[PATCH -stable] workqueue: UNBOUND -> REBIND morphing inrebind_workers() should be atomic

[Tejun Heo]

From: Lai Jiangshan <laijs@xxxxxxxxxxxxxx>

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.

stable: Idle worker rebinding doesn't apply for -stable and
        WORKER_UNBOUND used to be WORKER_ROGUE.  Updated accordingly.

Signed-off-by: Lai Jiangshan <laijs@xxxxxxxxxxxxxx>
Signed-off-by: Tejun Heo <tj@xxxxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx
---
 kernel/workqueue.c |    9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/kernel/workqueue.c b/kernel/workqueue.c
index 9a3128d..d7eb05a 100644
--- a/kernel/workqueue.c
+++ b/kernel/workqueue.c
@@ -3441,14 +3441,17 @@ static int __cpuinit trustee_thread(void *__gcwq)
 
 	for_each_busy_worker(worker, i, pos, gcwq) {
 		struct work_struct *rebind_work = &worker->rebind_work;
+		unsigned long worker_flags = worker->flags;
 
 		/*
 		 * Rebind_work may race with future cpu hotplug
 		 * operations.  Use a separate flag to mark that
-		 * rebinding is scheduled.
+		 * rebinding is scheduled.  The morphing should
+		 * be atomic.
 		 */
-		worker->flags |= WORKER_REBIND;
-		worker->flags &= ~WORKER_ROGUE;
+		worker_flags |= WORKER_REBIND;
+		worker_flags &= ~WORKER_ROGUE;
+		ACCESS_ONCE(worker->flags) = worker_flags;
 
 		/* queue rebind_work, wq doesn't matter, use the default one */
 		if (test_and_set_bit(WORK_STRUCT_PENDING_BIT,
--
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