[PATCH 3/4] pids: fix fork_idle() to setup ->pids correctly

[Oleg Nesterov <oleg@redhat.com>]

copy_process(pid => &init_struct_pid) doesn't do attach_pid/etc.

It shouldn't, but this means that the idle threads run with the wrong
pids copied from the caller's task_struct. In x86 case the caller is
either kernel_init() thread or keventd.

In particular, this means that after the series of cpu_up/cpu_down an
idle thread (which never exits) can run with .pid pointing to nowhere.

Change fork_idle() to initialize idle->pids[] correctly. We only set
.pid = &init_struct_pid but do not add .node to list, INIT_TASK() does
the same for the boot-cpu idle thread (swapper).

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

 kernel/fork.c |   14 +++++++++++++-
 1 file changed, 13 insertions(+), 1 deletion(-)

--- 34-rc1/kernel/fork.c~3_FORK_IDLE_SET_PIDS	2010-03-24 18:07:03.000000000 +0100
+++ 34-rc1/kernel/fork.c	2010-05-10 20:45:33.000000000 +0200
@@ -1339,6 +1339,16 @@ noinline struct pt_regs * __cpuinit __at
 	return regs;
 }
 
+static inline void init_idle_pids(struct pid_link *links)
+{
+	enum pid_type type;
+
+	for (type = PIDTYPE_PID; type < PIDTYPE_MAX; ++type) {
+		INIT_HLIST_NODE(&links[type].node); /* not really needed */
+		links[type].pid = &init_struct_pid;
+	}
+}
+
 struct task_struct * __cpuinit fork_idle(int cpu)
 {
 	struct task_struct *task;
@@ -1346,8 +1356,10 @@ struct task_struct * __cpuinit fork_idle
 
 	task = copy_process(CLONE_VM, 0, idle_regs(&regs), 0, NULL,
 			    &init_struct_pid, 0);
-	if (!IS_ERR(task))
+	if (!IS_ERR(task)) {
+		init_idle_pids(task->pids);
 		init_idle(task, cpu);
+	}
 
 	return task;
 }