.

Thumbs up! Thanks for the clear analysis in the bug.

Dan


> 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