rfork(RFPROC|RFMEM)

[Igor Sysoev]

On Mon, 5 May 2003, Terry Lambert wrote:

> Igor Sysoev wrote:
> > On Mon, 5 May 2003, Terry Lambert wrote:
> > What is stack glue ?
> 
> See the code in fork1() in /sys/kern/kern_fork.c.

I do not see any stack manipulation in kern_fork.c except the creating
alternate kstack for KSE thread in 5.0. And rfork(2) can not create
such stack - it passes 0 to fork1().
In 4.x there's no stack code at all.

> > I use rfork_thread(3) wrapper that allows to setup another stack for
> > rfork()ed process.

By the way I found the bug in x86 rfork_thread(3)'s error handling:

--- /usr/src/lib/libc/i386/gen/rfork_thread.S   Wed Feb  7 03:12:45 2001
+++ /usr/src/lib/libc/i386/gen/rfork_thread.S   Tue May  6 17:45:14 2003
@@ -108,5 +108,8 @@
         * Branch here if the thread creation fails:
         */
 2:
+       popl    %esi
+       movl    %ebp, %esp
+       popl    %ebp
        PIC_PROLOGUE
        jmp     PIC_PLT(HIDENAME(cerror))


> > What RFTHREAD flag does ?
> 
> See the code.  It basically sets up for kernel threading, rather
> than merely for processes sharing the same address space and/or
> file descriptor table and/or heap, which is what rfork was
> intended to be able to do.  It also ensures propagation of any
> SIGKILL to all peers, so they die all at once, in exit1() in 
> /sys/kern/kern_exit.c.

It seems that the single purpose of the RFTHREAD flag is to kill peers
when the leader got SIGKILL.
And in 4.8-STABLE and 5.0-CURRENT (after 5.0-RELEASE) the leader also
holds P_ADVLOCK flag.

> > By the way linuxthreads port always uses RFTHREAD flag.
> 
> They don't know any other way than the moral equivalent of
> the Linux "clone" system call, so that's what they use; it's
> technically not necessary.  See also the source code in the

Now it's necessary. Otherwise rfork() returns EINVAL.

> directory /usr/src/lib/libpthread, which doesn't use rfork()
> at all.

I know it.


Igor Sysoev
http://sysoev.ru/en/