1 files changed, 29 insertions, 0 deletions

diff --git a/kernel/workqueue.c b/kernel/workqueue.c
index 2232ae3e3ad6..3bfdff06eea7 100644
--- a/kernel/workqueue.c
+++ b/kernel/workqueue.c
@@ -666,6 +666,35 @@ static void set_work_pool_and_clear_pending(struct work_struct *work,
          */
         smp_wmb();
         set_work_data(work, (unsigned long)pool_id << WORK_OFFQ_POOL_SHIFT, 0);
+        /*
+         * The following mb guarantees that previous clear of a PENDING bit
+         * will not be reordered with any speculative LOADS or STORES from
+         * work->current_func, which is executed afterwards.  This possible
+         * reordering can lead to a missed execution on attempt to qeueue
+         * the same @work.  E.g. consider this case:
+         *
+         *   CPU#0                         CPU#1
+         *   ----------------------------  --------------------------------
+         *
+         * 1  STORE event_indicated
+         * 2  queue_work_on() {
+         * 3    test_and_set_bit(PENDING)
+         * 4 }                             set_..._and_clear_pending() {
+         * 5                                 set_work_data() # clear bit
+         * 6                                 smp_mb()
+         * 7                               work->current_func() {
+         * 8                                  LOAD event_indicated
+         *                                 }
+         *
+         * Without an explicit full barrier speculative LOAD on line 8 can
+         * be executed before CPU#0 does STORE on line 1.  If that happens,
+         * CPU#0 observes the PENDING bit is still set and new execution of
+         * a @work is not queued in a hope, that CPU#1 will eventually
+         * finish the queued @work.  Meanwhile CPU#1 does not see
+         * event_indicated is set, because speculative LOAD was executed
+         * before actual STORE.
+         */
+        smp_mb();
 }
 
 static void clear_work_data(struct work_struct *work)