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

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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>
This commit is contained in:
Eric W. Biederman 2006-09-27 01:51:06 -07:00 committed by Linus Torvalds
parent 35fa2048ab
commit c18258c6f0
3 changed files with 15 additions and 7 deletions
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11
fs/exec.c
View file

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

2
include/linux/pid.h
View file

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

9
kernel/pid.c
View file

@ -252,6 +252,15 @@ void fastcall detach_pid(struct task_struct *task, enum pid_type type)
free_pid(pid); 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 * fastcall pid_task(struct pid *pid, enum pid_type type)
{ {
struct task_struct *result = NULL; struct task_struct *result = NULL;