On Tue, Sep 01, 2009 at 03:37:09PM +0200, Oleg Nesterov wrote:
>On 09/01, Ingo Molnar wrote:
>>
>> * Oleg Nesterov <oleg@xxxxxxxxxx> wrote:
>>
>> > Yes, this should work. But I _think_ we can make the better fix...
>> >
>> > I'll try to make the patch soon. Afaics we don't need
>> > kthreadd_task_init_done.
>>
>> ok.
>
>Just in case, the patch is ready. I need to re-check my thinking
>and test it somehow...
>
>- remove kthreadd_task initialization from rest_init()
>
>- change kthreadd() to initialize kthreadd_task = current
>
>- change the main loop in kthreadd() to take kthread_create_lock
> before the first schedule() (just shift schedule() down)
This is the only part that I can't understand, why moving it down?
>
>This way, if kthreadd_task needs the wakeup, kthread_create()
>must see kthreadd_task != NULL after unlock(kthread_create_lock).
>
>If kthread_create() sees kthreadd_task == NULL we can just sleep
>on create.done, kthreadd() must notice the new request before
>it calls schedule().
>
>Note that with this change it is possible to use kthread_create()
>at any time, but the caller will sleep until rest_init() creates
>kthreadd.
>
What a nice patch!
Thanks!
>Oleg.
>
> init/main.c | 5 +----
> kernel/kthread.c | 30 ++++++++++++++++++------------
> 2 files changed, 19 insertions(+), 16 deletions(-)
>
>
>--- a/init/main.c
>+++ b/init/main.c
>@@ -449,12 +449,9 @@ static void __init setup_command_line(ch
> static noinline void __init_refok rest_init(void)
> __releases(kernel_lock)
> {
>- int pid;
>-
> kernel_thread(kernel_init, NULL, CLONE_FS | CLONE_SIGHAND);
> numa_default_policy();
>- pid = kernel_thread(kthreadd, NULL, CLONE_FS | CLONE_FILES);
>- kthreadd_task = find_task_by_pid_ns(pid, &init_pid_ns);
>+ kernel_thread(kthreadd, NULL, CLONE_FS | CLONE_FILES);
> unlock_kernel();
>
> /*
>--- a/kernel/kthread.c
>+++ b/kernel/kthread.c
>@@ -128,8 +128,14 @@ struct task_struct *kthread_create(int (
> spin_lock(&kthread_create_lock);
> list_add_tail(&create.list, &kthread_create_list);
> spin_unlock(&kthread_create_lock);
>-
>- wake_up_process(kthreadd_task);
>+ /*
>+ * If kthreadd was not created yet, kthreadd() must see the result
>+ * of list_add_tail() later, it takes kthread_create_lock before the
>+ * first schedule(). If kthreadd() locked kthread_create_lock at
>+ * least once, we must see kthreadd_task != NULL.
>+ */
>+ if (likely(kthreadd_task))
>+ wake_up_process(kthreadd_task);
> wait_for_completion(&create.done);
>
> if (!IS_ERR(create.result)) {
>@@ -216,23 +222,18 @@ EXPORT_SYMBOL(kthread_stop);
>
> int kthreadd(void *unused)
> {
>- struct task_struct *tsk = current;
>+ kthreadd_task = current;
>
> /* Setup a clean context for our children to inherit. */
>- set_task_comm(tsk, "kthreadd");
>- ignore_signals(tsk);
>- set_user_nice(tsk, KTHREAD_NICE_LEVEL);
>- set_cpus_allowed_ptr(tsk, cpu_all_mask);
>+ set_task_comm(kthreadd_task, "kthreadd");
>+ ignore_signals(kthreadd_task);
>+ set_user_nice(kthreadd_task, KTHREAD_NICE_LEVEL);
>+ set_cpus_allowed_ptr(kthreadd_task, cpu_all_mask);
> set_mems_allowed(node_possible_map);
>
> current->flags |= PF_NOFREEZE | PF_FREEZER_NOSIG;
>
> for (;;) {
>- set_current_state(TASK_INTERRUPTIBLE);
>- if (list_empty(&kthread_create_list))
>- schedule();
>- __set_current_state(TASK_RUNNING);
>-
> spin_lock(&kthread_create_lock);
> while (!list_empty(&kthread_create_list)) {
> struct kthread_create_info *create;
>@@ -247,6 +248,11 @@ int kthreadd(void *unused)
> spin_lock(&kthread_create_lock);
> }
> spin_unlock(&kthread_create_lock);
>+
>+ set_current_state(TASK_INTERRUPTIBLE);
>+ if (list_empty(&kthread_create_list))
>+ schedule();
>+ __set_current_state(TASK_RUNNING);
> }
>
> return 0;
>
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