workqueue: Warn attempt to flush system-wide workqueues.

Based on commit c4f135d6 ("workqueue: Wrap flush_workqueue() using
a macro"), all in-tree users stopped flushing system-wide workqueues.
Therefore, start emitting runtime message so that all out-of-tree users
will understand that they need to update their code.
Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Signed-off-by: Tejun Heo <tj@kernel.org>

include/linux/workqueue.h

@@ -569,6 +569,7 @@ static inline bool schedule_work(struct work_struct *work)
 
 /*
  * Detect attempt to flush system-wide workqueues at compile time when possible.
+ * Warn attempt to flush system-wide workqueues at runtime.
  *
  * See https://lkml.kernel.org/r/49925af7-78a8-a3dd-bce6-cfc02e1a9236@I-love.SAKURA.ne.jp
  * for reasons and steps for converting system-wide workqueues into local workqueues.
@@ -576,52 +577,13 @@ static inline bool schedule_work(struct work_struct *work)
 extern void __warn_flushing_systemwide_wq(void)
 	__compiletime_warning("Please avoid flushing system-wide workqueues.");
 
-/**
- * flush_scheduled_work - ensure that any scheduled work has run to completion.
- *
- * Forces execution of the kernel-global workqueue and blocks until its
- * completion.
- *
- * It's very easy to get into trouble if you don't take great care.
- * Either of the following situations will lead to deadlock:
- *
- *	One of the work items currently on the workqueue needs to acquire
- *	a lock held by your code or its caller.
- *
- *	Your code is running in the context of a work routine.
- *
- * They will be detected by lockdep when they occur, but the first might not
- * occur very often.  It depends on what work items are on the workqueue and
- * what locks they need, which you have no control over.
- *
- * In most situations flushing the entire workqueue is overkill; you merely
- * need to know that a particular work item isn't queued and isn't running.
- * In such cases you should use cancel_delayed_work_sync() or
- * cancel_work_sync() instead.
- *
- * Please stop calling this function! A conversion to stop flushing system-wide
- * workqueues is in progress. This function will be removed after all in-tree
- * users stopped calling this function.
- */
-/*
- * The background of commit 771c035372a036f8 ("deprecate the
- * '__deprecated' attribute warnings entirely and for good") is that,
- * since Linus builds all modules between every single pull he does,
- * the standard kernel build needs to be _clean_ in order to be able to
- * notice when new problems happen. Therefore, don't emit warning while
- * there are in-tree users.
- */
+/* Please stop using this function, for this function will be removed in near future. */
 #define flush_scheduled_work()						\
 ({									\
-	if (0)								\
-		__warn_flushing_systemwide_wq();			\
+	__warn_flushing_systemwide_wq();				\
 	__flush_workqueue(system_wq);					\
 })
 
-/*
- * Although there is no longer in-tree caller, for now just emit warning
- * in order to give out-of-tree callers time to update.
- */
 #define flush_workqueue(wq)						\
 ({									\
 	struct workqueue_struct *_wq = (wq);				\

kernel/workqueue.c

@@ -6571,10 +6571,9 @@ void __init workqueue_init(void)
 	wq_watchdog_init();
 }
 
-/*
- * Despite the naming, this is a no-op function which is here only for avoiding
- * link error. Since compile-time warning may fail to catch, we will need to
- * emit run-time warning from __flush_workqueue().
- */
-void __warn_flushing_systemwide_wq(void) { }
+void __warn_flushing_systemwide_wq(void)
+{
+	pr_warn("WARNING: Flushing system-wide workqueues will be prohibited in near future.\n");
+	dump_stack();
+}
 EXPORT_SYMBOL(__warn_flushing_systemwide_wq);