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

From: Eric W. Biederman
Date: Mon Jul 10 2006 - 18:40:29 EST



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@xxxxxxxxxxxx>
---
fs/exec.c | 11 ++++-------
include/linux/pid.h | 2 ++
kernel/pid.c | 10 ++++++++++
3 files changed, 16 insertions(+), 7 deletions(-)

diff --git a/fs/exec.c b/fs/exec.c
index 208a027..1dd7d49 100644
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -698,23 +698,20 @@ static int de_thread(struct task_struct
*/

/* 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);
diff --git a/include/linux/pid.h b/include/linux/pid.h
index 29960b0..93da7e2 100644
--- a/include/linux/pid.h
+++ b/include/linux/pid.h
@@ -76,6 +76,8 @@ extern int FASTCALL(attach_pid(struct ta
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
diff --git a/kernel/pid.c b/kernel/pid.c
index 37851b0..3153e96 100644
--- a/kernel/pid.c
+++ b/kernel/pid.c
@@ -253,6 +253,16 @@ void fastcall detach_pid(struct task_str
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;
--
1.4.1.gac83a

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