pids: Move the pgrp and session pid pointers from task_struct to signal_struct

To access these fields the code always has to go to group leader so
going to signal struct is no loss and is actually a fundamental simplification.

This saves a little bit of memory by only allocating the pid pointer array
once instead of once for every thread, and even better this removes a
few potential races caused by the fact that group_leader can be changed
by de_thread, while signal_struct can not.

Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>

1 files changed, 22 insertions, 23 deletions

diff --git a/kernel/pid.c b/kernel/pid.c
index d0de2b59f86f..f8486d2e2346 100644
--- a/kernel/pid.c
+++ b/kernel/pid.c
@@ -265,27 +265,35 @@ struct pid *find_vpid(int nr)
 }
 EXPORT_SYMBOL_GPL(find_vpid);
 
+static struct pid **task_pid_ptr(struct task_struct *task, enum pid_type type)
+{
+	return (type == PIDTYPE_PID) ?
+		&task->thread_pid :
+		(type == __PIDTYPE_TGID) ?
+		&task->signal->leader_pid :
+		&task->signal->pids[type];
+}
+
 /*
  * attach_pid() must be called with the tasklist_lock write-held.
  */
 void attach_pid(struct task_struct *task, enum pid_type type)
 {
-	struct pid_link *link = &task->pids[type];
-	hlist_add_head_rcu(&link->node, &link->pid->tasks[type]);
+	struct pid *pid = *task_pid_ptr(task, type);
+	hlist_add_head_rcu(&task->pid_links[type], &pid->tasks[type]);
 }
 
 static void __change_pid(struct task_struct *task, enum pid_type type,
 			struct pid *new)
 {
-	struct pid_link *link;
+	struct pid **pid_ptr = task_pid_ptr(task, type);
 	struct pid *pid;
 	int tmp;
 
-	link = &task->pids[type];
-	pid = link->pid;
+	pid = *pid_ptr;
 
-	hlist_del_rcu(&link->node);
-	link->pid = new;
+	hlist_del_rcu(&task->pid_links[type]);
+	*pid_ptr = new;
 
 	for (tmp = PIDTYPE_MAX; --tmp >= 0; )
 		if (!hlist_empty(&pid->tasks[tmp]))
@@ -310,8 +318,9 @@ void change_pid(struct task_struct *task, enum pid_type type,
 void transfer_pid(struct task_struct *old, struct task_struct *new,
 			   enum pid_type type)
 {
-	new->pids[type].pid = old->pids[type].pid;
-	hlist_replace_rcu(&old->pids[type].node, &new->pids[type].node);
+	if (type == PIDTYPE_PID)
+		new->thread_pid = old->thread_pid;
+	hlist_replace_rcu(&old->pid_links[type], &new->pid_links[type]);
 }
 
 struct task_struct *pid_task(struct pid *pid, enum pid_type type)
@@ -322,7 +331,7 @@ struct task_struct *pid_task(struct pid *pid, enum pid_type type)
 		first = rcu_dereference_check(hlist_first_rcu(&pid->tasks[type]),
 					      lockdep_tasklist_lock_is_held());
 		if (first)
-			result = hlist_entry(first, struct task_struct, pids[(type)].node);
+			result = hlist_entry(first, struct task_struct, pid_links[(type)]);
 	}
 	return result;
 }
@@ -360,9 +369,7 @@ struct pid *get_task_pid(struct task_struct *task, enum pid_type type)
 {
 	struct pid *pid;
 	rcu_read_lock();
-	if (type != PIDTYPE_PID)
-		task = task->group_leader;
-	pid = get_pid(rcu_dereference(task->pids[type].pid));
+	pid = get_pid(rcu_dereference(*task_pid_ptr(task, type)));
 	rcu_read_unlock();
 	return pid;
 }
@@ -420,16 +427,8 @@ pid_t __task_pid_nr_ns(struct task_struct *task, enum pid_type type,
 	rcu_read_lock();
 	if (!ns)
 		ns = task_active_pid_ns(current);
-	if (likely(pid_alive(task))) {
-		struct pid *pid;
-		if (type == PIDTYPE_PID)
-			pid = task_pid(task);
-		else if (type == __PIDTYPE_TGID)
-			pid = task_tgid(task);
-		else
-			pid = rcu_dereference(task->group_leader->pids[type].pid);
-		nr = pid_nr_ns(pid, ns);
-	}
+	if (likely(pid_alive(task)))
+		nr = pid_nr_ns(rcu_dereference(*task_pid_ptr(task, type)), ns);
 	rcu_read_unlock();
 
 	return nr;