workqueue: drop CPU_DYING notifier operation

Workqueue used CPU_DYING notification to mark GCWQ_DISASSOCIATED.
This was necessary because workqueue's CPU_DOWN_PREPARE happened
before other DOWN_PREPARE notifiers and workqueue needed to stay
associated across the rest of DOWN_PREPARE.

After the previous patch, workqueue's DOWN_PREPARE happens after
others and can set GCWQ_DISASSOCIATED directly.  Drop CPU_DYING and
let the trustee set GCWQ_DISASSOCIATED after disabling concurrency
management.
Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: "Rafael J. Wysocki" <rjw@sisk.pl>

kernel/workqueue.c

@@ -1250,11 +1250,11 @@ static void worker_leave_idle(struct worker *worker)
  * verbatim as it's best effort and blocking and gcwq may be
  * [dis]associated in the meantime.
  *
- * This function tries set_cpus_allowed() and locks gcwq and verifies
- * the binding against GCWQ_DISASSOCIATED which is set during
- * CPU_DYING and cleared during CPU_ONLINE, so if the worker enters
- * idle state or fetches works without dropping lock, it can guarantee
- * the scheduling requirement described in the first paragraph.
+ * This function tries set_cpus_allowed() and locks gcwq and verifies the
+ * binding against %GCWQ_DISASSOCIATED which is set during
+ * %CPU_DOWN_PREPARE and cleared during %CPU_ONLINE, so if the worker
+ * enters idle state or fetches works without dropping lock, it can
+ * guarantee the scheduling requirement described in the first paragraph.
  *
  * CONTEXT:
  * Might sleep.  Called without any lock but returns with gcwq->lock
@@ -3349,6 +3349,12 @@ static int __cpuinit trustee_thread(void *__gcwq)
 	rc = trustee_wait_event(!gcwq_is_managing_workers(gcwq));
 	BUG_ON(rc < 0);
 
+	/*
+	 * We've claimed all manager positions.  Make all workers unbound
+	 * and set DISASSOCIATED.  Before this, all workers except for the
+	 * ones which are still executing works from before the last CPU
+	 * down must be on the cpu.  After this, they may become diasporas.
+	 */
 	for_each_worker_pool(pool, gcwq) {
 		pool->flags |= POOL_MANAGING_WORKERS;
 
@@ -3359,6 +3365,8 @@ static int __cpuinit trustee_thread(void *__gcwq)
 	for_each_busy_worker(worker, i, pos, gcwq)
 		worker->flags |= WORKER_ROGUE;
 
+	gcwq->flags |= GCWQ_DISASSOCIATED;
+
 	/*
 	 * Call schedule() so that we cross rq->lock and thus can
 	 * guarantee sched callbacks see the rogue flag.  This is
@@ -3582,16 +3590,6 @@ static int __devinit workqueue_cpu_callback(struct notifier_block *nfb,
 		}
 		break;
 
-	case CPU_DYING:
-		/*
-		 * Before this, the trustee and all workers except for
-		 * the ones which are still executing works from
-		 * before the last CPU down must be on the cpu.  After
-		 * this, they'll all be diasporas.
-		 */
-		gcwq->flags |= GCWQ_DISASSOCIATED;
-		break;
-
 	case CPU_POST_DEAD:
 		gcwq->trustee_state = TRUSTEE_BUTCHER;
 		/* fall through */
@@ -3672,7 +3670,6 @@ static int __devinit workqueue_cpu_down_callback(struct notifier_block *nfb,
 {
 	switch (action & ~CPU_TASKS_FROZEN) {
 	case CPU_DOWN_PREPARE:
-	case CPU_DYING:
 	case CPU_POST_DEAD:
 		return workqueue_cpu_callback(nfb, action, hcpu);
 	}