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

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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
This commit is contained in:
Lai Jiangshan 2012-09-02 00:28:19 +08:00 committed by Tejun Heo
parent 0d7614f09c
commit 96e65306b8
1 changed files with 11 additions and 6 deletions
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17
kernel/workqueue.c
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@ -1396,12 +1396,15 @@ retry:
/* 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 @@ retry:
/* 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)))