kernel/fork.c:copy_process(): consolidate the lockless CLONE_THREAD checks

copy_process() does a lot of "chaotic" initializations and checks
CLONE_THREAD twice before it takes tasklist.  In particular it sets
"p->group_leader = p" and then changes it again under tasklist if
!thread_group_leader(p).

This looks a bit confusing, lets create a single "if (CLONE_THREAD)" block
which initializes ->exit_signal, ->group_leader, and ->tgid.
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Cc: "Eric W. Biederman" <ebiederm@xmission.com>
Cc: Michal Hocko <mhocko@suse.cz>
Cc: Pavel Emelyanov <xemul@parallels.com>
Cc: Sergey Dyasly <dserrg@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

kernel/fork.c

@@ -1360,11 +1360,6 @@ static struct task_struct *copy_process(unsigned long clone_flags,
 			goto bad_fork_cleanup_io;
 	}
 
-	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;
 	/*
 	 * Clear TID on mm_release()?
@@ -1400,12 +1395,19 @@ static struct task_struct *copy_process(unsigned long clone_flags,
 	clear_all_latency_tracing(p);
 
 	/* ok, now we should be set up.. */
-	if (clone_flags & CLONE_THREAD)
+	p->pid = pid_nr(pid);
+	if (clone_flags & CLONE_THREAD) {
 		p->exit_signal = -1;
-	else if (clone_flags & CLONE_PARENT)
-		p->exit_signal = current->group_leader->exit_signal;
-	else
-		p->exit_signal = (clone_flags & CSIGNAL);
+		p->group_leader = current->group_leader;
+		p->tgid = current->tgid;
+	} else {
+		if (clone_flags & CLONE_PARENT)
+			p->exit_signal = current->group_leader->exit_signal;
+		else
+			p->exit_signal = (clone_flags & CSIGNAL);
+		p->group_leader = p;
+		p->tgid = p->pid;
+	}
 
 	p->pdeath_signal = 0;
 	p->exit_state = 0;
@@ -1414,15 +1416,13 @@ static struct task_struct *copy_process(unsigned long clone_flags,
 	p->nr_dirtied_pause = 128 >> (PAGE_SHIFT - 10);
 	p->dirty_paused_when = 0;
 
-	/*
-	 * 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);
 	p->task_works = NULL;
 
-	/* Need tasklist lock for parent etc handling! */
+	/*
+	 * Make it visible to the rest of the system, but dont wake it up yet.
+	 * Need tasklist lock for parent etc handling!
+	 */
 	write_lock_irq(&tasklist_lock);
 
 	/* CLONE_PARENT re-uses the old parent */
@@ -1476,7 +1476,6 @@ static struct task_struct *copy_process(unsigned long clone_flags,
 			current->signal->nr_threads++;
 			atomic_inc(&current->signal->live);
 			atomic_inc(&current->signal->sigcnt);
-			p->group_leader = current->group_leader;
 			list_add_tail_rcu(&p->thread_group,
 					  &p->group_leader->thread_group);
 		}