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Subject[PATCH 1/1] kthreads: simplify !kthreadd_task logic, kill kthreadd_task_init_done
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@redhat.com>
---
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;


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