[PATCH v2] workqueue: introduce queue_delayed_work_on_offline_safe.

[Imran Khan]

Currently users of queue_delayed_work_on, need to ensure
that specified cpu is and remains online. The failure to
do so may result in delayed_work getting queued on an
offlined cpu and hence never getting executed.

The current users of queue_delayed_work_on, seem to ensure
the above mentioned criteria but for those, unknown amongst
current users or new users, who can't confirm to this
we need another interface.

So introduce queue_delayed_work_on_offline_safe, which
is a wrapper around queue_delayed_work_one to ensure that
the specified cpu is and remains online.

Signed-off-by: Imran Khan <imran.f.khan@xxxxxxxxxx>
---
v1 --> v2:
  - Remove RFC tag
  - For cases where dwork can't be put on specified CPU,
    let caller decide the next CPU to try with.

 include/linux/workqueue.h |  3 +++
 kernel/workqueue.c        | 42 +++++++++++++++++++++++++++++++++++++++
 2 files changed, 45 insertions(+)

diff --git a/include/linux/workqueue.h b/include/linux/workqueue.h
index b0dc957c3e560..cefcf9e89be6f 100644
--- a/include/linux/workqueue.h
+++ b/include/linux/workqueue.h
@@ -589,6 +589,9 @@ extern bool queue_work_node(int node, struct workqueue_struct *wq,
 			    struct work_struct *work);
 extern bool queue_delayed_work_on(int cpu, struct workqueue_struct *wq,
 			struct delayed_work *work, unsigned long delay);
+extern bool queue_delayed_work_on_offline_safe(int cpu,
+			struct workqueue_struct *wq, struct delayed_work *work,
+			unsigned long delay, bool *online);
 extern bool mod_delayed_work_on(int cpu, struct workqueue_struct *wq,
 			struct delayed_work *dwork, unsigned long delay);
 extern bool queue_rcu_work(struct workqueue_struct *wq, struct rcu_work *rwork);
diff --git a/kernel/workqueue.c b/kernel/workqueue.c
index 9362484a653c4..b3c030e6c6b17 100644
--- a/kernel/workqueue.c
+++ b/kernel/workqueue.c
@@ -2565,6 +2565,48 @@ bool queue_delayed_work_on(int cpu, struct workqueue_struct *wq,
 }
 EXPORT_SYMBOL(queue_delayed_work_on);
 
+/**
+ * queue_delayed_work_on_offline_safe - queue work on specific online CPU after
+ *					delay,
+ *
+ * @cpu: CPU number to execute work on
+ * @wq: workqueue to use
+ * @dwork: work to queue
+ * @delay: number of jiffies to wait before queueing
+ * @online: online status of @cpu, for caller
+ *
+ * a wrapper, around queue_delayed_work_on, that checks and ensures that
+ * specified @cpu is online. If @cpu is found to be offline or if its online
+ * status can't be reliably determined, set @online to false and return
+ * false, leaving the decision, of selecting new cpu for delayed_work, to
+ * the caller.
+ *
+ * If caller sees @online as false, it can try submitting work on a
+ * different @cpu, but if it sees @online as true, it can check the return
+ * value to determine if the work was really submitted or not.
+ */
+bool queue_delayed_work_on_offline_safe(int cpu, struct workqueue_struct *wq,
+			   struct delayed_work *dwork, unsigned long delay,
+			   bool *online)
+{
+	bool ret = false;
+	int locked = cpus_read_trylock();
+
+	if (locked && cpu_online(cpu)) {
+		ret = queue_delayed_work_on(cpu, wq, dwork, delay);
+		*online = true;
+	} else {
+		*online = false;
+	}
+
+	if (locked)
+		cpus_read_unlock();
+
+	return ret;
+}
+EXPORT_SYMBOL(queue_delayed_work_on_offline_safe);
+
+
 /**
  * mod_delayed_work_on - modify delay of or queue a delayed work on specific CPU
  * @cpu: CPU number to execute work on

base-commit: 5bc55a333a2f7316b58edc7573e8e893f7acb532
-- 
2.34.1