workqueue: separate out drain_workqueue() from destroy_workqueue()

There are users which want to drain workqueues without destroying it.
Separate out drain functionality from destroy_workqueue() into
drain_workqueue() and make it accessible to workqueue users.

To guarantee forward-progress, only chain queueing is allowed while
drain is in progress.  If a new work item which isn't chained from the
running or pending work items is queued while draining is in progress,
WARN_ON_ONCE() is triggered.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: James Bottomley <James.Bottomley@hansenpartnership.com>

1 files changed, 53 insertions, 28 deletions

diff --git a/kernel/workqueue.c b/kernel/workqueue.c
index e3378e8d3a5..25c8afeaeae 100644
--- a/kernel/workqueue.c
+++ b/kernel/workqueue.c
@@ -221,7 +221,7 @@ typedef unsigned long mayday_mask_t;
  * per-CPU workqueues:
  */
 struct workqueue_struct {
-        unsigned int            flags;          /* I: WQ_* flags */
+        unsigned int            flags;          /* W: WQ_* flags */
         union {
                 struct cpu_workqueue_struct __percpu    *pcpu;
                 struct cpu_workqueue_struct             *single;
@@ -240,6 +240,7 @@ struct workqueue_struct {
         mayday_mask_t           mayday_mask;    /* cpus requesting rescue */
         struct worker           *rescuer;       /* I: rescue worker */
 
+        int                     nr_drainers;    /* W: drain in progress */
         int                     saved_max_active; /* W: saved cwq max_active */
         const char              *name;          /* I: workqueue name */
 #ifdef CONFIG_LOCKDEP
@@ -990,7 +991,7 @@ static void __queue_work(unsigned int cpu, struct workqueue_struct *wq,
         debug_work_activate(work);
 
         /* if dying, only works from the same workqueue are allowed */
-        if (unlikely(wq->flags & WQ_DYING) &&
+        if (unlikely(wq->flags & WQ_DRAINING) &&
             WARN_ON_ONCE(!is_chained_work(wq)))
                 return;
 
@@ -2381,6 +2382,54 @@ out_unlock:
 }
 EXPORT_SYMBOL_GPL(flush_workqueue);
 
+/**
+ * drain_workqueue - drain a workqueue
+ * @wq: workqueue to drain
+ *
+ * Wait until the workqueue becomes empty.  While draining is in progress,
+ * only chain queueing is allowed.  IOW, only currently pending or running
+ * work items on @wq can queue further work items on it.  @wq is flushed
+ * repeatedly until it becomes empty.  The number of flushing is detemined
+ * by the depth of chaining and should be relatively short.  Whine if it
+ * takes too long.
+ */
+void drain_workqueue(struct workqueue_struct *wq)
+{
+        unsigned int flush_cnt = 0;
+        unsigned int cpu;
+
+        /*
+         * __queue_work() needs to test whether there are drainers, is much
+         * hotter than drain_workqueue() and already looks at @wq->flags.
+         * Use WQ_DRAINING so that queue doesn't have to check nr_drainers.
+         */
+        spin_lock(&workqueue_lock);
+        if (!wq->nr_drainers++)
+                wq->flags |= WQ_DRAINING;
+        spin_unlock(&workqueue_lock);
+reflush:
+        flush_workqueue(wq);
+
+        for_each_cwq_cpu(cpu, wq) {
+                struct cpu_workqueue_struct *cwq = get_cwq(cpu, wq);
+
+                if (!cwq->nr_active && list_empty(&cwq->delayed_works))
+                        continue;
+
+                if (++flush_cnt == 10 ||
+                    (flush_cnt % 100 == 0 && flush_cnt <= 1000))
+                        pr_warning("workqueue %s: flush on destruction isn't complete after %u tries\n",
+                                   wq->name, flush_cnt);
+                goto reflush;
+        }
+
+        spin_lock(&workqueue_lock);
+        if (!--wq->nr_drainers)
+                wq->flags &= ~WQ_DRAINING;
+        spin_unlock(&workqueue_lock);
+}
+EXPORT_SYMBOL_GPL(drain_workqueue);
+
 static bool start_flush_work(struct work_struct *work, struct wq_barrier *barr,
                              bool wait_executing)
 {
@@ -3011,34 +3060,10 @@ EXPORT_SYMBOL_GPL(__alloc_workqueue_key);
  */
 void destroy_workqueue(struct workqueue_struct *wq)
 {
-        unsigned int flush_cnt = 0;
         unsigned int cpu;
 
-        /*
-         * Mark @wq dying and drain all pending works.  Once WQ_DYING is
-         * set, only chain queueing is allowed.  IOW, only currently
-         * pending or running work items on @wq can queue further work
-         * items on it.  @wq is flushed repeatedly until it becomes empty.
-         * The number of flushing is detemined by the depth of chaining and
-         * should be relatively short.  Whine if it takes too long.
-         */
-        wq->flags |= WQ_DYING;
-reflush:
-        flush_workqueue(wq);
-
-        for_each_cwq_cpu(cpu, wq) {
-                struct cpu_workqueue_struct *cwq = get_cwq(cpu, wq);
-
-                if (!cwq->nr_active && list_empty(&cwq->delayed_works))
-                        continue;
-
-                if (++flush_cnt == 10 ||
-                    (flush_cnt % 100 == 0 && flush_cnt <= 1000))
-                        printk(KERN_WARNING "workqueue %s: flush on "
-                               "destruction isn't complete after %u tries\n",
-                               wq->name, flush_cnt);
-                goto reflush;
-        }
+        /* drain it before proceeding with destruction */
+        drain_workqueue(wq);
 
         /*
          * wq list is used to freeze wq, remove from list after