[PATCH v4 10/22] kthread: Allow to modify delayed kthread work

[Petr Mladek]

There are situations when we need to modify the delay of a delayed kthread
work. For example, when the work depends on an event and the initial delay
means a timeout. Then we want to queue the work immediately when the event
happens.

This patch implements mod_delayed_kthread_work() as inspired workqueues.
It tries to cancel the pending work and queue it again with the
given timeout.

A very special case is when the work is being canceled at the same time.
cancel_*kthread_work_sync() operation blocks queuing until the running
work finishes. Therefore we do nothing and let cancel() win. This should
not normally happen as the caller is supposed to synchronize these
operations a reasonable way.

Signed-off-by: Petr Mladek <pmladek@xxxxxxxx>
---
 include/linux/kthread.h |  4 ++++
 kernel/kthread.c        | 50 +++++++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 54 insertions(+)

diff --git a/include/linux/kthread.h b/include/linux/kthread.h
index dd2a587a2bd7..f501dfeaa0e3 100644
--- a/include/linux/kthread.h
+++ b/include/linux/kthread.h
@@ -168,6 +168,10 @@ bool queue_delayed_kthread_work(struct kthread_worker *worker,
 				struct delayed_kthread_work *dwork,
 				unsigned long delay);
 
+bool mod_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 6e2eeca08d5f..ebb91848685f 100644
--- a/kernel/kthread.c
+++ b/kernel/kthread.c
@@ -1013,6 +1013,56 @@ out:
 	return ret;
 }
 
+/**
+ * mod_delayed_kthread_work - modify delay of or queue a delayed kthread work
+ * @worker: kthread worker to use
+ * @dwork: delayed kthread work to queue
+ * @delay: number of jiffies to wait before queuing
+ *
+ * If @dwork is idle, equivalent to queue_delayed_kthread work(). Otherwise,
+ * modify @dwork's timer so that it expires after @delay. If @delay is zero,
+ * @work is guaranteed to be queued immediately.
+ *
+ * Return: %false if @dwork was idle and queued. Return %true if @dwork was
+ * pending and its timer was modified.
+ *
+ * A special case is when cancel_work_sync() is running in parallel.
+ * It blocks further queuing. We let the cancel() win and return %false.
+ * The caller is supposed to synchronize these operations a reasonable way.
+ *
+ * This function is safe to call from any context including IRQ handler.
+ * See try_to_grab_pending_kthread_work() for details.
+ */
+bool mod_delayed_kthread_work(struct kthread_worker *worker,
+			      struct delayed_kthread_work *dwork,
+			      unsigned long delay)
+{
+	struct kthread_work *work = &dwork->work;
+	unsigned long flags;
+	int ret = 0;
+
+try_again:
+	spin_lock_irqsave(&worker->lock, flags);
+	WARN_ON_ONCE(work->worker && work->worker != worker);
+
+	if (work->canceling)
+		goto out;
+
+	ret = try_to_cancel_kthread_work(work, &worker->lock, &flags);
+	if (ret == -EAGAIN)
+		goto try_again;
+
+	if (work->canceling)
+		ret = 0;
+	else
+		__queue_delayed_kthread_work(worker, dwork, delay);
+
+out:
+	spin_unlock_irqrestore(&worker->lock, flags);
+	return ret;
+}
+EXPORT_SYMBOL_GPL(mod_delayed_kthread_work);
+
 static bool __cancel_kthread_work_sync(struct kthread_work *work)
 {
 	struct kthread_worker *worker;
-- 
1.8.5.6