[PATCH] workqueue: Prevent a new work item from queueing into a destruction wq

[Richard Clark]

Currently the __WQ_DRAINING is used to prevent a new work item from queueing
to a draining workqueue, but this flag will be cleared before the end of a
RCU grace period. Because the workqueue instance is actually freed after
the RCU grace period, this fact results in an opening window in which a new
work item can be queued into a destorying workqueue and be scheduled
consequently, for instance, the below code snippet demos this accident:

	static void work_func(struct work_struct *work)
	{
		pr_info("%s scheduled\n", __func__);
	}
	static DECLARE_WORK(w0, work_func);

	struct workqueue_struct * wq0 = alloc_workqueue("wq0", 0, 0);
	destroy_workqueue(wq0);
	queue_work_on(1, wq0, &w0);

The above work_func(...) can be scheduled by a destroying workqueue.

This patch will close that window by introducing a new flag __WQ_DESTROYING,
which will keep valid until the end of a RCU grace period. With this commit,
the above code will trigger a WARNING message and return directly from
__queue_work(...), like this:

WARNING: CPU: 7 PID: 3994 at kernel/workqueue.c:1438 __queue_work+0x3ec/0x580

Signed-off-by: Richard Clark <richard.xnu.clark@xxxxxxxxx>
---
 include/linux/workqueue.h |  1 +
 kernel/workqueue.c        | 11 ++++++++---
 2 files changed, 9 insertions(+), 3 deletions(-)

diff --git a/include/linux/workqueue.h b/include/linux/workqueue.h
index a0143dd24430..ac551b8ee7d9 100644
--- a/include/linux/workqueue.h
+++ b/include/linux/workqueue.h
@@ -335,6 +335,7 @@ enum {
 	 */
 	WQ_POWER_EFFICIENT	= 1 << 7,
 
+	__WQ_DESTROYING		= 1 << 15, /* internal: workqueue is destroying */
 	__WQ_DRAINING		= 1 << 16, /* internal: workqueue is draining */
 	__WQ_ORDERED		= 1 << 17, /* internal: workqueue is ordered */
 	__WQ_LEGACY		= 1 << 18, /* internal: create*_workqueue() */
diff --git a/kernel/workqueue.c b/kernel/workqueue.c
index 39060a5d0905..09527c9db9cb 100644
--- a/kernel/workqueue.c
+++ b/kernel/workqueue.c
@@ -1433,9 +1433,9 @@ static void __queue_work(int cpu, struct workqueue_struct *wq,
 	lockdep_assert_irqs_disabled();
 
 
-	/* if draining, only works from the same workqueue are allowed */
-	if (unlikely(wq->flags & __WQ_DRAINING) &&
-	    WARN_ON_ONCE(!is_chained_work(wq)))
+	/* if destroying or draining, only works from the same workqueue are allowed */
+	if (unlikely(wq->flags & (__WQ_DESTROYING | __WQ_DRAINING)
+		&& WARN_ON_ONCE(!is_chained_work(wq))))
 		return;
 	rcu_read_lock();
 retry:
@@ -4414,6 +4414,11 @@ void destroy_workqueue(struct workqueue_struct *wq)
 	 */
 	workqueue_sysfs_unregister(wq);
 
+	/* mark the workqueue destruction is in progress */
+	mutex_lock(&wq->mutex);
+	wq->flags |= __WQ_DESTROYING;
+	mutex_unlock(&wq->mutex);
+
 	/* drain it before proceeding with destruction */
 	drain_workqueue(wq);
 
-- 
2.37.2