Subject: Re: 2.6.23-mm1 thread exit_group issue

From: Mathieu Desnoyers <[email-masked]>
Date: Sun, 14 Oct 2007 00:04:33 -0400

* Oleg Nesterov ([email-masked]) wrote:
> On 10/12, Andrew Morton wrote:
> >
> > On Fri, 12 Oct 2007 15:47:59 -0400
> > Mathieu Desnoyers <[email-masked]> wrote:
> > 
> > > Hi Andrew,
> > > 
> > > I noticed a regression between 2.6.23-rc8-mm2 and 2.6.23-mm1 (with your
> > > hotfixes). User space threads seems to receive a ERESTART_RESTARTBLOCK
> > > as soon as a thread does a pthread_join on them. The previous behavior
> > > was to wait for them to exit by taking a futex.
> 
> No, the reason is that pthread_join() succeeds while it shouldn't. The main
> thread does exit_group() and kills the sub-thread sleeping in nanosleep.
> ERESTART_RESTARTBLOCK is not delivered to the user-space (sub-thread is dying),
> it is just reported by gdb.
> 
> > > I provide a toy program that shows the problem. On 2.6.23-rc8-mm2, it
> > > loops forever (as it should). On 2.6.23-mm1, it exits after 10 seconds.
> 
> I bet something like this
> 
> 	void *threda(void *arg)
> 	{
> 		for (;;)
> 			pause();
> 		return NULL;
> 	}
> 
> 	int main(void)
> 	{
> 		pthread_t tid;
> 
> 		pthread_create(&tid, NULL, thread, NULL);
> 		pthread_join(tid, NULL);
> 
> 		return 0;
> 	}
> 
> won't work as well.
> 
> > > Any idea on what may cause this problem ?
> 
> Because do_fork() doesn't use parent_tidptr. At all! So it is very clear
> why 2.6.23-mm1 is broken.
> 
> > Bisection shows that this problem is caused by these two patches:
> >
> > pid-namespaces-allow-cloning-of-new-namespace.patch
> 
> This? http://marc.info/?l=linux-mm-commits&m=118712242002039
> 
> Pavel, this patch has a subtle difference compared to what we discussed on
> containers list. It moves put_user(parent_tidptr) from copy_process() to
> do_fork(), so we don't report child's pid if copy_process() failed. I do
> not think this is bad, but Eric seems to disagree with such a change.
> 
> But I can't understand why Andrew sees the same problem _after_ this patch!
> 
> And which patch removed the "put_user(nr, parent_tidptr)" chunk?
> 
> Andrew, could I get the kernel source after bisection somehow? (I am not
> familiar with guilt, will try to study it later)
> 
> Mathieu, could you try the patch below?
> 

Hi Oleg,

Yes, it runs fine with this patch.

Thanks,

Mathieu

> Oleg.
> 
> --- kernel/fork.c~	2007-10-13 15:41:35.000000000 +0400
> +++ kernel/fork.c	2007-10-13 15:41:41.000000000 +0400
> @@ -1443,6 +1443,9 @@ long do_fork(unsigned long clone_flags,
>  			task_pid_nr_ns(p, current->nsproxy->pid_ns) :
>  				task_pid_vnr(p);
>  
> +		if (clone_flags & CLONE_PARENT_SETTID)
> +			put_user(nr, parent_tidptr);
> +
>  		if (clone_flags & CLONE_VFORK) {
>  			p->vfork_done = &vfork;
>  			init_completion(&vfork);
> 

-- 
Mathieu Desnoyers
Computer Engineering Ph.D. Student, Ecole Polytechnique de Montreal
OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F  BA06 3F25 A8FE 3BAE 9A68