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workqueue: fix possible stall on try_to_grab_pending() of a delayed work item 

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>
Cc: stable@kernel.org
This commit is contained in:
Lai Jiangshan 2012-09-18 10:40:00 -07:00 committed by Tejun Heo
parent a5b4e57d7c
commit 3aa6249759
1 changed files with 22 additions and 3 deletions
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25
kernel/workqueue.c
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@ -977,10 +977,9 @@ static void move_linked_works(struct work_struct *work, struct list_head *head,
*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);
trace_workqueue_activate_work(work);
move_linked_works(work, &cwq->pool->worklist, NULL);
@ -988,6 +987,14 @@ static void cwq_activate_first_delayed(struct cpu_workqueue_struct *cwq)
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
@ -1106,6 +1113,18 @@ static int try_to_grab_pending(struct work_struct *work, bool is_dwork,
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),