kthread: don't depend on work queues

Currently there is a circular reference between work queue initialization
and kthread initialization.  This prevents the kthread infrastructure from
initializing until after work queues have been initialized.

We want the properties of tasks created with kthread_create to be as close
as possible to the init_task and to not be contaminated by user processes.
The later we start our kthreadd that creates these tasks the harder it is
to avoid contamination from user processes and the more of a mess we have
to clean up because the defaults have changed on us.

So this patch modifies the kthread support to not use work queues but to
instead use a simple list of structures, and to have kthreadd start from
init_task immediately after our kernel thread that execs /sbin/init.

By being a true child of init_task we only have to change those process
settings that we want to have different from init_task, such as our process
name, the cpus that are allowed, blocking all signals and setting SIGCHLD
to SIG_IGN so that all of our children are reaped automatically.

By being a true child of init_task we also naturally get our ppid set to 0
and do not wind up as a child of PID == 1.  Ensuring that tasks generated
by kthread_create will not slow down the functioning of the wait family of
functions.

[akpm@linux-foundation.org: use interruptible sleeps]
Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>
Cc: Oleg Nesterov <oleg@tv-sign.ru>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/kthread.h

@@ -30,4 +30,7 @@ void kthread_bind(struct task_struct *k, unsigned int cpu);
 int kthread_stop(struct task_struct *k);
 int kthread_should_stop(void);
 
+int kthreadd(void *unused);
+extern struct task_struct *kthreadd_task;
+
 #endif /* _LINUX_KTHREAD_H */

init/main.c

@@ -54,6 +54,7 @@
 #include <linux/lockdep.h>
 #include <linux/pid_namespace.h>
 #include <linux/device.h>
+#include <linux/kthread.h>
 
 #include <asm/io.h>
 #include <asm/bugs.h>
@@ -425,8 +426,12 @@ static void __init setup_command_line(char *command_line)
 static void noinline 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(pid);
 	unlock_kernel();
 
 	/*

kernel/kthread.c

@@ -1,7 +1,7 @@
 /* Kernel thread helper functions.
  *   Copyright (C) 2004 IBM Corporation, Rusty Russell.
  *
- * Creation is done via keventd, so that we get a clean environment
+ * Creation is done via kthreadd, so that we get a clean environment
  * even if we're invoked from userspace (think modprobe, hotplug cpu,
  * etc.).
  */
@@ -15,24 +15,22 @@
 #include <linux/mutex.h>
 #include <asm/semaphore.h>
 
-/*
- * We dont want to execute off keventd since it might
- * hold a semaphore our callers hold too:
- */
-static struct workqueue_struct *helper_wq;
+static DEFINE_SPINLOCK(kthread_create_lock);
+static LIST_HEAD(kthread_create_list);
+struct task_struct *kthreadd_task;
 
 struct kthread_create_info
 {
-	/* Information passed to kthread() from keventd. */
+	/* Information passed to kthread() from kthreadd. */
 	int (*threadfn)(void *data);
 	void *data;
 	struct completion started;
 
-	/* Result passed back to kthread_create() from keventd. */
+	/* Result passed back to kthread_create() from kthreadd. */
 	struct task_struct *result;
 	struct completion done;
 
-	struct work_struct work;
+	struct list_head list;
 };
 
 struct kthread_stop_info
@@ -60,42 +58,17 @@ int kthread_should_stop(void)
 }
 EXPORT_SYMBOL(kthread_should_stop);
 
-static void kthread_exit_files(void)
-{
-	struct fs_struct *fs;
-	struct task_struct *tsk = current;
-
-	exit_fs(tsk);		/* current->fs->count--; */
-	fs = init_task.fs;
-	tsk->fs = fs;
-	atomic_inc(&fs->count);
- 	exit_files(tsk);
-	current->files = init_task.files;
-	atomic_inc(&tsk->files->count);
-}
-
 static int kthread(void *_create)
 {
 	struct kthread_create_info *create = _create;
 	int (*threadfn)(void *data);
 	void *data;
-	sigset_t blocked;
 	int ret = -EINTR;
 
-	kthread_exit_files();
-
-	/* Copy data: it's on keventd's stack */
+	/* Copy data: it's on kthread's stack */
 	threadfn = create->threadfn;
 	data = create->data;
 
-	/* Block and flush all signals (in case we're not from keventd). */
-	sigfillset(&blocked);
-	sigprocmask(SIG_BLOCK, &blocked, NULL);
-	flush_signals(current);
-
-	/* By default we can run anywhere, unlike keventd. */
-	set_cpus_allowed(current, CPU_MASK_ALL);
-
 	/* OK, tell user we're spawned, wait for stop or wakeup */
 	__set_current_state(TASK_INTERRUPTIBLE);
 	complete(&create->started);
@@ -112,11 +85,8 @@ static int kthread(void *_create)
 	return 0;
 }
 
-/* We are keventd: create a thread. */
-static void keventd_create_kthread(struct work_struct *work)
+static void create_kthread(struct kthread_create_info *create)
 {
-	struct kthread_create_info *create =
-		container_of(work, struct kthread_create_info, work);
 	int pid;
 
 	/* We want our own signal handler (we take no signals by default). */
@@ -162,17 +132,14 @@ struct task_struct *kthread_create(int (*threadfn)(void *data),
 	create.data = data;
 	init_completion(&create.started);
 	init_completion(&create.done);
-	INIT_WORK(&create.work, keventd_create_kthread);
-
-	/*
-	 * The workqueue needs to start up first:
-	 */
-	if (!helper_wq)
-		create.work.func(&create.work);
-	else {
-		queue_work(helper_wq, &create.work);
-		wait_for_completion(&create.done);
-	}
+
+	spin_lock(&kthread_create_lock);
+	list_add_tail(&create.list, &kthread_create_list);
+	wake_up_process(kthreadd_task);
+	spin_unlock(&kthread_create_lock);
+
+	wait_for_completion(&create.done);
+
 	if (!IS_ERR(create.result)) {
 		va_list args;
 		va_start(args, namefmt);
@@ -180,7 +147,6 @@ struct task_struct *kthread_create(int (*threadfn)(void *data),
 			  namefmt, args);
 		va_end(args);
 	}
-
 	return create.result;
 }
 EXPORT_SYMBOL(kthread_create);
@@ -245,12 +211,58 @@ int kthread_stop(struct task_struct *k)
 }
 EXPORT_SYMBOL(kthread_stop);
 
-static __init int helper_init(void)
+
+static __init void kthreadd_setup(void)
 {
-	helper_wq = create_singlethread_workqueue("kthread");
-	BUG_ON(!helper_wq);
+	struct task_struct *tsk = current;
+	struct k_sigaction sa;
+	sigset_t blocked;
 
-	return 0;
+	set_task_comm(tsk, "kthreadd");
+
+	/* Block and flush all signals */
+	sigfillset(&blocked);
+	sigprocmask(SIG_BLOCK, &blocked, NULL);
+	flush_signals(tsk);
+
+	/* SIG_IGN makes children autoreap: see do_notify_parent(). */
+	sa.sa.sa_handler = SIG_IGN;
+	sa.sa.sa_flags = 0;
+	siginitset(&sa.sa.sa_mask, sigmask(SIGCHLD));
+	do_sigaction(SIGCHLD, &sa, (struct k_sigaction *)0);
+
+	set_user_nice(current, -5);
+	set_cpus_allowed(current, CPU_MASK_ALL);
 }
 
-core_initcall(helper_init);
+int kthreadd(void *unused)
+{
+	/* Setup a clean context for our children to inherit. */
+	kthreadd_setup();
+
+	current->flags |= PF_NOFREEZE;
+
+	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;
+
+			create = list_entry(kthread_create_list.next,
+					    struct kthread_create_info, list);
+			list_del_init(&create->list);
+			spin_unlock(&kthread_create_lock);
+
+			create_kthread(create);
+
+			spin_lock(&kthread_create_lock);
+		}
+		spin_unlock(&kthread_create_lock);
+	}
+
+	return 0;
+}