workqueue: use generic attach/detach routine for rescuers

There are several problems with the code that rescuers use to bind
themselve to the target pool's cpumask.

  1) It is very different from how the normal workers bind to cpumask,
     increasing code complexity and maintenance overhead.

  2) The code of cpu-binding for rescuers is complicated.

  3) If one or more cpu hotplugs happen while a rescuer is processing
     its scheduled work items, the rescuer may not stay bound to the
     cpumask of the pool. This is an allowed behavior, but is still
     hairy. It will be better if the cpumask of the rescuer is always
     kept synchronized with the pool across cpu hotplugs.

Using generic attach/detach routine will solve the above problems and
results in much simpler code.

tj: Minor description updates.
Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

kernel/workqueue.c

@@ -1603,70 +1603,6 @@ static void worker_leave_idle(struct worker *worker)
 	list_del_init(&worker->entry);
 }
 
-/**
- * worker_maybe_bind_and_lock - try to bind %current to worker_pool and lock it
- * @pool: target worker_pool
- *
- * Bind %current to the cpu of @pool if it is associated and lock @pool.
- *
- * Works which are scheduled while the cpu is online must at least be
- * scheduled to a worker which is bound to the cpu so that if they are
- * flushed from cpu callbacks while cpu is going down, they are
- * guaranteed to execute on the cpu.
- *
- * This function is to be used by unbound workers and rescuers to bind
- * themselves to the target cpu and may race with cpu going down or
- * coming online.  kthread_bind() can't be used because it may put the
- * worker to already dead cpu and set_cpus_allowed_ptr() can't be used
- * verbatim as it's best effort and blocking and pool may be
- * [dis]associated in the meantime.
- *
- * This function tries set_cpus_allowed() and locks pool and verifies the
- * binding against %POOL_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 pool->lock
- * held.
- *
- * Return:
- * %true if the associated pool is online (@worker is successfully
- * bound), %false if offline.
- */
-static bool worker_maybe_bind_and_lock(struct worker_pool *pool)
-__acquires(&pool->lock)
-{
-	while (true) {
-		/*
-		 * The following call may fail, succeed or succeed
-		 * without actually migrating the task to the cpu if
-		 * it races with cpu hotunplug operation.  Verify
-		 * against POOL_DISASSOCIATED.
-		 */
-		if (!(pool->flags & POOL_DISASSOCIATED))
-			set_cpus_allowed_ptr(current, pool->attrs->cpumask);
-
-		spin_lock_irq(&pool->lock);
-		if (pool->flags & POOL_DISASSOCIATED)
-			return false;
-		if (task_cpu(current) == pool->cpu &&
-		    cpumask_equal(&current->cpus_allowed, pool->attrs->cpumask))
-			return true;
-		spin_unlock_irq(&pool->lock);
-
-		/*
-		 * We've raced with CPU hot[un]plug.  Give it a breather
-		 * and retry migration.  cond_resched() is required here;
-		 * otherwise, we might deadlock against cpu_stop trying to
-		 * bring down the CPU on non-preemptive kernel.
-		 */
-		cpu_relax();
-		cond_resched();
-	}
-}
-
 static struct worker *alloc_worker(void)
 {
 	struct worker *worker;
@@ -2361,8 +2297,9 @@ static int rescuer_thread(void *__rescuer)
 
 		spin_unlock_irq(&wq_mayday_lock);
 
-		/* migrate to the target cpu if possible */
-		worker_maybe_bind_and_lock(pool);
+		worker_attach_to_pool(rescuer, pool);
+
+		spin_lock_irq(&pool->lock);
 		rescuer->pool = pool;
 
 		/*
@@ -2375,6 +2312,11 @@ static int rescuer_thread(void *__rescuer)
 				move_linked_works(work, scheduled, &n);
 
 		process_scheduled_works(rescuer);
+		spin_unlock_irq(&pool->lock);
+
+		worker_detach_from_pool(rescuer, pool);
+
+		spin_lock_irq(&pool->lock);
 
 		/*
 		 * Put the reference grabbed by send_mayday().  @pool won't