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>

3 files changed, 32 insertions, 60 deletions

diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
index d5be1693ac93..d67989b8ba44 100644
--- a/fs/fs-writeback.c
+++ b/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)
diff --git a/include/linux/backing-dev.h b/include/linux/backing-dev.h
index e9aec0d099df..50f146146169 100644
--- a/include/linux/backing-dev.h
+++ b/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;
 
diff --git a/mm/backing-dev.c b/mm/backing-dev.c
index 123bcef13e51..6c2a09c8922c 100644
--- a/mm/backing-dev.c
+++ b/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++;
-        }
+        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);