Why is fork1() exported from hotspot?

[Daniel D. Daugherty]

On 10/20/15, 1:14 AM, Thomas Stüfe wrote:
> Hi all,
>
> on Linux we define and export "fork1()" - a stub for fork() - and "fork1"
> also appears in linker mapfiles for bsd and AIX. The latter, I am sure, is
> just a copy-paste-effect.
>
> Why do we need to define and export fork1() for non-solaris platforms? We
> only ever use it on Solaris.
>
> The comment in os_linux.cpp is not really enlightening:
> "// Something to do with the numa-aware allocator needs these symbols"
>
> Does anyone know why this is needed?
>
> Regards, Thomas

$ hg -R hotspot annot hotspot/src/os/linux/vm/os_linux.cpp | grep fork1
2072: extern "C" JNIEXPORT int fork1() { return fork(); }

$ hg -R hotspot log -r 2072 hotspot/src/os/linux/vm/os_linux.cpp
changeset:   2072:d70fe6ab4436
parent:      2062:3582bf76420e
user:        coleenp
date:        Tue Feb 01 11:23:19 2011 -0500
summary:     6588413: Use -fvisibility=hidden for gcc compiles

Here's the review thread: 
http://mail.openjdk.java.net/pipermail/hotspot-runtime-dev/2011-January/001726.html


Here's the diff for just that file:

$ hg -R hotspot diff -r 2062 -r 2072 hotspot/src/os/linux/vm/os_linux.cpp
diff -r 3582bf76420e -r d70fe6ab4436 src/os/linux/vm/os_linux.cpp
--- a/src/os/linux/vm/os_linux.cpp      Thu Jan 27 16:11:27 2011 -0800
+++ b/src/os/linux/vm/os_linux.cpp      Tue Feb 01 11:23:19 2011 -0500
@@ -2509,8 +2509,10 @@ char *os::scan_pages(char *start, char*
    return end;
  }

-extern "C" void numa_warn(int number, char *where, ...) { }
-extern "C" void numa_error(char *where) { }
+// Something to do with the numa-aware allocator needs these symbols
+extern "C" JNIEXPORT void numa_warn(int number, char *where, ...) { }
+extern "C" JNIEXPORT void numa_error(char *where) { }
+extern "C" JNIEXPORT int fork1() { return fork(); }


  // If we are running with libnuma version > 2, then we should
@@ -3483,7 +3485,7 @@ bool os::is_interrupted(Thread* thread,
  // Note that the VM will print warnings if it detects conflicting signal
  // handlers, unless invoked with the option 
"-XX:+AllowUserSignalHandlers".
  //
-extern "C" int
+extern "C" JNIEXPORT int
  JVM_handle_linux_signal(int signo, siginfo_t* siginfo,
                          void* ucontext, int abort_if_unrecognized);

@@ -4678,44 +4680,6 @@ void os::pause() {
    }
  }

-extern "C" {
-
-/**
- * NOTE: the following code is to keep the green threads code
- * in the libjava.so happy. Once the green threads is removed,
- * these code will no longer be needed.
- */
-int
-jdk_waitpid(pid_t pid, int* status, int options) {
-    return waitpid(pid, status, options);
-}
-
-int
-fork1() {
-    return fork();
-}
-
-int
-jdk_sem_init(sem_t *sem, int pshared, unsigned int value) {
-    return sem_init(sem, pshared, value);
-}
-
-int
-jdk_sem_post(sem_t *sem) {
-    return sem_post(sem);
-}
-
-int
-jdk_sem_wait(sem_t *sem) {
-    return sem_wait(sem);
-}
-
-int
-jdk_pthread_sigmask(int how , const sigset_t* newmask, sigset_t* oldmask) {
-    return pthread_sigmask(how , newmask, oldmask);
-}
-
-}

  // Refer to the comments in os_solaris.cpp park-unpark.
  //


So fork1() used to be an "extern C" wrapper around fork() on Linux
and now it is a JNIEXPORT'ed wrapper.

I don't think the addition of this comment:

+// Something to do with the numa-aware allocator needs these symbols

was meant to apply to the new JNIEXPORT of fork1(). However, it
seems very strange that fork1() moved from a section that was
associated with "green threads" to a section associated with
"numa-aware allocator"...

Volker and Tom R are credited with doing some perf runs in the
code review thread... Perhaps either of them can shed some light
here?

Dan