writeback: remove wb_list

The wb_list member of struct backing_device_info always has exactly one
element.  Just use the direct bdi->wb pointer instead and simplify some
code.

Also remove bdi_task_init which is now trivial to prepare for the next
patch.
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <jaxboe@fusionio.com>

fs/fs-writeback.c

@@ -73,9 +73,9 @@ static void bdi_queue_work(struct backing_dev_info *bdi,
 	 * If the default thread isn't there, make sure we add it. When
 	 * it gets created and wakes up, we'll run this work.
 	 */
-	if (unlikely(list_empty_careful(&bdi->wb_list)))
+	if (unlikely(!bdi->wb.task)) {
 		wake_up_process(default_backing_dev_info.wb.task);
-	else {
+	} else {
 		struct bdi_writeback *wb = &bdi->wb;
 
 		if (wb->task)

include/linux/backing-dev.h

@@ -45,8 +45,6 @@ enum bdi_stat_item {
 #define BDI_STAT_BATCH (8*(1+ilog2(nr_cpu_ids)))
 
 struct bdi_writeback {
-	struct list_head list;			/* hangs off the bdi */
-
 	struct backing_dev_info *bdi;		/* our parent bdi */
 	unsigned int nr;
 
@@ -80,8 +78,7 @@ struct backing_dev_info {
 	unsigned int max_ratio, max_prop_frac;
 
 	struct bdi_writeback wb;  /* default writeback info for this bdi */
-	spinlock_t wb_lock;	  /* protects update side of wb_list */
-	struct list_head wb_list; /* the flusher threads hanging off this bdi */
+	spinlock_t wb_lock;	  /* protects work_list */
 
 	struct list_head work_list;
 

mm/backing-dev.c

@@ -65,28 +65,21 @@ static void bdi_debug_init(void)
 static int bdi_debug_stats_show(struct seq_file *m, void *v)
 {
 	struct backing_dev_info *bdi = m->private;
-	struct bdi_writeback *wb;
+	struct bdi_writeback *wb = &bdi->wb;
 	unsigned long background_thresh;
 	unsigned long dirty_thresh;
 	unsigned long bdi_thresh;
 	unsigned long nr_dirty, nr_io, nr_more_io, nr_wb;
 	struct inode *inode;
 
-	/*
-	 * inode lock is enough here, the bdi->wb_list is protected by
-	 * RCU on the reader side
-	 */
 	nr_wb = nr_dirty = nr_io = nr_more_io = 0;
 	spin_lock(&inode_lock);
-	list_for_each_entry(wb, &bdi->wb_list, list) {
-		nr_wb++;
 	list_for_each_entry(inode, &wb->b_dirty, i_list)
 		nr_dirty++;
 	list_for_each_entry(inode, &wb->b_io, i_list)
 		nr_io++;
 	list_for_each_entry(inode, &wb->b_more_io, i_list)
 		nr_more_io++;
-	}
 	spin_unlock(&inode_lock);
 
 	get_dirty_limits(&background_thresh, &dirty_thresh, &bdi_thresh, bdi);
@@ -98,19 +91,16 @@ static int bdi_debug_stats_show(struct seq_file *m, void *v)
 		   "BdiDirtyThresh:   %8lu kB\n"
 		   "DirtyThresh:      %8lu kB\n"
 		   "BackgroundThresh: %8lu kB\n"
-		   "WritebackThreads: %8lu\n"
 		   "b_dirty:          %8lu\n"
 		   "b_io:             %8lu\n"
 		   "b_more_io:        %8lu\n"
 		   "bdi_list:         %8u\n"
-		   "state:            %8lx\n"
-		   "wb_list:          %8u\n",
+		   "state:            %8lx\n",
 		   (unsigned long) K(bdi_stat(bdi, BDI_WRITEBACK)),
 		   (unsigned long) K(bdi_stat(bdi, BDI_RECLAIMABLE)),
 		   K(bdi_thresh), K(dirty_thresh),
-		   K(background_thresh), nr_wb, nr_dirty, nr_io, nr_more_io,
-		   !list_empty(&bdi->bdi_list), bdi->state,
-		   !list_empty(&bdi->wb_list));
+		   K(background_thresh), nr_dirty, nr_io, nr_more_io,
+		   !list_empty(&bdi->bdi_list), bdi->state);
 #undef K
 
 	return 0;
@@ -270,24 +260,6 @@ static void bdi_wb_init(struct bdi_writeback *wb, struct backing_dev_info *bdi)
 	INIT_LIST_HEAD(&wb->b_more_io);
 }
 
-static void bdi_task_init(struct backing_dev_info *bdi,
-			  struct bdi_writeback *wb)
-{
-	struct task_struct *tsk = current;
-
-	spin_lock(&bdi->wb_lock);
-	list_add_tail_rcu(&wb->list, &bdi->wb_list);
-	spin_unlock(&bdi->wb_lock);
-
-	tsk->flags |= PF_FLUSHER | PF_SWAPWRITE;
-	set_freezable();
-
-	/*
-	 * Our parent may run at a different priority, just set us to normal
-	 */
-	set_user_nice(tsk, 0);
-}
-
 static int bdi_start_fn(void *ptr)
 {
 	struct bdi_writeback *wb = ptr;
@@ -301,7 +273,13 @@ static int bdi_start_fn(void *ptr)
 	list_add_rcu(&bdi->bdi_list, &bdi_list);
 	spin_unlock_bh(&bdi_lock);
 
-	bdi_task_init(bdi, wb);
+	current->flags |= PF_FLUSHER | PF_SWAPWRITE;
+	set_freezable();
+
+	/*
+	 * Our parent may run at a different priority, just set us to normal
+	 */
+	set_user_nice(current, 0);
 
 	/*
 	 * Clear pending bit and wakeup anybody waiting to tear us down
@@ -312,12 +290,7 @@ static int bdi_start_fn(void *ptr)
 
 	ret = bdi_writeback_task(wb);
 
-	/*
-	 * Remove us from the list
-	 */
-	spin_lock(&bdi->wb_lock);
-	list_del_rcu(&wb->list);
-	spin_unlock(&bdi->wb_lock);
+	wb->task = NULL;
 
 	/*
 	 * Flush any work that raced with us exiting. No new work
@@ -326,7 +299,6 @@ static int bdi_start_fn(void *ptr)
 	if (!list_empty(&bdi->work_list))
 		wb_do_writeback(wb, 1);
 
-	wb->task = NULL;
 	return ret;
 }
 
@@ -391,7 +363,13 @@ static int bdi_forker_task(void *ptr)
 {
 	struct bdi_writeback *me = ptr;
 
-	bdi_task_init(me->bdi, me);
+	current->flags |= PF_FLUSHER | PF_SWAPWRITE;
+	set_freezable();
+
+	/*
+	 * Our parent may run at a different priority, just set us to normal
+	 */
+	set_user_nice(current, 0);
 
 	for (;;) {
 		struct backing_dev_info *bdi, *tmp;
@@ -598,8 +576,6 @@ EXPORT_SYMBOL(bdi_register_dev);
  */
 static void bdi_wb_shutdown(struct backing_dev_info *bdi)
 {
-	struct bdi_writeback *wb;
-
 	if (!bdi_cap_writeback_dirty(bdi))
 		return;
 
@@ -615,14 +591,14 @@ static void bdi_wb_shutdown(struct backing_dev_info *bdi)
 	bdi_remove_from_list(bdi);
 
 	/*
-	 * Finally, kill the kernel threads. We don't need to be RCU
+	 * Finally, kill the kernel thread. We don't need to be RCU
 	 * safe anymore, since the bdi is gone from visibility. Force
 	 * unfreeze of the thread before calling kthread_stop(), otherwise
 	 * it would never exet if it is currently stuck in the refrigerator.
 	 */
-	list_for_each_entry(wb, &bdi->wb_list, list) {
-		thaw_process(wb->task);
-		kthread_stop(wb->task);
+	if (bdi->wb.task) {
+		thaw_process(bdi->wb.task);
+		kthread_stop(bdi->wb.task);
 	}
 }
 
@@ -667,7 +643,6 @@ int bdi_init(struct backing_dev_info *bdi)
 	spin_lock_init(&bdi->wb_lock);
 	INIT_RCU_HEAD(&bdi->rcu_head);
 	INIT_LIST_HEAD(&bdi->bdi_list);
-	INIT_LIST_HEAD(&bdi->wb_list);
 	INIT_LIST_HEAD(&bdi->work_list);
 
 	bdi_wb_init(&bdi->wb, bdi);