[PATCH 1/2] kthread_create

Rusty Russell
2003-12-31T04:20:31Z

Hi all,

Ingo read through this before and liked it: this is the basis
of the Hotplug CPU patch, and as such has been stressed fairly well.
Tested stand-alone, and included here for wider review.

Feedback welcome!
Rusty.
--
Anyone who quotes me in their sig is an idiot. -- Rusty Russell.

Name: Kernel Thread Control Primitives
Author: Rusty Russell
Status: Tested on 2.6.0-bk3

D: The hotplug CPU code introduces two major problems:
D:
D: 1) Threads which previously never stopped (migration thread,
D: ksoftirqd, keventd) have to be stopped cleanly as CPUs go offline.
D: 2) Threads which previously never had to be created now have
D: to be created when a CPU goes online.
D:
D: Unfortunately, stopping a thread is fairly baroque, involving memory
D: barriers, a completion and spinning until the task is actually dead.
D:
D: There are also three problems in starting a thread:
D: 1) Doing it from a random process context risks environment contamination:
D: better to do it from keventd to guarantee a clean environment, a-la
D: call_usermodehelper.
D: 2) Getting the task struct without races is a hard: see kernel/sched.c
D: migration_call(), kernel/workqueue.c create_workqueue_thread().
D: 3) There are races in starting a thread for a CPU which is not yet
D: online: migration thread does a complex dance at the moment for
D: a similar reason (there may be no migration thread to migrate us).
D:
D: Place all this logic in some primitives to make life easier:
D: kthread_create(), kthread_start() and kthread_destroy(). These
D: primitives require no extra data-structures in the caller: they operate
D: on normal "struct task_struct"s.
D:
D: Other changes:
D: - Expose keventd_up(), as keventd and migration threads will use
D: kthread to launch, and kthread normally uses workqueues and must
D: recognize this case.

diff -urpN --exclude TAGS -X /home/rusty/devel/kernel/kernel-patches/current-dontdiff --minimal .32392-linux-2.6.0-test11-bk6/include/linux/kthread.h .32392-linux-2.6.0-test11-bk6.updated/include/linux/kthread.h
--- .32392-linux-2.6.0-test11-bk6/include/linux/kthread.h 1970-01-01 10:00:00.000000000 +1000
+++ .32392-linux-2.6.0-test11-bk6.updated/include/linux/kthread.h 2003-12-09 17:42:29.000000000 +1100
@@ -0,0 +1,38 @@
+#ifndef _LINUX_KTHREAD_H
+#define _LINUX_KTHREAD_H
+/* Simple abstraction for kernel thread usage: the initfn is called at
+ * the start, if that's successful (ie. returns 0), then the thread
+ * sleeps.
+ *
+ * Every time the thread is woken, it will run corefn, until that
+ * returns an error. The thread must be ended by calling
+ * kthread_destroy().
+ */
+#include <linux/err.h>
+struct task_struct;
+
+/* Part I: create a kthread: if fork fails return ERR_PTR(-errno). */
+struct task_struct *kthread_create(int (*initfn)(void *data),
+ int (*corefn)(void *data),
+ void *data,
+ const char namefmt[], ...);
+
+/* Part II: have thread call initfn(); return thread if successful,
+ otherwise ERR_PTR(-errno). */
+struct task_struct *kthread_start(struct task_struct *k);
+
+/* Convenient wrapper for both of the above. */
+#define kthread_run(initfn, corefn, data, namefmt, ...) \
+({ \
+ struct task_struct *__k \
+ = kthread_create(initfn,corefn,data,namefmt, ## __VA_ARGS__); \
+ if (!IS_ERR(__k)) \
+ __k = kthread_start(__k); \
+ __k; \
+})
+
+/* Stop the thread. Return value is last return of corefn() (ie. zero
+ * if exited as normal). Can be called before kthread_start(). */
+int kthread_destroy(struct task_struct *k);
+
+#endif /* _LINUX_KTHREAD_H */
diff -urpN --exclude TAGS -X /home/rusty/devel/kernel/kernel-patches/current-dontdiff --minimal .32392-linux-2.6.0-test11-bk6/include/linux/workqueue.h .32392-linux-2.6.0-test11-bk6.updated/include/linux/workqueue.h
--- .32392-linux-2.6.0-test11-bk6/include/linux/workqueue.h 2003-09-22 10:07:08.000000000 +1000
+++ .32392-linux-2.6.0-test11-bk6.updated/include/linux/workqueue.h 2003-12-09 17:42:29.000000000 +1100
@@ -60,6 +60,7 @@ extern int FASTCALL(schedule_work(struct
extern int FASTCALL(schedule_delayed_work(struct work_struct *work, unsigned long delay));
extern void flush_scheduled_work(void);
extern int current_is_keventd(void);
+extern int keventd_up(void);

extern void init_workqueues(void);

diff -urpN --exclude TAGS -X /home/rusty/devel/kernel/kernel-patches/current-dontdiff --minimal .32392-linux-2.6.0-test11-bk6/kernel/Makefile .32392-linux-2.6.0-test11-bk6.updated/kernel/Makefile
--- .32392-linux-2.6.0-test11-bk6/kernel/Makefile 2003-10-09 18:03:02.000000000 +1000
+++ .32392-linux-2.6.0-test11-bk6.updated/kernel/Makefile 2003-12-09 17:42:29.000000000 +1100
@@ -6,7 +6,8 @@ obj-y = sched.o fork.o exec_domain.o
exit.o itimer.o time.o softirq.o resource.o \
sysctl.o capability.o ptrace.o timer.o user.o \
signal.o sys.o kmod.o workqueue.o pid.o \
- rcupdate.o intermodule.o extable.o params.o posix-timers.o
+ rcupdate.o intermodule.o extable.o params.o posix-timers.o \
+ kthread.o

obj-$(CONFIG_FUTEX) += futex.o
obj-$(CONFIG_GENERIC_ISA_DMA) += dma.o
diff -urpN --exclude TAGS -X /home/rusty/devel/kernel/kernel-patches/current-dontdiff --minimal .32392-linux-2.6.0-test11-bk6/kernel/kthread.c .32392-linux-2.6.0-test11-bk6.updated/kernel/kthread.c
--- .32392-linux-2.6.0-test11-bk6/kernel/kthread.c 1970-01-01 10:00:00.000000000 +1000
+++ .32392-linux-2.6.0-test11-bk6.updated/kernel/kthread.c 2003-12-09 17:42:29.000000000 +1100
@@ -0,0 +1,228 @@
+/* Kernel thread helper functions.
+ * Copyright (C) 2003 IBM Corporation, Rusty Russell.
+ *
+ * Everything uses keventd, so that we get a clean environment even if
+ * we're invoked from userspace (think modprobe, hotplug cpu).
+ */
+#include <linux/sched.h>
+#include <linux/kthread.h>
+#include <linux/completion.h>
+#include <linux/err.h>
+#include <asm/semaphore.h>
+
+/* This makes sure only one kthread is being talked to at once. */
+static DECLARE_MUTEX(kthread_control);
+
+/* This coordinates communication between the kthread and routine
+ * controlling it. Strictly unneccessary, but without it it's barrier
+ * hell. */
+static spinlock_t ktm_lock = SPIN_LOCK_UNLOCKED;
+
+/* All thread comms is command -> ack, so we keep it simple. */
+struct kt_message
+{
+ struct task_struct *from, *to;
+ void *info;
+};
+
+static struct kt_message ktm;
+
+static void ktm_send(struct task_struct *to, void *info)
+{
+ spin_lock(&ktm_lock);
+ ktm.to = to;
+ ktm.from = current;
+ ktm.info = info;
+ if (ktm.to)
+ wake_up_process(ktm.to);
+ spin_unlock(&ktm_lock);
+}
+
+static struct kt_message ktm_receive(void)
+{
+ struct kt_message m;
+
+ for (;;) {
+ spin_lock(&ktm_lock);
+ if (ktm.to == current)
+ break;
+ current->state = TASK_INTERRUPTIBLE;
+ spin_unlock(&ktm_lock);
+ schedule();
+ }
+ m = ktm;
+ spin_unlock(&ktm_lock);
+ return m;
+}
+
+struct kthread
+{
+ int (*initfn)(void *data);
+ int (*corefn)(void *data);
+ void *data;
+ char *name;
+};
+
+/* Check if we're being told to stop. */
+static int time_to_die(struct kt_message *m)
+{
+ int ret = 0;
+
+ spin_lock(&ktm_lock);
+ if (ktm.to == current && ktm.info == NULL) {
+ *m = ktm;
+ ret = 1;
+ }
+ spin_unlock(&ktm_lock);
+ return ret;
+}
+
+static int kthread(void *data)
+{
+ /* Copy data: it's on keventd_init's stack */
+ struct kthread k = *(struct kthread *)data;
+ struct kt_message m;
+ int ret = 0;
+ sigset_t blocked;
+
+ strcpy(current->comm, k.name);
+
+ /* Block and flush all signals. */
+ sigfillset(&blocked);
+ sigprocmask(SIG_BLOCK, &blocked, NULL);
+ flush_signals(current);
+
+ /* Send to spawn_kthread, so it knows who we are. */
+ ktm_send(ktm.info, current);
+
+ /* Receive from kthread_start or kthread_destroy */
+ m = ktm_receive();
+ if (!m.info)
+ goto stop;
+ if (k.initfn && (ret = k.initfn(k.data)) < 0)
+ goto stop;
+ ktm_send(m.from, current);
+
+ for (;;) {
+ if (time_to_die(&m))
+ break;
+
+ /* If it fails, just wait until kthread_destroy. */
+ if (k.corefn && (ret = k.corefn(k.data)) < 0)
+ k.corefn = NULL;
+
+ if (time_to_die(&m))
+ break;
+
+ schedule();
+ }
+
+ current->state = TASK_RUNNING;
+stop:
+ ktm_send(m.from, ERR_PTR(ret));
+ return ret;
+}
+
+struct kthread_create
+{
+ struct task_struct *result;
+ struct kthread k;
+ struct completion done;
+};
+
+/* We are keventd(). We create a thread. */
+static void spawn_kthread(void *data)
+{
+ struct kthread_create *kc = data;
+ int ret;
+
+ /* Set up message so they know who their parent is. */
+ ktm_send(NULL, current);
+
+ /* We want our own signal handler (we take no signals by default). */
+ ret = kernel_thread(kthread, &kc->k, CLONE_FS | CLONE_FILES | SIGCHLD);
+ if (ret < 0)
+ kc->result = ERR_PTR(ret);
+ else {
+ /* They tell us who they are. */
+ struct kt_message m = ktm_receive();
+ kc->result = m.info;
+ }
+ complete(&kc->done);
+}
+
+struct task_struct *kthread_create(int (*initfn)(void *data),
+ int (*corefn)(void *data),
+ void *data,
+ const char namefmt[],
+ ...)
+{
+ va_list args;
+ struct kthread_create kc;
+ DECLARE_WORK(work, spawn_kthread, &kc);
+ /* Or, as we like to say, 16. */
+ char name[sizeof(((struct task_struct *)0)->comm)];
+
+ va_start(args, namefmt);
+ vsnprintf(name, sizeof(name), namefmt, args);
+ va_end(args);
+
+ init_completion(&kc.done);
+ kc.k.initfn = initfn;
+ kc.k.corefn = corefn;
+ kc.k.data = data;
+ kc.k.name = name;
+
+ down(&kthread_control);
+ /* If we're being called to start the first workqueue, we
+ * can't use keventd. */
+ if (!keventd_up())
+ work.func(work.data);
+ else {
+ schedule_work(&work);
+ wait_for_completion(&kc.done);
+ }
+ up(&kthread_control);
+ return kc.result;
+}
+
+static void wait_for_death(struct task_struct *k)
+{
+ while (!(k->state & TASK_ZOMBIE) && !(k->state & TASK_DEAD))
+ yield();
+}
+
+struct task_struct *kthread_start(struct task_struct *k)
+{
+ struct kt_message m;
+
+ get_task_struct(k);
+
+ down(&kthread_control);
+ ktm_send(k, k);
+ m = ktm_receive();
+ up(&kthread_control);
+
+ if (IS_ERR(m.info))
+ wait_for_death(k);
+ put_task_struct(k);
+
+ return m.info;
+}
+
+int kthread_destroy(struct task_struct *k)
+{
+ struct kt_message m;
+
+ get_task_struct(k);
+
+ down(&kthread_control);
+ ktm_send(k, NULL);
+ m = ktm_receive();
+ up(&kthread_control);
+
+ wait_for_death(k);
+ put_task_struct(k);
+
+ return PTR_ERR(m.info);
+}
diff -urpN --exclude TAGS -X /home/rusty/devel/kernel/kernel-patches/current-dontdiff --minimal .32392-linux-2.6.0-test11-bk6/kernel/workqueue.c .32392-linux-2.6.0-test11-bk6.updated/kernel/workqueue.c
--- .32392-linux-2.6.0-test11-bk6/kernel/workqueue.c 2003-09-22 10:27:38.000000000 +1000
+++ .32392-linux-2.6.0-test11-bk6.updated/kernel/workqueue.c 2003-12-09 17:42:29.000000000 +1100
@@ -359,6 +359,11 @@ void flush_scheduled_work(void)
flush_workqueue(keventd_wq);
}

+int keventd_up(void)
+{
+ return keventd_wq != NULL;
+}
+
int current_is_keventd(void)
{
struct cpu_workqueue_struct *cwq;