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workqueue: UNBOUND -> REBIND morphing in rebind_workers() should be atomic

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commit 96e65306b8 upstream.

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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
This commit is contained in:
Lai Jiangshan
2012-09-02 00:28:19 +08:00
committed by Greg Kroah-Hartman
parent e2471ec3e8
commit bc713e264e
1 changed files with 6 additions and 3 deletions
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9
kernel/workqueue.c
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@@ -3412,14 +3412,17 @@ 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,