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SubjectRe: [patch] O(1) sys_exit(), threading, scalable-exit-2.5.31-A6
On Tue, Aug 27, 2002 at 11:39:57AM -0400, Daniel Jacobowitz wrote:
> On Tue, Aug 20, 2002 at 04:36:19PM +0200, Ingo Molnar wrote:
> >
> > the attached patch ontop of BK-curr fixes the ptrace wait4() anomaly that
> > can be observed in any previous Linux kernel i could get my hands at. So
> > in fact ->ptrace_children, besides being a speedup, also fixed a bug that
> > couldnt be fixed in any satisfactory way before.
> >
> > Ingo
> >
> > --- linux/kernel/exit.c.orig Tue Aug 20 16:28:57 2002
> > +++ linux/kernel/exit.c Tue Aug 20 16:30:13 2002
> > @@ -731,7 +731,7 @@
> > tsk = next_thread(tsk);
> > } while (tsk != current);
> > read_unlock(&tasklist_lock);
> > - if (flag) {
> > + if (flag || !list_empty(&current->ptrace_children)) {
> > retval = 0;
> > if (options & WNOHANG)
> > goto end_wait4;
>
> Ingo,
>
> At this point your ptrace changes have completely broken both
> _TRACEME/exec and _ATTACH debugging. If an attached process finishes
> while a debugger is attached, its parent no longer gets the proper wait
> result for it:
>
> wait4(-1, [WIFEXITED(s) && WEXITSTATUS(s) == 0], WNOHANG|WUNTRACED, NULL) = 478
> wait4(-1, [WIFEXITED(s) && WEXITSTATUS(s) == 0], WNOHANG|WUNTRACED, NULL) = 478
> wait4(-1, [WIFEXITED(s) && WEXITSTATUS(s) == 0], WNOHANG|WUNTRACED, NULL) = 478
> wait4(-1, [WIFEXITED(s) && WEXITSTATUS(s) == 0], WNOHANG|WUNTRACED, NULL) = 478
> wait4(-1, [WIFEXITED(s) && WEXITSTATUS(s) == 0], WNOHANG|WUNTRACED, NULL) = 478
> wait4(-1, [WIFEXITED(s) && WEXITSTATUS(s) == 0], WNOHANG|WUNTRACED, NULL) = 478
>
> etc. It is never removed from the list. _TRACEME/exec debugging
> appears to have the same problem but it's harder to tell, since one can
> not strace GDB in 2.5 without the patch I posted here two weeks ago.
> If you don't have a chance to look at this I'll investigate later
> today.

The problem is the line:
retval = p->pid;
!!! if (p->real_parent != p->parent || p->ptrace) {
write_lock_irq(&tasklist_lock);
remove_parent(p);
p->parent = p->real_parent;
add_parent(p, p->parent);
do_notify_parent(p, SIGCHLD);
write_unlock_irq(&tasklist_lock);
} else
release_task(p);

p->ptrace continues to be true, even if the real parent is waiting for
it. So we go through this code time after time once p->real_parent ==
p->parent, sending it extra SIGCHLDs and not releasing the task. I
don't think that change is correct but I'm not clear what you're doing
with the reparenting right now, so I don't know the right fix.

You might want to add:
gdb /bin/ls
run
run

to your stress testing in the future.


--
Daniel Jacobowitz
MontaVista Software Debian GNU/Linux Developer
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