[RFC PATCH 06/28] lkl: kernel threads support

[Octavian Purdila]

LKL does not support user processes but it must support kernel threads
as part as the normal kernel work-flow. It uses host operations to
create and terminate host threads that are going to run the kernel
threads. It also uses semaphores to synchronize those threads and to
allow the Linux kernel scheduler to control how the kernel threads
run.

Each kernel thread runs in a host threads and has a host semaphore
associated with it - the thread's scheduling semaphore. The semaphore
counter is initialized to 0. The first thing a kernel thread does
after getting spawned, before running any kernel code, is to perform a
down operation to block the thread.

The kernel controls host threads scheduling by performing up and down
operations on the scheduling semaphore. In __switch_context an up
operation on the next thread is performed to wake up a blocked thread,
and a down operation is performed on the prev thread to block it.

A thread is terminated by marking it in free_thread_info and
performing an up operation on the scheduling semaphore at which point
the marked thread will terminate itself.

Signed-off-by: Octavian Purdila <octavian.purdila@xxxxxxxxx>
---
 arch/lkl/include/asm/thread_info.h |  82 +++++++++++++
 arch/lkl/kernel/threads.c          | 235 +++++++++++++++++++++++++++++++++++++
 2 files changed, 317 insertions(+)
 create mode 100644 arch/lkl/include/asm/thread_info.h
 create mode 100644 arch/lkl/kernel/threads.c

diff --git a/arch/lkl/include/asm/thread_info.h b/arch/lkl/include/asm/thread_info.h
new file mode 100644
index 0000000..7636227
--- /dev/null
+++ b/arch/lkl/include/asm/thread_info.h
@@ -0,0 +1,82 @@
+#ifndef _ASM_LKL_THREAD_INFO_H
+#define _ASM_LKL_THREAD_INFO_H
+
+#define THREAD_SIZE	       (4096)
+
+#ifndef __ASSEMBLY__
+#include <asm/types.h>
+#include <asm/processor.h>
+
+typedef struct {
+	unsigned long seg;
+} mm_segment_t;
+
+struct thread_exit_info {
+	bool dead;
+	void *sched_sem;
+};
+
+struct thread_info {
+	struct task_struct *task;
+	unsigned long flags;
+	int preempt_count;
+	mm_segment_t addr_limit;
+	void *sched_sem;
+	struct thread_exit_info *exit_info;
+	struct task_struct *prev_sched;
+	unsigned long stackend;
+};
+
+#define INIT_THREAD_INFO(tsk)				\
+{							\
+	.task		= &tsk,				\
+	.preempt_count	= INIT_PREEMPT_COUNT,		\
+	.flags		= 0,				\
+	.addr_limit	= KERNEL_DS,			\
+}
+
+#define init_thread_info	(init_thread_union.thread_info)
+#define init_stack		(init_thread_union.stack)
+
+/* how to get the thread information struct from C */
+extern struct thread_info *_current_thread_info;
+static inline struct thread_info *current_thread_info(void)
+{
+	return _current_thread_info;
+}
+
+/* thread information allocation */
+struct thread_info *alloc_thread_info_node(struct task_struct *, int node);
+void free_thread_info(struct thread_info *);
+
+int threads_init(void);
+void threads_cleanup(void);
+
+#define TIF_SYSCALL_TRACE		0
+#define TIF_NOTIFY_RESUME		1
+#define TIF_SIGPENDING			2
+#define TIF_NEED_RESCHED		3
+#define TIF_RESTORE_SIGMASK		4
+#define TIF_MEMDIE			5
+#define TIF_NOHZ			6
+
+#define __HAVE_THREAD_FUNCTIONS
+
+#define task_thread_info(task)	((struct thread_info *)(task)->stack)
+#define task_stack_page(task)	((task)->stack)
+
+/*
+ * Nothing to do here. The only new tasks created are kernel threads that have a
+ * predefined starting point thus no stack copy is required as for regular
+ * forked tasks.
+ */
+static inline void setup_thread_stack(struct task_struct *p,
+				      struct task_struct *org)
+{
+}
+
+#define end_of_stack(p) (&task_thread_info(p)->stackend)
+
+#endif /* __ASSEMBLY__ */
+
+#endif
diff --git a/arch/lkl/kernel/threads.c b/arch/lkl/kernel/threads.c
new file mode 100644
index 0000000..aa13e57
--- /dev/null
+++ b/arch/lkl/kernel/threads.c
@@ -0,0 +1,235 @@
+#include <linux/module.h>
+#include <linux/slab.h>
+#include <linux/sched.h>
+#include <asm/host_ops.h>
+
+static int threads_counter;
+static void *threads_counter_lock;
+
+static inline void threads_counter_inc(void)
+{
+	lkl_ops->sem_down(threads_counter_lock);
+	threads_counter++;
+	lkl_ops->sem_up(threads_counter_lock);
+}
+
+static inline void threads_counter_dec(void)
+{
+	lkl_ops->sem_down(threads_counter_lock);
+	threads_counter--;
+	lkl_ops->sem_up(threads_counter_lock);
+}
+
+static inline int threads_counter_get(void)
+{
+	int counter;
+
+	lkl_ops->sem_down(threads_counter_lock);
+	counter = threads_counter;
+	lkl_ops->sem_up(threads_counter_lock);
+
+	return counter;
+}
+
+struct thread_info *alloc_thread_info_node(struct task_struct *task, int node)
+{
+	struct thread_info *ti;
+
+	ti = kmalloc(sizeof(*ti), GFP_KERNEL);
+	if (!ti)
+		return NULL;
+
+	ti->exit_info = NULL;
+	ti->prev_sched = NULL;
+	ti->sched_sem = lkl_ops->sem_alloc(0);
+	ti->task = task;
+	if (!ti->sched_sem) {
+		kfree(ti);
+		return NULL;
+	}
+
+	return ti;
+}
+
+static void kill_thread(struct thread_exit_info *ei)
+{
+	if (WARN_ON(!ei))
+		return;
+
+	ei->dead = true;
+	lkl_ops->sem_up(ei->sched_sem);
+}
+
+void free_thread_info(struct thread_info *ti)
+{
+	struct thread_exit_info *ei = ti->exit_info;
+
+	kfree(ti);
+	kill_thread(ei);
+}
+
+struct thread_info *_current_thread_info = &init_thread_union.thread_info;
+
+struct task_struct *__switch_to(struct task_struct *prev,
+				struct task_struct *next)
+{
+	struct thread_info *_prev = task_thread_info(prev);
+	struct thread_info *_next = task_thread_info(next);
+	/*
+	 * schedule() expects the return of this function to be the task that we
+	 * switched away from. Returning prev is not going to work because we
+	 * are actually going to return the previous taks that was scheduled
+	 * before the task we are going to wake up, and not the current task,
+	 * e.g.:
+	 *
+	 * swapper -> init: saved prev on swapper stack is swapper
+	 * init -> ksoftirqd0: saved prev on init stack is init
+	 * ksoftirqd0 -> swapper: returned prev is swapper
+	 */
+	static struct task_struct *abs_prev = &init_task;
+	/*
+	 * We need to free the thread_info structure in free_thread_info to
+	 * avoid races between the dying thread and other threads. We also need
+	 * to cleanup sched_sem and signal to the prev thread that it needs to
+	 * exit, and we use this stack varible to pass this info.
+	 */
+	struct thread_exit_info ei = {
+		.dead = false,
+		.sched_sem = _prev->sched_sem,
+	};
+
+	_current_thread_info = task_thread_info(next);
+	_next->prev_sched = prev;
+	abs_prev = prev;
+	_prev->exit_info = &ei;
+
+	lkl_ops->sem_up(_next->sched_sem);
+	/* _next may be already gone so use ei instead */
+	lkl_ops->sem_down(ei.sched_sem);
+
+	if (ei.dead) {
+		lkl_ops->sem_free(ei.sched_sem);
+		threads_counter_dec();
+		lkl_ops->thread_exit();
+	}
+
+	_prev->exit_info = NULL;
+
+	return abs_prev;
+}
+
+struct thread_bootstrap_arg {
+	struct thread_info *ti;
+	int (*f)(void *);
+	void *arg;
+};
+
+static void thread_bootstrap(void *_tba)
+{
+	struct thread_bootstrap_arg *tba = (struct thread_bootstrap_arg *)_tba;
+	struct thread_info *ti = tba->ti;
+	int (*f)(void *) = tba->f;
+	void *arg = tba->arg;
+
+	lkl_ops->sem_down(ti->sched_sem);
+	kfree(tba);
+	if (ti->prev_sched)
+		schedule_tail(ti->prev_sched);
+
+	f(arg);
+	do_exit(0);
+}
+
+int copy_thread(unsigned long clone_flags, unsigned long esp,
+		unsigned long unused, struct task_struct *p)
+{
+	struct thread_info *ti = task_thread_info(p);
+	struct thread_bootstrap_arg *tba;
+	int ret;
+
+	tba = kmalloc(sizeof(*tba), GFP_KERNEL);
+	if (!tba)
+		return -ENOMEM;
+
+	tba->f = (int (*)(void *))esp;
+	tba->arg = (void *)unused;
+	tba->ti = ti;
+
+	ret = lkl_ops->thread_create(thread_bootstrap, tba);
+	if (ret) {
+		kfree(tba);
+		return -ENOMEM;
+	}
+
+	threads_counter_inc();
+
+	return 0;
+}
+
+void show_stack(struct task_struct *task, unsigned long *esp)
+{
+}
+
+static inline void pr_early(const char *str)
+{
+	if (lkl_ops->print)
+		lkl_ops->print(str, strlen(str));
+}
+
+/**
+ * This is called before the kernel initializes, so no kernel calls (including
+ * printk) can't be made yet.
+ */
+int threads_init(void)
+{
+	struct thread_info *ti = &init_thread_union.thread_info;
+	int ret = 0;
+
+	ti->exit_info = NULL;
+	ti->prev_sched = NULL;
+
+	ti->sched_sem = lkl_ops->sem_alloc(0);
+	if (!ti->sched_sem) {
+		pr_early("lkl: failed to allocate init schedule semaphore\n");
+		ret = -ENOMEM;
+		goto out;
+	}
+
+	threads_counter_lock = lkl_ops->sem_alloc(1);
+	if (!threads_counter_lock) {
+		pr_early("lkl: failed to alllocate threads counter lock\n");
+		ret = -ENOMEM;
+		goto out_free_init_sched_sem;
+	}
+
+	return 0;
+
+out_free_init_sched_sem:
+	lkl_ops->sem_free(ti->sched_sem);
+
+out:
+	return ret;
+}
+
+void threads_cleanup(void)
+{
+	struct task_struct *p;
+
+	for_each_process(p) {
+		struct thread_info *ti = task_thread_info(p);
+
+		if (p->pid != 1)
+			WARN(!(p->flags & PF_KTHREAD),
+			     "non kernel thread task %p\n", p->comm);
+		WARN(p->state == TASK_RUNNING,
+		     "thread %s still running while halting\n", p->comm);
+
+		kill_thread(ti->exit_info);
+	}
+
+	while (threads_counter_get())
+		;
+
+	lkl_ops->sem_free(init_thread_union.thread_info.sched_sem);
+	lkl_ops->sem_free(threads_counter_lock);
+}
-- 
2.1.0

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