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

David Holmes david.holmes at oracle.com
Tue Apr 14 05:56:18 UTC 2015

Hi Thomas,

On 10/04/2015 8:36 PM, Thomas Stüfe wrote:

Hi David,

it is difficult to understand Linux implementation of os::forkandexec() 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): pthreadatfork() handlers get called in the context of the thread calling the fork(). Depending on the pthreadatfork handlers, this may make the code non-reentrant, but async-unsafe?

As the comment says:

// fork() in LinuxThreads/NPTL is not async-safe.

but I suspect this is an extremely old comment and no longer applicable to NPTL. So I think you and Martin are right that there's no reason not to call fork() and execve() using the glibc calls rather than the direct syscalls.

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.

Again as stated in the comments this was a "feature" of the LinuxThreads implementation. I don't know the details - before my time :)

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

I don't think at_fork handlers are specifically an issue - this is not trying to solve every problem in every circumstance.

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

"It is implementation-defined whether the fork handlers are run when posix_spawn() or posix_spawnp() is called." - POSIX

Of course Linux may have made a determination one way or another.

But given Martin's experience with using fork() and execve() at Google, I would propose just adopting that approach.

Aside: it was also suggested that we simply drop this "onError" mechanism and replace it with one that simply halts the VM process and thereby allow the user to run arbitrary external commands. That may be worth an RFE in itself, but I need something in the short term.

Thanks, David

Regards, Thomas

On Fri, Apr 10, 2015 at 9:47 AM, David Holmes <david.holmes at oracle.com_ _<mailto: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 <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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