[PATCH 7/9] workqueue: Move pwq_dec_nr_in_flight() to the end of work item handling

[Tejun Heo]

The planned shared nr_active handling for unbound workqueues will make
pwq_dec_nr_active() sometimes drop the pool lock temporarily to acquire
other pool locks, which is necessary as retirement of an nr_active count
from one pool may need kick off an inactive work item in another pool.

This patch moves pwq_dec_nr_in_flight() call in try_to_grab_pending() to the
end of work item handling so that work item state changes stay atomic.
process_one_work() which is the other user of pwq_dec_nr_in_flight() already
calls it at the end of work item handling. Comments are added to both call
sites and pwq_dec_nr_in_flight().

This shouldn't cause any behavior changes.

Signed-off-by: Tejun Heo <tj@xxxxxxxxxx>
---
 kernel/workqueue.c | 11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/kernel/workqueue.c b/kernel/workqueue.c
index 3f45baa96d51..522ef63088de 100644
--- a/kernel/workqueue.c
+++ b/kernel/workqueue.c
@@ -1566,6 +1566,11 @@ static void pwq_dec_nr_active(struct pool_workqueue *pwq)
  * A work either has completed or is removed from pending queue,
  * decrement nr_in_flight of its pwq and handle workqueue flushing.
  *
+ * NOTE:
+ * For unbound workqueues, this function may temporarily drop @pwq->pool->lock
+ * and thus should be called after all other state updates for the in-flight
+ * work item is complete.
+ *
  * CONTEXT:
  * raw_spin_lock_irq(pool->lock).
  */
@@ -1690,11 +1695,13 @@ static int try_to_grab_pending(struct work_struct *work, bool is_dwork,
 		pwq_activate_work(pwq, work);
 
 		list_del_init(&work->entry);
-		pwq_dec_nr_in_flight(pwq, *work_data_bits(work));
 
 		/* work->data points to pwq iff queued, point to pool */
 		set_work_pool_and_keep_pending(work, pool->id);
 
+		/* must be the last step, see the function comment */
+		pwq_dec_nr_in_flight(pwq, *work_data_bits(work));
+
 		raw_spin_unlock(&pool->lock);
 		rcu_read_unlock();
 		return 1;
@@ -2759,6 +2766,8 @@ __acquires(&pool->lock)
 	worker->current_func = NULL;
 	worker->current_pwq = NULL;
 	worker->current_color = INT_MAX;
+
+	/* must be the last step, see the function comment */
 	pwq_dec_nr_in_flight(pwq, work_data);
 }
 
-- 
2.43.0