[ 032/147] workqueue: fix possible stall on try_to_grab_pending() of a delayed work item

Ben Hutchings
2012-10-14T14:48:03Z

From: Lai Jiangshan <laijs@cn.fujitsu.com>

commit 3aa62497594430ea522050b75c033f71f2c60ee6 upstream.

Currently, when try_to_grab_pending() grabs a delayed work item, it
leaves its linked work items alone on the delayed_works. The linked
work items are always NO_COLOR and will cause future
cwq_activate_first_delayed() increase cwq->nr_active incorrectly, and
may cause the whole cwq to stall. For example,

state: cwq->max_active = 1, cwq->nr_active = 1
one work in cwq->pool, many in cwq->delayed_works.

step1: try_to_grab_pending() removes a work item from delayed_works
but leaves its NO_COLOR linked work items on it.

step2: Later on, cwq_activate_first_delayed() activates the linked
work item increasing ->nr_active.

step3: cwq->nr_active = 1, but all activated work items of the cwq are
NO_COLOR. When they finish, cwq->nr_active will not be
decreased due to NO_COLOR, and no further work items will be
activated from cwq->delayed_works. the cwq stalls.

Fix it by ensuring the target work item is activated before stealing
PENDING in try_to_grab_pending(). This ensures that all the linked
work items are activated without incorrectly bumping cwq->nr_active.

tj: Updated comment and description.

Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
[bwh: Backported to 3.2: adjust context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
---
kernel/workqueue.c | 25 ++++++++++++++++++++++---
1 file changed, 22 insertions(+), 3 deletions(-)

--- a/kernel/workqueue.c
+++ b/kernel/workqueue.c
@@ -1726,10 +1726,9 @@ static void move_linked_works(struct wor
*nextp = n;
}

-static void cwq_activate_first_delayed(struct cpu_workqueue_struct *cwq)
+static void cwq_activate_delayed_work(struct work_struct *work)
{
- struct work_struct *work = list_first_entry(&cwq->delayed_works,
- struct work_struct, entry);
+ struct cpu_workqueue_struct *cwq = get_work_cwq(work);
struct list_head *pos = gcwq_determine_ins_pos(cwq->gcwq, cwq);

trace_workqueue_activate_work(work);
@@ -1738,6 +1737,14 @@ static void cwq_activate_first_delayed(s
cwq->nr_active++;
}

+static void cwq_activate_first_delayed(struct cpu_workqueue_struct *cwq)
+{
+ struct work_struct *work = list_first_entry(&cwq->delayed_works,
+ struct work_struct, entry);
+
+ cwq_activate_delayed_work(work);
+}
+
/**
* cwq_dec_nr_in_flight - decrement cwq's nr_in_flight
* @cwq: cwq of interest
@@ -2628,6 +2635,18 @@ static int try_to_grab_pending(struct wo
smp_rmb();
if (gcwq == get_work_gcwq(work)) {
debug_work_deactivate(work);
+
+ /*
+ * A delayed work item cannot be grabbed directly
+ * because it might have linked NO_COLOR work items
+ * which, if left on the delayed_list, will confuse
+ * cwq->nr_active management later on and cause
+ * stall. Make sure the work item is activated
+ * before grabbing.
+ */
+ if (*work_data_bits(work) & WORK_STRUCT_DELAYED)
+ cwq_activate_delayed_work(work);
+
list_del_init(&work->entry);
cwq_dec_nr_in_flight(get_work_cwq(work),
get_work_color(work),