workqueue: warn if delayed_work is queued to an offlined cpu.

delayed_work submitted to an offlined cpu, will not get executed,
after the specified delay if the cpu remains offline. If the cpu
never comes online the work will never get executed.
checking for online cpu in __queue_delayed_work, does not sound
like a good idea because to do this reliably we need hotplug lock
and since work may be submitted from atomic contexts, we would
have to use cpus_read_trylock. But if trylock fails we would queue
the work on any cpu and this may not be optimal because our intended
cpu might still be online.

Putting a WARN_ON_ONCE for an already offlined cpu, will indicate users
of queue_delayed_work_on, if they are (wrongly) trying to queue
delayed_work on offlined cpu. Also indicate the problem of using
offlined cpu with queue_delayed_work_on, in its description.

Signed-off-by: Imran Khan <imran.f.khan@oracle.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

Author: imran-kn

kernel/workqueue.c

@@ -2508,6 +2508,7 @@ static void __queue_delayed_work(int cpu, struct workqueue_struct *wq,
 		return;
 	}
 
+	WARN_ON_ONCE(cpu != WORK_CPU_UNBOUND && !cpu_online(cpu));
 	dwork->wq = wq;
 	dwork->cpu = cpu;
 	timer->expires = jiffies + delay;
@@ -2533,6 +2534,12 @@ static void __queue_delayed_work(int cpu, struct workqueue_struct *wq,
  * @dwork: work to queue
  * @delay: number of jiffies to wait before queueing
  *
+ * We queue the delayed_work to a specific CPU, for non-zero delays the
+ * caller must ensure it is online and can't go away. Callers that fail
+ * to ensure this, may get @dwork->timer queued to an offlined CPU and
+ * this will prevent queueing of @dwork->work unless the offlined CPU
+ * becomes online again.
+ *
  * Return: %false if @work was already on a queue, %true otherwise.  If
  * @delay is zero and @dwork is idle, it will be scheduled for immediate
  * execution.