Revert "[PATCH] kthreads: Fix startup synchronization boot crash", it was the minimal fix for stable. Then: - change kthread_create() to check kthreadd_task != NULL before wake_up_process(kthreadd_task) - 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 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 .done, kthreadd() must notice the new request before it calls schedule(). IOW both checks, !kthreadd_task and list_empty(), are done after lock+unlock of the same lock. The 2nd task which takes the lock must see the changes which were done by the 1st task which locked this lock. Signed-off-by: Oleg Nesterov <oleg@xxxxxxxxxx> --- include/linux/kthread.h | 1 - init/main.c | 7 +------ kernel/kthread.c | 35 ++++++++++++++++++----------------- 3 files changed, 19 insertions(+), 24 deletions(-) --- WAIT/include/linux/kthread.h~KT_KTHREADD_NULL_FIX 2009-09-01 18:25:50.000000000 +0200 +++ WAIT/include/linux/kthread.h 2009-09-01 18:31:39.000000000 +0200 @@ -33,6 +33,5 @@ int kthread_should_stop(void); int kthreadd(void *unused); extern struct task_struct *kthreadd_task; -extern struct completion kthreadd_task_init_done; #endif /* _LINUX_KTHREAD_H */ --- WAIT/init/main.c~KT_KTHREADD_NULL_FIX 2009-09-01 18:25:50.000000000 +0200 +++ WAIT/init/main.c 2009-09-01 18:32:05.000000000 +0200 @@ -453,14 +453,9 @@ 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); - complete(&kthreadd_task_init_done); - + kernel_thread(kthreadd, NULL, CLONE_FS | CLONE_FILES); unlock_kernel(); /* --- WAIT/kernel/kthread.c~KT_KTHREADD_NULL_FIX 2009-09-01 18:25:50.000000000 +0200 +++ WAIT/kernel/kthread.c 2009-09-01 18:32:05.000000000 +0200 @@ -20,9 +20,7 @@ static DEFINE_SPINLOCK(kthread_create_lock); static LIST_HEAD(kthread_create_list); - struct task_struct *kthreadd_task; -DECLARE_COMPLETION(kthreadd_task_init_done); struct kthread_create_info { @@ -130,11 +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); - - if (unlikely(!kthreadd_task)) - wait_for_completion(&kthreadd_task_init_done); - - 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)) { @@ -219,23 +220,18 @@ 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; @@ -250,6 +246,11 @@ 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; -- To unsubscribe from this list: send the line "unsubscribe linux-tip-commits" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html