[PATCH] pid: Implement transfer_pid and use it to simplify de_thread

In de_thread we move pids from one process to another, a rather ugly case.
The function transfer_pid makes it clear what we are doing, and makes the
action atomic.  This is useful we ever want to atomically traverse the
process group and session lists, in a rcu safe manner.

Even if the atomic properties this change should be a win as transfer_pid
should be less code to execute than executing both attach_pid and
detach_pid, and this should make de_thread slightly smaller as only a
single function call needs to be emitted.  The only downside is that the
code might be slower to execute as the odds are against transfer_pid being
in cache.

Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>
Cc: Oleg Nesterov <oleg@tv-sign.ru>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

fs/exec.c

@@ -696,23 +696,20 @@ static int de_thread(struct task_struct *tsk)
 		 */
 
 		/* Become a process group leader with the old leader's pid.
-		 * Note: The old leader also uses thispid until release_task
+		 * The old leader becomes a thread of the this thread group.
+		 * Note: The old leader also uses this pid until release_task
 		 *       is called.  Odd but simple and correct.
 		 */
 		detach_pid(current, PIDTYPE_PID);
 		current->pid = leader->pid;
 		attach_pid(current, PIDTYPE_PID,  current->pid);
-		attach_pid(current, PIDTYPE_PGID, current->signal->pgrp);
-		attach_pid(current, PIDTYPE_SID,  current->signal->session);
+		transfer_pid(leader, current, PIDTYPE_PGID);
+		transfer_pid(leader, current, PIDTYPE_SID);
 		list_replace_rcu(&leader->tasks, &current->tasks);
 
 		current->group_leader = current;
 		leader->group_leader = current;
 
-		/* Reduce leader to a thread */
-		detach_pid(leader, PIDTYPE_PGID);
-		detach_pid(leader, PIDTYPE_SID);
-
 		current->exit_signal = SIGCHLD;
 
 		BUG_ON(leader->exit_state != EXIT_ZOMBIE);

include/linux/pid.h

@@ -76,6 +76,8 @@ extern int FASTCALL(attach_pid(struct task_struct *task,
 				enum pid_type type, int nr));
 
 extern void FASTCALL(detach_pid(struct task_struct *task, enum pid_type));
+extern void FASTCALL(transfer_pid(struct task_struct *old,
+				  struct task_struct *new, enum pid_type));
 
 /*
  * look up a PID in the hash table. Must be called with the tasklist_lock

kernel/pid.c

@@ -252,6 +252,15 @@ void fastcall detach_pid(struct task_struct *task, enum pid_type type)
 	free_pid(pid);
 }
 
+/* transfer_pid is an optimization of attach_pid(new), detach_pid(old) */
+void fastcall 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);
+	old->pids[type].pid = NULL;
+}
+
 struct task_struct * fastcall pid_task(struct pid *pid, enum pid_type type)
 {
 	struct task_struct *result = NULL;