[PATCH] kthread: Don't depend on work queues

[ebiederm@xmission.com (Eric W. Biederman)]

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

For kernel threads we want something that is as close as possible to the
init_task and is not contaminated by user processes.  The later we start
our kthreadd that forks the rest of the kernel threads the harder this is
to do 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, blocking all signals and setting SIGCHLD to SIG_IGN
so that all of our children are reaped automatically.

By being a tre 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 kernel threads
will not slow down the functioning of the wait family of functions.

Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>
---
 include/linux/kthread.h |    2 +
 init/main.c             |    2 +
 kernel/kthread.c        |  121 +++++++++++++++++++++++++----------------------
 3 files changed, 69 insertions(+), 56 deletions(-)

diff --git a/include/linux/kthread.h b/include/linux/kthread.h
index 1c65e7a..113bd7c 100644
--- a/include/linux/kthread.h
+++ b/include/linux/kthread.h
@@ -30,4 +30,6 @@ 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);
+
 #endif /* _LINUX_KTHREAD_H */
diff --git a/init/main.c b/init/main.c
index a92989e..12a8aba 100644
--- a/init/main.c
+++ b/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>
@@ -437,6 +438,7 @@ static void noinline rest_init(void)
 {
 	kernel_thread(init, NULL, CLONE_FS | CLONE_SIGHAND);
 	numa_default_policy();
+	kernel_thread(kthreadd, NULL, CLONE_FS | CLONE_FILES);
 	unlock_kernel();
 
 	/*
diff --git a/kernel/kthread.c b/kernel/kthread.c
index 87c50cc..8638a04 100644
--- a/kernel/kthread.c
+++ b/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);
+static DECLARE_WAIT_QUEUE_HEAD(kthread_create_work);
 
 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(&kthread_create_work);
+	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 +211,55 @@ 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;
+	strcpy(tsk->comm, "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);
 }
 
-core_initcall(helper_init);
+int kthreadd(void *unused)
+{
+	/* Setup a clean context for our children to inherit. */
+	kthreadd_setup();
+
+	current->flags |= PF_NOFREEZE;
+
+	for (;;) {
+		wait_event(kthread_create_work,
+			   !list_empty(&kthread_create_list));
+
+		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;
+}
-- 
1.5.0.g53756