workqueue: implement mod_delayed_work[_on]()

Workqueue was lacking a mechanism to modify the timeout of an already
pending delayed_work.  delayed_work users have been working around
this using several methods - using an explicit timer + work item,
messing directly with delayed_work->timer, and canceling before
re-queueing, all of which are error-prone and/or ugly.

This patch implements mod_delayed_work[_on]() which behaves similarly
to mod_timer() - if the delayed_work is idle, it's queued with the
given delay; otherwise, its timeout is modified to the new value.
Zero @delay guarantees immediate execution.

v2: Updated to reflect try_to_grab_pending() changes.  Now safe to be
    called from bh context.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Ingo Molnar <mingo@redhat.com>

include/linux/workqueue.h

@@ -390,6 +390,10 @@ extern bool queue_delayed_work_on(int cpu, struct workqueue_struct *wq,
 			struct delayed_work *work, unsigned long delay);
 extern bool queue_delayed_work(struct workqueue_struct *wq,
 			struct delayed_work *work, unsigned long delay);
+extern bool mod_delayed_work_on(int cpu, struct workqueue_struct *wq,
+			struct delayed_work *dwork, unsigned long delay);
+extern bool mod_delayed_work(struct workqueue_struct *wq,
+			struct delayed_work *dwork, unsigned long delay);
 
 extern void flush_workqueue(struct workqueue_struct *wq);
 extern void drain_workqueue(struct workqueue_struct *wq);

kernel/workqueue.c

@@ -1413,6 +1413,59 @@ bool queue_delayed_work(struct workqueue_struct *wq,
 }
 EXPORT_SYMBOL_GPL(queue_delayed_work);
 
+/**
+ * mod_delayed_work_on - modify delay of or queue a delayed work on specific CPU
+ * @cpu: CPU number to execute work on
+ * @wq: workqueue to use
+ * @dwork: work to queue
+ * @delay: number of jiffies to wait before queueing
+ *
+ * If @dwork is idle, equivalent to queue_delayed_work_on(); otherwise,
+ * modify @dwork's timer so that it expires after @delay.  If @delay is
+ * zero, @work is guaranteed to be scheduled immediately regardless of its
+ * current state.
+ *
+ * Returns %false if @dwork was idle and queued, %true if @dwork was
+ * pending and its timer was modified.
+ *
+ * This function is safe to call from any context other than IRQ handler.
+ * See try_to_grab_pending() for details.
+ */
+bool mod_delayed_work_on(int cpu, struct workqueue_struct *wq,
+			 struct delayed_work *dwork, unsigned long delay)
+{
+	unsigned long flags;
+	int ret;
+
+	do {
+		ret = try_to_grab_pending(&dwork->work, true, &flags);
+	} while (unlikely(ret == -EAGAIN));
+
+	if (likely(ret >= 0)) {
+		__queue_delayed_work(cpu, wq, dwork, delay);
+		local_irq_restore(flags);
+	}
+
+	/* -ENOENT from try_to_grab_pending() becomes %true */
+	return ret;
+}
+EXPORT_SYMBOL_GPL(mod_delayed_work_on);
+
+/**
+ * mod_delayed_work - modify delay of or queue a delayed work
+ * @wq: workqueue to use
+ * @dwork: work to queue
+ * @delay: number of jiffies to wait before queueing
+ *
+ * mod_delayed_work_on() on local CPU.
+ */
+bool mod_delayed_work(struct workqueue_struct *wq, struct delayed_work *dwork,
+		      unsigned long delay)
+{
+	return mod_delayed_work_on(WORK_CPU_UNBOUND, wq, dwork, delay);
+}
+EXPORT_SYMBOL_GPL(mod_delayed_work);
+
 /**
  * worker_enter_idle - enter idle state
  * @worker: worker which is entering idle state