[PATCH] kthread: add kthread_mod_pending_delayed_work api

[Yiwei Zhang <zzyiwei@android.com>]

The existing kthread_mod_delayed_work api will queue a new work if
failing to cancel the current work due to no longer being pending.
However, there's a case that the same work can be enqueued from both
an async request and a delayed work, and a racing could happen if the
async request comes right after the timeout delayed work gets scheduled,
because the clean up work may not be safe to run twice.

Signed-off-by: Yiwei Zhang <zzyiwei@android.com>
---
 include/linux/kthread.h |  3 +++
 kernel/kthread.c        | 48 +++++++++++++++++++++++++++++++++++++++++
 2 files changed, 51 insertions(+)

diff --git a/include/linux/kthread.h b/include/linux/kthread.h
index 65b81e0c494d..250cdc5ff2a5 100644
--- a/include/linux/kthread.h
+++ b/include/linux/kthread.h
@@ -192,6 +192,9 @@ bool kthread_queue_delayed_work(struct kthread_worker *worker,
 bool kthread_mod_delayed_work(struct kthread_worker *worker,
 			      struct kthread_delayed_work *dwork,
 			      unsigned long delay);
+bool kthread_mod_pending_delayed_work(struct kthread_worker *worker,
+				      struct kthread_delayed_work *dwork,
+				      unsigned long delay);
 
 void kthread_flush_work(struct kthread_work *work);
 void kthread_flush_worker(struct kthread_worker *worker);
diff --git a/kernel/kthread.c b/kernel/kthread.c
index a5eceecd4513..13881076afdd 100644
--- a/kernel/kthread.c
+++ b/kernel/kthread.c
@@ -1142,6 +1142,54 @@ bool kthread_mod_delayed_work(struct kthread_worker *worker,
 }
 EXPORT_SYMBOL_GPL(kthread_mod_delayed_work);
 
+/**
+ * kthread_mod_pending_delayed_work - modify delay of a pending delayed work
+ * @worker: kthread worker to use
+ * @dwork: kthread delayed work to queue
+ * @delay: number of jiffies to wait before queuing
+ *
+ * If @dwork is still pending modify @dwork's timer so that it expires after
+ * @delay. If @dwork is still pending and @delay is zero, @work is guaranteed to
+ * be queued immediately.
+ *
+ * Return: %true if @dwork was pending and its timer was modified,
+ * %false otherwise.
+ *
+ * A special case is when the work is being canceled in parallel.
+ * It might be caused either by the real kthread_cancel_delayed_work_sync()
+ * or yet another kthread_mod_delayed_work() call. We let the other command
+ * win and return %false here. The caller is supposed to synchronize these
+ * operations a reasonable way.
+ *
+ * This function is safe to call from any context including IRQ handler.
+ * See __kthread_cancel_work() and kthread_delayed_work_timer_fn()
+ * for details.
+ */
+bool kthread_mod_pending_delayed_work(struct kthread_worker *worker,
+				      struct kthread_delayed_work *dwork,
+				      unsigned long delay)
+{
+	struct kthread_work *work = &dwork->work;
+	unsigned long flags;
+	int ret = true;
+
+	raw_spin_lock_irqsave(&worker->lock, flags);
+	if (!work->worker || work->canceling ||
+	    !__kthread_cancel_work(work, true, &flags)) {
+		ret = false;
+		goto out;
+	}
+
+	/* Work must not be used with >1 worker, see kthread_queue_work() */
+	WARN_ON_ONCE(work->worker != worker);
+
+	__kthread_queue_delayed_work(worker, dwork, delay);
+out:
+	raw_spin_unlock_irqrestore(&worker->lock, flags);
+	return ret;
+}
+EXPORT_SYMBOL_GPL(kthread_mod_pending_delayed_work);
+
 static bool __kthread_cancel_work_sync(struct kthread_work *work, bool is_dwork)
 {
 	struct kthread_worker *worker = work->worker;
-- 
2.30.0.478.g8a0d178c01-goog