workqueue: async worker destruction

worker destruction includes these parts of code: adjust pool's stats remove the worker from idle list detach the worker from the pool kthread_stop() to wait for the worker's task exit free the worker struct We can find out that there is no essential work to do after kthread_stop(), which means destroy_worker() doesn't need to wait for the worker's task exit, so we can remove kthread_stop() and free the worker struct in the worker exiting path. However, put_unbound_pool() still needs to sync the all the workers' destruction before destroying the pool; otherwise, the workers may access to the invalid pool when they are exiting. So we also move the code of "detach the worker" to the exiting path and let put_unbound_pool() to sync with this code via detach_completion. The code of "detach the worker" is wrapped in a new function "worker_detach_from_pool()" although worker_detach_from_pool() is only called once (in worker_thread()) after this patch, but we need to wrap it for these reasons: 1) The code of "detach the worker" is not short enough to unfold them in worker_thread(). 2) the name of "worker_detach_from_pool()" is self-comment, and we add some comments above the function. 3) it will be shared by rescuer in later patch which allows rescuer and normal thread use the same attach/detach frameworks. The worker id is freed when detaching which happens before the worker is fully dead, but this id of the dying worker may be re-used for a new worker, so the dying worker's task name is changed to "worker/dying" to avoid two or several workers having the same name. Since "detach the worker" is moved out from destroy_worker(), destroy_worker() doesn't require manager_mutex, so the "lockdep_assert_held(&pool->manager_mutex)" in destroy_worker() is removed, and destroy_worker() is not protected by manager_mutex in put_unbound_pool(). tj: Minor description updates.

LJLai Jiangshan committed 12 years ago