Replacing syscall(_NR_fork) on linux with syscall(_NR_clone) (original) (raw)

Thomas Stüfe thomas.stuefe at gmail.com
Fri Apr 10 10:36:19 UTC 2015

Hi David,

it is difficult to understand Linux implementation of os::fork_and_exec() fully. Comment claims fork() would not be async safe for use in signal handler, but I do not see why (maybe I am being slow here): pthread_atfork() handlers get called in the context of the thread calling the fork(). Depending on the pthread_atfork handlers, this may make the code non-reentrant, but async-unsafe?

Also the comment above the execve syscall is a bit of a riddle. "execve() will instead kill every thread in the parent process.". I would love to understand why.

But if pthread_atfork handlers are the problem, how about just deactivating pthread_atfork handlers for the duration of os::fork_and_exec? Note that the jdk itself does not install pthread_atfork handlers, so this only affects user handlers.

Another alternative to using raw syscalls would be to use posix_spawn(), which I believe does not call pthread_atfork handlers.

Regards, Thomas

On Fri, Apr 10, 2015 at 9:47 AM, David Holmes <david.holmes at oracle.com> wrote:

In os::forkandexec we provide a per platform fork/exec implementation for use by the OnError and OnOutOfMemory JVM options.

On linux this is implemented with direct syscalls to NRfork (sysfork) and NRexecve. We already encountered a problem on linux-sparc: http://mail.openjdk.java.net/pipermail/hotspot-runtime-dev/ 2015-February/013811.html due to a different register usage for the return value, which is so far restricted to the 7u train but will need addressing in 8u and 9. Further it seems that the fork syscall has actually been deprecated for quite some time and we are now seeing kernels on some platforms which are no longer providing any of the deprecated syscalls - we simply get an error ENOSYS. The fork() library was updated to use clone a long time ago, but as I understand it we can't use the fork() library call because we might be executing in a signal-handling context within the error handler. So I'm considering switching to use the clone syscall for all linux systems. Thoughts/comments? David

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