workqueue: UNBOUND -> REBIND morphing in rebind_workers() should be atomic
The compiler may compile the following code into TWO write/modify
instructions.
worker->flags &= ~WORKER_UNBOUND;
worker->flags |= WORKER_REBIND;
so the other CPU may temporarily see worker->flags which doesn't have
either WORKER_UNBOUND or WORKER_REBIND set and perform local wakeup
prematurely.
Fix it by using single explicit assignment via ACCESS_ONCE().
Because idle workers have another WORKER_NOT_RUNNING flag, this bug
doesn't exist for them; however, update it to use the same pattern for
consistency.
tj: Applied the change to idle workers too and updated comments and
patch description a bit.
Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: stable@vger.kernel.org
diff --git a/kernel/workqueue.c b/kernel/workqueue.c
index 692d976..c462cd6 100644
--- a/kernel/workqueue.c
+++ b/kernel/workqueue.c
@@ -1396,12 +1396,15 @@
/* set REBIND and kick idle ones, we'll wait for these later */
for_each_worker_pool(pool, gcwq) {
list_for_each_entry(worker, &pool->idle_list, entry) {
+ unsigned long worker_flags = worker->flags;
+
if (worker->flags & WORKER_REBIND)
continue;
- /* morph UNBOUND to REBIND */
- worker->flags &= ~WORKER_UNBOUND;
- worker->flags |= WORKER_REBIND;
+ /* morph UNBOUND to REBIND atomically */
+ worker_flags &= ~WORKER_UNBOUND;
+ worker_flags |= WORKER_REBIND;
+ ACCESS_ONCE(worker->flags) = worker_flags;
idle_rebind.cnt++;
worker->idle_rebind = &idle_rebind;
@@ -1434,10 +1437,12 @@
/* rebind busy workers */
for_each_busy_worker(worker, i, pos, gcwq) {
struct work_struct *rebind_work = &worker->rebind_work;
+ unsigned long worker_flags = worker->flags;
- /* morph UNBOUND to REBIND */
- worker->flags &= ~WORKER_UNBOUND;
- worker->flags |= WORKER_REBIND;
+ /* morph UNBOUND to REBIND atomically */
+ worker_flags &= ~WORKER_UNBOUND;
+ worker_flags |= WORKER_REBIND;
+ ACCESS_ONCE(worker->flags) = worker_flags;
if (test_and_set_bit(WORK_STRUCT_PENDING_BIT,
work_data_bits(rebind_work)))