workqueue: fix possible pool stall bug in wq_unbind_fn()

Since multiple pools per cpu have been introduced, wq_unbind_fn() has
a subtle bug which may theoretically stall work item processing.  The
problem is two-fold.

* wq_unbind_fn() depends on the worker executing wq_unbind_fn() itself
  to start unbound chain execution, which works fine when there was
  only single pool.  With multiple pools, only the pool which is
  running wq_unbind_fn() - the highpri one - is guaranteed to have
  such kick-off.  The other pool could stall when its busy workers
  block.

* The current code is setting WORKER_UNBIND / POOL_DISASSOCIATED of
  the two pools in succession without initiating work execution
  inbetween.  Because setting the flags requires grabbing assoc_mutex
  which is held while new workers are created, this could lead to
  stalls if a pool's manager is waiting for the previous pool's work
  items to release memory.  This is almost purely theoretical tho.

Update wq_unbind_fn() such that it sets WORKER_UNBIND /
POOL_DISASSOCIATED, goes over schedule() and explicitly kicks off
execution for a pool and then moves on to the next one.

tj: Updated comments and description.

Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: stable@vger.kernel.org

diff --git a/kernel/workqueue.c b/kernel/workqueue.c
index 81f2457..604801b 100644 (file)
--- a/kernel/workqueue.c
+++ b/kernel/workqueue.c
@@ -3446,28 +3446,34 @@ static void wq_unbind_fn(struct work_struct *work)
 
                spin_unlock_irq(&pool->lock);
                mutex_unlock(&pool->assoc_mutex);
-       }
 
-       /*
-        * Call schedule() so that we cross rq->lock and thus can guarantee
-        * sched callbacks see the %WORKER_UNBOUND flag.  This is necessary
-        * as scheduler callbacks may be invoked from other cpus.
-        */
-       schedule();
+               /*
+                * Call schedule() so that we cross rq->lock and thus can
+                * guarantee sched callbacks see the %WORKER_UNBOUND flag.
+                * This is necessary as scheduler callbacks may be invoked
+                * from other cpus.
+                */
+               schedule();
 
-       /*
-        * Sched callbacks are disabled now.  Zap nr_running.  After this,
-        * nr_running stays zero and need_more_worker() and keep_working()
-        * are always true as long as the worklist is not empty.  Pools on
-        * @cpu now behave as unbound (in terms of concurrency management)
-        * pools which are served by workers tied to the CPU.
-        *
-        * On return from this function, the current worker would trigger
-        * unbound chain execution of pending work items if other workers
-        * didn't already.
-        */
-       for_each_std_worker_pool(pool, cpu)
+               /*
+                * Sched callbacks are disabled now.  Zap nr_running.
+                * After this, nr_running stays zero and need_more_worker()
+                * and keep_working() are always true as long as the
+                * worklist is not empty.  This pool now behaves as an
+                * unbound (in terms of concurrency management) pool which
+                * are served by workers tied to the pool.
+                */
                atomic_set(&pool->nr_running, 0);
+
+               /*
+                * With concurrency management just turned off, a busy
+                * worker blocking could lead to lengthy stalls.  Kick off
+                * unbound chain execution of currently pending work items.
+                */
+               spin_lock_irq(&pool->lock);
+               wake_up_worker(pool);
+               spin_unlock_irq(&pool->lock);
+       }
 }
 
 /*