[PATCH 1/1] do not place sub-threads on task_struct->children list

Oleg Nesterov
2009-07-02T18:14:53Z

Thanks to Roland who pointed out de_thread() issues.

Currently we add sub-threads to ->real_parent->children list. This
buys nothing but slows down do_wait().

With this patch ->children contains only main threads (group leaders).
The only complication is that forget_original_parent() should iterate
over sub-threads by hand, and de_thread() needs another list_replace()
when it changes ->group_leader.

>From now do_wait_thread() can never see task_detached() && !EXIT_DEAD
tasks, we can remove this check (and we can unify do_wait_thread() and
ptrace_do_wait()).

This change can confuse the optimistic search in mm_update_next_owner(),
but this is fixable and minor.

Perhaps badness() and oom_kill_process() should be updated, but they
should be fixed in any case.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
---

fs/exec.c | 2 ++
kernel/fork.c | 2 +-
kernel/exit.c | 36 +++++++++++++++++-------------------
3 files changed, 20 insertions(+), 20 deletions(-)

--- WAIT/fs/exec.c~4_CHILDREN_NO_THREADS 2009-07-01 20:01:54.000000000 +0200
+++ WAIT/fs/exec.c 2009-07-02 19:27:36.000000000 +0200
@@ -827,7 +827,9 @@ static int de_thread(struct task_struct
attach_pid(tsk, PIDTYPE_PID, task_pid(leader));
transfer_pid(leader, tsk, PIDTYPE_PGID);
transfer_pid(leader, tsk, PIDTYPE_SID);
+
list_replace_rcu(&leader->tasks, &tsk->tasks);
+ list_replace_init(&leader->sibling, &tsk->sibling);

tsk->group_leader = tsk;
leader->group_leader = tsk;
--- WAIT/kernel/fork.c~4_CHILDREN_NO_THREADS 2009-07-01 20:01:54.000000000 +0200
+++ WAIT/kernel/fork.c 2009-07-02 19:27:36.000000000 +0200
@@ -1245,7 +1245,6 @@ static struct task_struct *copy_process(
}

if (likely(p->pid)) {
- list_add_tail(&p->sibling, &p->real_parent->children);
tracehook_finish_clone(p, clone_flags, trace);

if (thread_group_leader(p)) {
@@ -1257,6 +1256,7 @@ static struct task_struct *copy_process(
p->signal->tty = tty_kref_get(current->signal->tty);
attach_pid(p, PIDTYPE_PGID, task_pgrp(current));
attach_pid(p, PIDTYPE_SID, task_session(current));
+ list_add_tail(&p->sibling, &p->real_parent->children);
list_add_tail_rcu(&p->tasks, &init_task.tasks);
__get_cpu_var(process_counts)++;
}
--- WAIT/kernel/exit.c~4_CHILDREN_NO_THREADS 2009-07-02 18:44:43.000000000 +0200
+++ WAIT/kernel/exit.c 2009-07-02 19:56:33.000000000 +0200
@@ -68,10 +68,10 @@ static void __unhash_process(struct task
detach_pid(p, PIDTYPE_SID);

list_del_rcu(&p->tasks);
+ list_del_init(&p->sibling);
__get_cpu_var(process_counts)--;
}
list_del_rcu(&p->thread_group);
- list_del_init(&p->sibling);
}

/*
@@ -736,12 +736,9 @@ static struct task_struct *find_new_reap
/*
* Any that need to be release_task'd are put on the @dead list.
*/
-static void reparent_thread(struct task_struct *father, struct task_struct *p,
+static void reparent_leader(struct task_struct *father, struct task_struct *p,
struct list_head *dead)
{
- if (p->pdeath_signal)
- group_send_sig_info(p->pdeath_signal, SEND_SIG_NOINFO, p);
-
list_move_tail(&p->sibling, &p->real_parent->children);

if (task_detached(p))
@@ -780,12 +777,18 @@ static void forget_original_parent(struc
reaper = find_new_reaper(father);

list_for_each_entry_safe(p, n, &father->children, sibling) {
- p->real_parent = reaper;
- if (p->parent == father) {
- BUG_ON(task_ptrace(p));
- p->parent = p->real_parent;
- }
- reparent_thread(father, p, &dead_children);
+ struct task_struct *t = p;
+ do {
+ t->real_parent = reaper;
+ if (t->parent == father) {
+ BUG_ON(task_ptrace(t));
+ t->parent = t->real_parent;
+ }
+ if (t->pdeath_signal)
+ group_send_sig_info(t->pdeath_signal,
+ SEND_SIG_NOINFO, t);
+ } while_each_thread(p, t);
+ reparent_leader(father, p, &dead_children);
}
write_unlock_irq(&tasklist_lock);

@@ -1533,14 +1536,9 @@ static int do_wait_thread(struct wait_op
struct task_struct *p;

list_for_each_entry(p, &tsk->children, sibling) {
- /*
- * Do not consider detached threads.
- */
- if (!task_detached(p)) {
- int ret = wait_consider_task(wo, tsk, 0, p);
- if (ret)
- return ret;
- }
+ int ret = wait_consider_task(wo, tsk, 0, p);
+ if (ret)
+ return ret;
}

return 0;