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From: Oleg Nesterov on 10 May 2010 16:00
"statically initialize struct pid for swapper" commit 820e45db says:

Statically initialize a struct pid for the swapper process (pid_t == 0)
and attach it to init_task. This is needed so task_pid(), task_pgrp()
and task_session() interfaces work on the swapper process also.

OK, but:

- it doesn't make sense to add init_task.pids[].node into
init_struct_pid.tasks[], and in fact this just wrong.

idle threads are special, they shouldn't be visible on any
global list. In particular do_each_pid_task(init_struct_pid)
shouldn't see swapper.

This is the actual reason why kill(0, SIGKILL) from /sbin/init
(which starts with 0,0 special pids) crashes the kernel. The
signal sent to pgid/sid == 0 must never see idle threads, even
if the previous patch fixed the crash itself.

- we have other idle threads running on the non-boot CPUs, see
the next patch.

Change INIT_STRUCT_PID/INIT_PID_LINK to create the empty/unhashed
hlist_head/hlist_node. Like any other idle thread swapper can never exit,
so detach_pid()->__hlist_del() is not possible, but we could change
INIT_PID_LINK() to set pprev = &next if needed.

All we need is the valid swapper->pids[].pid == &init_struct_pid.

Reported-by: Mathias Krause <mathias.krause(a)secunet.com>
Signed-off-by: Oleg Nesterov <oleg(a)redhat.com>
---

include/linux/init_task.h | 8 ++++----
1 file changed, 4 insertions(+), 4 deletions(-)

--- 34-rc1/include/linux/init_task.h~2_INIT_PID_DONT_LINK_SWAPPER 2010-05-10 19:45:27.000000000 +0200
+++ 34-rc1/include/linux/init_task.h 2010-05-10 20:26:08.000000000 +0200
@@ -53,9 +53,9 @@ extern struct group_info init_groups;
#define INIT_STRUCT_PID { \
.count = ATOMIC_INIT(1), \
.tasks = { \
- { .first = &init_task.pids[PIDTYPE_PID].node }, \
- { .first = &init_task.pids[PIDTYPE_PGID].node }, \
- { .first = &init_task.pids[PIDTYPE_SID].node }, \
+ { .first = NULL }, \
+ { .first = NULL }, \
+ { .first = NULL }, \
}, \
.rcu = RCU_HEAD_INIT, \
.level = 0, \
@@ -70,7 +70,7 @@ extern struct group_info init_groups;
{ \
.node = { \
.next = NULL, \
- .pprev = &init_struct_pid.tasks[type].first, \
+ .pprev = NULL, \
}, \
.pid = &init_struct_pid, \
}

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Prev: [PATCH 4/4] INIT_SIGHAND: use SIG_DFL instead of NULL
Next: [PATCH 0/4] swapper fixes (Was: kernel panic on kill(0, SIGTERM) with PGID == 0)