1 files changed, 52 insertions, 4 deletions

diff --git a/kernel/workqueue.c b/kernel/workqueue.c
index f28849394791..41ff75b478c6 100644
--- a/kernel/workqueue.c
+++ b/kernel/workqueue.c
@@ -2728,19 +2728,57 @@ bool flush_work(struct work_struct *work)
 }
 EXPORT_SYMBOL_GPL(flush_work);
 
+struct cwt_wait {
+        wait_queue_t            wait;
+        struct work_struct      *work;
+};
+
+static int cwt_wakefn(wait_queue_t *wait, unsigned mode, int sync, void *key)
+{
+        struct cwt_wait *cwait = container_of(wait, struct cwt_wait, wait);
+
+        if (cwait->work != key)
+                return 0;
+        return autoremove_wake_function(wait, mode, sync, key);
+}
+
 static bool __cancel_work_timer(struct work_struct *work, bool is_dwork)
 {
+        static DECLARE_WAIT_QUEUE_HEAD(cancel_waitq);
         unsigned long flags;
         int ret;
 
         do {
                 ret = try_to_grab_pending(work, is_dwork, &flags);
                 /*
-                 * If someone else is canceling, wait for the same event it
-                 * would be waiting for before retrying.
+                 * If someone else is already canceling, wait for it to
+                 * finish.  flush_work() doesn't work for PREEMPT_NONE
+                 * because we may get scheduled between @work's completion
+                 * and the other canceling task resuming and clearing
+                 * CANCELING - flush_work() will return false immediately
+                 * as @work is no longer busy, try_to_grab_pending() will
+                 * return -ENOENT as @work is still being canceled and the
+                 * other canceling task won't be able to clear CANCELING as
+                 * we're hogging the CPU.
+                 *
+                 * Let's wait for completion using a waitqueue.  As this
+                 * may lead to the thundering herd problem, use a custom
+                 * wake function which matches @work along with exclusive
+                 * wait and wakeup.
                  */
-                if (unlikely(ret == -ENOENT))
-                        flush_work(work);
+                if (unlikely(ret == -ENOENT)) {
+                        struct cwt_wait cwait;
+
+                        init_wait(&cwait.wait);
+                        cwait.wait.func = cwt_wakefn;
+                        cwait.work = work;
+
+                        prepare_to_wait_exclusive(&cancel_waitq, &cwait.wait,
+                                                  TASK_UNINTERRUPTIBLE);
+                        if (work_is_canceling(work))
+                                schedule();
+                        finish_wait(&cancel_waitq, &cwait.wait);
+                }
         } while (unlikely(ret < 0));
 
         /* tell other tasks trying to grab @work to back off */
@@ -2749,6 +2787,16 @@ static bool __cancel_work_timer(struct work_struct *work, bool is_dwork)
 
         flush_work(work);
         clear_work_data(work);
+
+        /*
+         * Paired with prepare_to_wait() above so that either
+         * waitqueue_active() is visible here or !work_is_canceling() is
+         * visible there.
+         */
+        smp_mb();
+        if (waitqueue_active(&cancel_waitq))
+                __wake_up(&cancel_waitq, TASK_NORMAL, 1, work);
+
         return ret;
 }