From 96e65306b81351b656835c15931d1d237b252f27 Mon Sep 17 00:00:00 2001 From: Lai Jiangshan Date: Sun, 2 Sep 2012 00:28:19 +0800 Subject: 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 Signed-off-by: Tejun Heo 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 @@ 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))) -- cgit v0.10.2