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

[coleen.phillimore at oracle.com]

Seems good.  I'll check it in for you.
Coleen

On 7/18/17 9:02 AM, Thomas Stüfe wrote:
> Hi Folks,
>
> may I please have a second review and a sponsor?
>
> This is a bit of an annoying issue, it causes endless recursions on AIX
> because error reporting stumbles over itself when calling Thread::current().
>
> Thanks! ..Thomas
>
> On Mon, Jul 17, 2017 at 9:00 AM, Thomas Stüfe <thomas.stuefe at gmail.com>
> 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/
>>
>> 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
>>