RFR: JDK-8283326: Implement SafeFetch statically

[Thomas Stuefe]

On Mon, 11 Apr 2022 09:58:16 GMT, Florian Weimer <fweimer at openjdk.org> wrote:

>> Ah, that explains the weird errors I got. I'll change the comment.
>
> `pthread_getspecific` and `pthread_setspecific` cannot be called from a signal handler, either. In particular, `pthread_setspecific` may call `malloc`.
> 
> On GNU/Linux, if you use initial-exec TLS, it is fully async-signal-safe. Less so for the other variants. But maybe this does not matter because the assembler implementation uses `os::Posix::ucontext_set_pc` to redirect the execution in a more direction fashion?

@fweimer-rh I'll move `pthread_setspecific` out of signal handling, thanks for the hint.

`pthread_getspecific` may not be signal safe, but it has been in use in JVM signal handlers for decades (signal handlers call `Thread::current`, which uses TLS) and it works in practice on all Posix platforms.

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PR: https://git.openjdk.java.net/jdk/pull/7865