workqueue: add missing wmb() in clear_work_data()

Any operation which clears PENDING should be preceded by a wmb to
guarantee that the next PENDING owner sees all the changes made before
PENDING release.

There are only two places where PENDING is cleared -
set_work_cpu_and_clear_pending() and clear_work_data().  The caller of
the former already does smp_wmb() but the latter doesn't have any.

Move the wmb above set_work_cpu_and_clear_pending() into it and add
one to clear_work_data().

There hasn't been any report related to this issue, and, given how
clear_work_data() is used, it is extremely unlikely to have caused any
actual problems on any architecture.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Oleg Nesterov <oleg@redhat.com>

1 files changed, 12 insertions, 7 deletions

diff --git a/kernel/workqueue.c b/kernel/workqueue.c
index 11723c5b2b2..4fef9527a62 100644
--- a/kernel/workqueue.c
+++ b/kernel/workqueue.c
@@ -570,11 +570,19 @@ static void set_work_cwq(struct work_struct *work,
 static void set_work_cpu_and_clear_pending(struct work_struct *work,
                                            unsigned int cpu)
 {
+        /*
+         * The following wmb is paired with the implied mb in
+         * test_and_set_bit(PENDING) and ensures all updates to @work made
+         * here are visible to and precede any updates by the next PENDING
+         * owner.
+         */
+        smp_wmb();
         set_work_data(work, (unsigned long)cpu << WORK_OFFQ_CPU_SHIFT, 0);
 }
 
 static void clear_work_data(struct work_struct *work)
 {
+        smp_wmb();      /* see set_work_cpu_and_clear_pending() */
         set_work_data(work, WORK_STRUCT_NO_CPU, 0);
 }
 
@@ -2182,14 +2190,11 @@ __acquires(&gcwq->lock)
                 wake_up_worker(pool);
 
         /*
-         * Record the last CPU and clear PENDING.  The following wmb is
-         * paired with the implied mb in test_and_set_bit(PENDING) and
-         * ensures all updates to @work made here are visible to and
-         * precede any updates by the next PENDING owner.  Also, clear
-         * PENDING inside @gcwq->lock so that PENDING and queued state
-         * changes happen together while IRQ is disabled.
+         * Record the last CPU and clear PENDING which should be the last
+         * update to @work.  Also, do this inside @gcwq->lock so that
+         * PENDING and queued state changes happen together while IRQ is
+         * disabled.
          */
-        smp_wmb();
         set_work_cpu_and_clear_pending(work, gcwq->cpu);
 
         spin_unlock_irq(&gcwq->lock);