workqueue: Wrap flush_workqueue() using a macro

Since flush operation synchronously waits for completion, flushing system-wide WQs (e.g. system_wq) might introduce possibility of deadlock due to unexpected locking dependency. Tejun Heo commented at [1] that it makes no sense at all to call flush_workqueue() on the shared WQs as the caller has no idea what it's gonna end up waiting for. Although there is flush_scheduled_work() which flushes system_wq WQ with "Think twice before calling this function! It's very easy to get into trouble if you don't take great care." warning message, syzbot found a circular locking dependency caused by flushing system_wq WQ [2]. Therefore, let's change the direction to that developers had better use their local WQs if flush_scheduled_work()/flush_workqueue(system_*_wq) is inevitable. Steps for converting system-wide WQs into local WQs are explained at [3], and a conversion to stop flushing system-wide WQs is in progress. Now we want some mechanism for preventing developers who are not aware of this conversion from again start flushing system-wide WQs. Since I found that WARN_ON() is complete but awkward approach for teaching developers about this problem, let's use __compiletime_warning() for incomplete but handy approach. For completeness, we will also insert WARN_ON() into __flush_workqueue() after all in-tree users stopped calling flush_scheduled_work().

Tetsuo Handa committed 4 years ago

Commit c4f135d1 parent e71e60c2 files · +68 −12Browse files

Showing the changes to kernel/workqueue.c (1 of 2 files). Show all

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kernel/workqueue.c+12−4

 }
 /**
- * flush_workqueue - ensure that any scheduled work has run to completion.
+ * __flush_workqueue - ensure that any scheduled work has run to completion.
  * @wq: workqueue to flush
  *
  * This function sleeps until all work items which were queued on entry
  * have finished execution, but it is not livelocked by new incoming ones.
  */
-void flush_workqueue(struct workqueue_struct *wq)
+void __flush_workqueue(struct workqueue_struct *wq)
 {
 	struct wq_flusher this_flusher = {
 		.list = LIST_HEAD_INIT(this_flusher.list),
 out_unlock:
 	mutex_unlock(&wq->mutex);
 }
-EXPORT_SYMBOL(flush_workqueue);
+EXPORT_SYMBOL(__flush_workqueue);
 /**
  * drain_workqueue - drain a workqueue
 		wq->flags |= __WQ_DRAINING;
 	mutex_unlock(&wq->mutex);
 reflush:
-	flush_workqueue(wq);
+	__flush_workqueue(wq);
 	mutex_lock(&wq->mutex);
 	wq_online = true;
 	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) { }
+EXPORT_SYMBOL(__warn_flushing_systemwide_wq);