+ struct backing_dev_info *bdi = inode_to_bdi(isw->inodes[0]);
+ struct bdi_writeback *old_wb = isw->inodes[0]->i_wb;
+ struct bdi_writeback *new_wb = isw->new_wb;
+ unsigned long nr_switched = 0;
+ struct inode **inodep;
+
+ /*
+ * If @inode switches cgwb membership while sync_inodes_sb() is
+ * being issued, sync_inodes_sb() might miss it. Synchronize.
+ */
+ down_read(&bdi->wb_switch_rwsem);
+
+ /*
+ * By the time control reaches here, RCU grace period has passed
+ * since I_WB_SWITCH assertion and all wb stat update transactions
+ * between unlocked_inode_to_wb_begin/end() are guaranteed to be
+ * synchronizing against the i_pages lock.
+ *
+ * Grabbing old_wb->list_lock, inode->i_lock and the i_pages lock
+ * gives us exclusion against all wb related operations on @inode
+ * including IO list manipulations and stat updates.
+ */
+ if (old_wb < new_wb) {
+ spin_lock(&old_wb->list_lock);
+ spin_lock_nested(&new_wb->list_lock, SINGLE_DEPTH_NESTING);
+ } else {
+ spin_lock(&new_wb->list_lock);
+ spin_lock_nested(&old_wb->list_lock, SINGLE_DEPTH_NESTING);
+ }
+
+ for (inodep = isw->inodes; *inodep; inodep++) {
+ WARN_ON_ONCE((*inodep)->i_wb != old_wb);
+ if (inode_do_switch_wbs(*inodep, old_wb, new_wb))
+ nr_switched++;
+ }
+
+ spin_unlock(&new_wb->list_lock);
+ spin_unlock(&old_wb->list_lock);
+
+ up_read(&bdi->wb_switch_rwsem);
+
+ if (nr_switched) {
+ wb_wakeup(new_wb);
+ wb_put_many(old_wb, nr_switched);
+ }