[PATCH v4 08/22] kthread: Initial support for delayed kthread work

[Petr Mladek]

We are going to use kthread_worker more widely and delayed works
will be pretty useful.

The implementation is inspired by workqueues. It uses a timer to
queue the work after the requested delay. If the delay is zero,
the work is queued immediately.

In compare with workqueues, each work is associated with a single
worker (kthread). Therefore the implementation could be much easier.
In particular, we use the worker->lock to synchronized all the
operations with the work. We do not need any atomic operation
with a flags variable.

We even do not need to maintain any state variable. Instead, we
add a pointer[*] to the timer into the struct kthread_work. Then
we could detect the pending status using a rather simple check,
see the new kthread_worker_pending() function.

Finally, the timer callback knows only about the struct work.
It is better be paranoid and try to get the worker->lock carefully.
The try_lock_thread_work() function will be later useful also when
canceling the work.

[*] I considered also adding the entire struct timer_list into
    struct kthread_work. But it would increase the size from
    40 to 120 bytes on x86_64 with an often unused stuff.

    Another alternative was to add a flags variable. But this
    would add an extra code to synchronize it with the state
    of the timer.

Signed-off-by: Petr Mladek <pmladek@xxxxxxxx>
---
 include/linux/kthread.h |  34 ++++++++++++
 kernel/kthread.c        | 142 +++++++++++++++++++++++++++++++++++++++++++++++-
 2 files changed, 174 insertions(+), 2 deletions(-)

diff --git a/include/linux/kthread.h b/include/linux/kthread.h
index c4a95a3ba500..1a5738dcdf8d 100644
--- a/include/linux/kthread.h
+++ b/include/linux/kthread.h
@@ -63,6 +63,7 @@ extern int tsk_fork_get_node(struct task_struct *tsk);
  */
 struct kthread_work;
 typedef void (*kthread_work_func_t)(struct kthread_work *work);
+void delayed_kthread_work_timer_fn(unsigned long __data);
 
 struct kthread_worker {
 	spinlock_t		lock;
@@ -75,6 +76,12 @@ struct kthread_work {
 	struct list_head	node;
 	kthread_work_func_t	func;
 	struct kthread_worker	*worker;
+	struct timer_list	*timer;
+};
+
+struct delayed_kthread_work {
+	struct kthread_work work;
+	struct timer_list timer;
 };
 
 #define KTHREAD_WORKER_INIT(worker)	{				\
@@ -87,12 +94,24 @@ struct kthread_work {
 	.func = (fn),							\
 	}
 
+#define DELAYED_KTHREAD_WORK_INIT(dwork, fn) {				\
+	.work = KTHREAD_WORK_INIT((dwork).work, (fn)),			\
+	.timer = __TIMER_INITIALIZER(delayed_kthread_work_timer_fn,	\
+				     0, (unsigned long)&(dwork),	\
+				     TIMER_IRQSAFE),			\
+	.work.timer = &(dwork).timer,			\
+	}
+
 #define DEFINE_KTHREAD_WORKER(worker)					\
 	struct kthread_worker worker = KTHREAD_WORKER_INIT(worker)
 
 #define DEFINE_KTHREAD_WORK(work, fn)					\
 	struct kthread_work work = KTHREAD_WORK_INIT(work, fn)
 
+#define DEFINE_DELAYED_KTHREAD_WORK(dwork, fn)				\
+	struct delayed_kthread_work dwork =				\
+		DELAYED_KTHREAD_WORK_INIT(dwork, fn)
+
 /*
  * kthread_worker.lock needs its own lockdep class key when defined on
  * stack with lockdep enabled.  Use the following macros in such cases.
@@ -122,6 +141,16 @@ extern void __init_kthread_worker(struct kthread_worker *worker,
 		(work)->func = (fn);					\
 	} while (0)
 
+#define init_delayed_kthread_work(dwork, fn)				\
+	do {								\
+		init_kthread_work(&(dwork)->work, (fn));		\
+		__setup_timer(&(dwork)->timer,				\
+			      delayed_kthread_work_timer_fn,		\
+			      (unsigned long)(dwork),			\
+			      TIMER_IRQSAFE);				\
+		(dwork)->work.timer = &(dwork)->timer;			\
+	} while (0)
+
 int kthread_worker_fn(void *worker_ptr);
 
 __printf(1, 2)
@@ -133,6 +162,11 @@ create_kthread_worker_on_cpu(int cpu, const char namefmt[]);
 
 bool queue_kthread_work(struct kthread_worker *worker,
 			struct kthread_work *work);
+
+bool queue_delayed_kthread_work(struct kthread_worker *worker,
+				struct delayed_kthread_work *dwork,
+				unsigned long delay);
+
 void flush_kthread_work(struct kthread_work *work);
 void flush_kthread_worker(struct kthread_worker *worker);
 
diff --git a/kernel/kthread.c b/kernel/kthread.c
index e12576bc0e39..6933d90c0fec 100644
--- a/kernel/kthread.c
+++ b/kernel/kthread.c
@@ -563,6 +563,20 @@ void __init_kthread_worker(struct kthread_worker *worker,
 }
 EXPORT_SYMBOL_GPL(__init_kthread_worker);
 
+/*
+ * Returns true when there is a pending operation for this work.
+ * In particular, it checks if the work is:
+ *	- queued
+ *	- a timer is running to queue this delayed work
+ *
+ * This function must be called with locked work.
+ */
+static inline bool kthread_work_pending(const struct kthread_work *work)
+{
+	return !list_empty(&work->node) ||
+	       (work->timer && timer_active(work->timer));
+}
+
 /**
  * kthread_worker_fn - kthread function to process kthread_worker
  * @worker_ptr: pointer to initialized kthread_worker
@@ -722,6 +736,15 @@ static void insert_kthread_work(struct kthread_worker *worker,
 		wake_up_process(worker->task);
 }
 
+/*
+ * Queue @work right into the worker queue.
+ */
+static void __queue_kthread_work(struct kthread_worker *worker,
+			  struct kthread_work *work)
+{
+	insert_kthread_work(worker, work, &worker->work_list);
+}
+
 /**
  * queue_kthread_work - queue a kthread_work
  * @worker: target kthread_worker
@@ -747,8 +770,8 @@ bool queue_kthread_work(struct kthread_worker *worker,
 	unsigned long flags;
 
 	spin_lock_irqsave(&worker->lock, flags);
-	if (list_empty(&work->node)) {
-		insert_kthread_work(worker, work, &worker->work_list);
+	if (!kthread_work_pending(work)) {
+		__queue_kthread_work(worker, work);
 		ret = true;
 	}
 	spin_unlock_irqrestore(&worker->lock, flags);
@@ -756,6 +779,121 @@ bool queue_kthread_work(struct kthread_worker *worker,
 }
 EXPORT_SYMBOL_GPL(queue_kthread_work);
 
+static bool try_lock_kthread_work(struct kthread_work *work)
+{
+	struct kthread_worker *worker;
+	int ret = false;
+
+try_again:
+	worker = work->worker;
+
+	if (!worker)
+		goto out;
+
+	spin_lock(&worker->lock);
+	if (worker != work->worker) {
+		spin_unlock(&worker->lock);
+		goto try_again;
+	}
+	ret = true;
+
+out:
+	return ret;
+}
+
+static inline void unlock_kthread_work(struct kthread_work *work)
+{
+	spin_unlock(&work->worker->lock);
+}
+
+/**
+ * delayed_kthread_work_timer_fn - callback that queues the associated delayed
+ *	kthread work when the timer expires.
+ * @__data: pointer to the data associated with the timer
+ *
+ * The format of the function is defined by struct timer_list.
+ * It should have been called from irqsafe timer with irq already off.
+ */
+void delayed_kthread_work_timer_fn(unsigned long __data)
+{
+	struct delayed_kthread_work *dwork =
+		(struct delayed_kthread_work *)__data;
+	struct kthread_work *work = &dwork->work;
+
+	if (!try_lock_kthread_work(work))
+		return;
+
+	__queue_kthread_work(work->worker, work);
+	unlock_kthread_work(work);
+}
+EXPORT_SYMBOL(delayed_kthread_work_timer_fn);
+
+void __queue_delayed_kthread_work(struct kthread_worker *worker,
+				struct delayed_kthread_work *dwork,
+				unsigned long delay)
+{
+	struct timer_list *timer = &dwork->timer;
+	struct kthread_work *work = &dwork->work;
+
+	WARN_ON_ONCE(timer->function != delayed_kthread_work_timer_fn ||
+		     timer->data != (unsigned long)dwork);
+	WARN_ON_ONCE(timer_pending(timer));
+
+	/*
+	 * If @delay is 0, queue @dwork->work immediately.  This is for
+	 * both optimization and correctness.  The earliest @timer can
+	 * expire is on the closest next tick and delayed_work users depend
+	 * on that there's no such delay when @delay is 0.
+	 */
+	if (!delay) {
+		__queue_kthread_work(worker, work);
+		return;
+	}
+
+	/* Be paranoid and try to detect possible races already now. */
+	insert_kthread_work_sanity_check(worker, work);
+
+	work->worker = worker;
+	timer_stats_timer_set_start_info(&dwork->timer);
+	timer->expires = jiffies + delay;
+	add_timer(timer);
+}
+
+/**
+ * queue_delayed_kthread_work - queue the associated kthread work
+ *	after a delay.
+ * @worker: target kthread_worker
+ * @work: kthread_work to queue
+ * delay: number of jiffies to wait before queuing
+ *
+ * If the work has not been pending it starts a timer that will queue
+ * the work after the given @delay. If @delay is zero, it queues the
+ * work immediately.
+ *
+ * Return: %false if the @work has already been pending. It means that
+ * either the timer was running or the work was queued. It returns %true
+ * otherwise.
+ */
+bool queue_delayed_kthread_work(struct kthread_worker *worker,
+				struct delayed_kthread_work *dwork,
+				unsigned long delay)
+{
+	struct kthread_work *work = &dwork->work;
+	unsigned long flags;
+	bool ret = false;
+
+	spin_lock_irqsave(&worker->lock, flags);
+
+	if (!kthread_work_pending(work)) {
+		__queue_delayed_kthread_work(worker, dwork, delay);
+		ret = true;
+	}
+
+	spin_unlock_irqrestore(&worker->lock, flags);
+	return ret;
+}
+EXPORT_SYMBOL_GPL(queue_delayed_kthread_work);
+
 struct kthread_flush_work {
 	struct kthread_work	work;
 	struct completion	done;
-- 
1.8.5.6