linux-kernel - [PATCH] Warn about flush_scheduled

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Date:	Wed, 16 Dec 2009 10:52:21 -0500 (EST)
From:	Alan Stern <stern@...land.harvard.edu>
To:	Tejun Heo <tj@...nel.org>
cc:	Oliver Neukum <oliver@...kum.org>,
	Kernel development list <linux-kernel@...r.kernel.org>
Subject: [PATCH] Warn about flush_scheduled_work()

This patch (as1319) adds kerneldoc and a pointed warning to
flush_scheduled_work().

Signed-off-by: Alan Stern <stern@...land.harvard.edu>

---

Index: usb-2.6/kernel/workqueue.c
===================================================================
--- usb-2.6.orig/kernel/workqueue.c
+++ usb-2.6/kernel/workqueue.c
@@ -720,6 +720,30 @@ int schedule_on_each_cpu(work_func_t fun
 	return 0;
 }
 
+/**
+ * flush_scheduled_work - ensure that any scheduled work has run to completion.
+ *
+ * Forces execution of the kernel-global workqueue and blocks until its
+ * completion.
+ *
+ * Think twice before calling this function!  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.
+ */
 void flush_scheduled_work(void)
 {
 	flush_workqueue(keventd_wq);

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