The major differences of creating a new thread from creating a
new process is that

1. newbie's tgid is set to leader's
2. newbie's leader is set to leader
3. newbie is added to leader's thread_list

So move the initialization of these in one place. This helps
in pid/tgid fields isolation.

Signed-off-by: Pavel Emelyanov <xemul@openvz.org>

---

diff --git a/kernel/fork.c b/kernel/fork.c
index 7517efe..0b282a8 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -950,6 +950,20 @@ static inline void rt_mutex_init_task(st
 #endif
 }
 
+static void setup_new_thread(struct task_struct *thr, struct task_struct *leader)
+{
+	thr->tgid = leader->tgid;
+	thr->group_leader = leader;
+	list_add_tail_rcu(&thr->thread_group, &leader->thread_group);
+}
+
+static void setup_new_leader(struct task_struct *tsk)
+{
+	tsk->tgid = task_pid_nr(tsk);
+	tsk->group_leader = tsk;
+	INIT_LIST_HEAD(&tsk->thread_group);
+}
+
 /*
  * This creates a new process as a copy of the old one,
  * but does not actually start it yet.
@@ -1147,9 +1161,6 @@ static struct task_struct *copy_process(
 	}
 
 	p->pid = pid_nr(pid);
-	p->tgid = p->pid;
-	if (clone_flags & CLONE_THREAD)
-		p->tgid = current->tgid;
 
 	p->set_child_tid = (clone_flags & CLONE_CHILD_SETTID) ? child_tidptr : NULL;
 	/*
@@ -1191,8 +1202,6 @@ static struct task_struct *copy_process(
 	 * Ok, make it visible to the rest of the system.
 	 * We dont wake it up yet.
 	 */
-	p->group_leader = p;
-	INIT_LIST_HEAD(&p->thread_group);
 	INIT_LIST_HEAD(&p->ptrace_children);
 	INIT_LIST_HEAD(&p->ptrace_list);
 
@@ -1251,8 +1260,7 @@ static struct task_struct *copy_process(
 	}
 
 	if (clone_flags & CLONE_THREAD) {
-		p->group_leader = current->group_leader;
-		list_add_tail_rcu(&p->thread_group, &p->group_leader->thread_group);
+		setup_new_thread(p, current->group_leader);
 
 		if (!cputime_eq(current->signal->it_virt_expires,
 				cputime_zero) ||
@@ -1268,7 +1276,8 @@ static struct task_struct *copy_process(
 			 */
 			p->it_prof_expires = jiffies_to_cputime(1);
 		}
-	}
+	} else
+		setup_new_leader(p);
 
 	if (likely(p->pid)) {
 		add_parent(p);
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On 08/24, Pavel Emelyanov wrote:
>
> The major differences of creating a new thread from creating a
> new process is that
>
> 1. newbie's tgid is set to leader's
> 2. newbie's leader is set to leader
> 3. newbie is added to leader's thread_list

(Surely, the are many other major differences, but from the pids virtualization
 POV - yes ;)

> +static void setup_new_thread(struct task_struct *thr, struct task_struct 
> *leader)
> +{
> +	thr->tgid = leader->tgid;
> +	thr->group_leader = leader;
> +	list_add_tail_rcu(&thr->thread_group, &leader->thread_group);
> +}

Imho, this name is a bit "too generic". Not that I can suggest something
better... copy_sub_thread/copy_group_leader ?

> @@ -1147,9 +1161,6 @@ static struct task_struct *copy_process(
> 	}
> 
> 	p->pid = pid_nr(pid);
> -	p->tgid = p->pid;
> -	if (clone_flags & CLONE_THREAD)
> -		p->tgid = current->tgid;

I agree, it is absoulutely not clear why should we set ->tgid here, and it
would be nice to consolidate "if (CLONE_THREAD)" checks, but do we really
need the helpers above? There are very simple, and have the only one caller.
Sometimes it is good to see what's going on without pressing C-]

Not that I against this patch, just I'm not sure it really simplifies things.
Perhaps I missed something else you have in mind.

Oleg.

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Oleg Nesterov wrote:
> On 08/24, Pavel Emelyanov wrote:
>> The major differences of creating a new thread from creating a
>> new process is that
>>
>> 1. newbie's tgid is set to leader's
>> 2. newbie's leader is set to leader
>> 3. newbie is added to leader's thread_list
> 
> (Surely, the are many other major differences, but from the pids virtualization
>  POV - yes ;)
> 
>> +static void setup_new_thread(struct task_struct *thr, struct task_struct 
>> *leader)
>> +{
>> +	thr->tgid = leader->tgid;
>> +	thr->group_leader = leader;
>> +	list_add_tail_rcu(&thr->thread_group, &leader->thread_group);
>> +}
> 
> Imho, this name is a bit "too generic". Not that I can suggest something
> better... copy_sub_thread/copy_group_leader ?
> 
>> @@ -1147,9 +1161,6 @@ static struct task_struct *copy_process(
>> 	}
>>
>> 	p->pid = pid_nr(pid);
>> -	p->tgid = p->pid;
>> -	if (clone_flags & CLONE_THREAD)
>> -		p->tgid = current->tgid;
> 
> I agree, it is absoulutely not clear why should we set ->tgid here, and it
> would be nice to consolidate "if (CLONE_THREAD)" checks, but do we really
> need the helpers above? There are very simple, and have the only one caller.
> Sometimes it is good to see what's going on without pressing C-]
> 
> Not that I against this patch, just I'm not sure it really simplifies things.
> Perhaps I missed something else you have in mind.

Me too, but while cleaning up the pid_t usage over the kernel I found
this place to be one of the most difficult from "how to make it better"
point of view. We need to hide the pid/tgid explicit usage somehow, but 
the problem is that pid and tgid are set in this place and de_thread() 
only and making helpers like set_task_tgid() doesn't sound reasonable.

> Oleg.
> 
> 

Thanks,
Pavel
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