commit: eb2834285cf172856cd12f66892fc7467935ebed From: Lai Jiangshan <laijs@xxxxxxxxxxxxxx> Date: Fri, 8 Mar 2013 15:18:28 -0800 Subject: 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@xxxxxxxxxxxxxx> Signed-off-by: Tejun Heo <tj@xxxxxxxxxx> Cc: stable@xxxxxxxxxxxxxxx --- kernel/workqueue.c | 44 +++++++++++++++++++++++++------------------- 1 file changed, 25 insertions(+), 19 deletions(-) diff --git a/kernel/workqueue.c b/kernel/workqueue.c index 81f2457..604801b 100644 --- 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); + } } /* -- 1.7.10.4 -- To unsubscribe from this list: send the line "unsubscribe stable" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html