Btrfs: fix possible empty list access when flushing the delalloc inodes

We didn't have a lock to protect the access to the delalloc inodes list, that is
we might access a empty delalloc inodes list if someone start flushing delalloc
inodes because the delalloc inodes were moved into a other list temporarily.
Fix it by wrapping the access with a lock.
Signed-off-by: Miao Xie <miaox@cn.fujitsu.com>
Signed-off-by: Josef Bacik <jbacik@fb.com>

fs/btrfs/ctree.h

@@ -1490,6 +1490,7 @@ struct btrfs_fs_info {
 	 */
 	struct list_head ordered_roots;
 
+	struct mutex delalloc_root_mutex;
 	spinlock_t delalloc_root_lock;
 	/* all fs/file tree roots that have delalloc inodes. */
 	struct list_head delalloc_roots;
@@ -1805,6 +1806,7 @@ struct btrfs_root {
 	spinlock_t root_item_lock;
 	atomic_t refs;
 
+	struct mutex delalloc_mutex;
 	spinlock_t delalloc_lock;
 	/*
 	 * all of the inodes that have delalloc bytes.  It is possible for

fs/btrfs/disk-io.c

@@ -1221,6 +1221,7 @@ static void __setup_root(u32 nodesize, u32 leafsize, u32 sectorsize,
 	mutex_init(&root->objectid_mutex);
 	mutex_init(&root->log_mutex);
 	mutex_init(&root->ordered_extent_mutex);
+	mutex_init(&root->delalloc_mutex);
 	init_waitqueue_head(&root->log_writer_wait);
 	init_waitqueue_head(&root->log_commit_wait[0]);
 	init_waitqueue_head(&root->log_commit_wait[1]);
@@ -2209,6 +2210,7 @@ int open_ctree(struct super_block *sb,
 	spin_lock_init(&fs_info->buffer_lock);
 	rwlock_init(&fs_info->tree_mod_log_lock);
 	mutex_init(&fs_info->reloc_mutex);
+	mutex_init(&fs_info->delalloc_root_mutex);
 	seqlock_init(&fs_info->profiles_lock);
 
 	init_completion(&fs_info->kobj_unregister);

fs/btrfs/inode.c

@@ -8450,6 +8450,7 @@ static int __start_delalloc_inodes(struct btrfs_root *root, int delay_iput,
 	INIT_LIST_HEAD(&works);
 	INIT_LIST_HEAD(&splice);
 
+	mutex_lock(&root->delalloc_mutex);
 	spin_lock(&root->delalloc_lock);
 	list_splice_init(&root->delalloc_inodes, &splice);
 	while (!list_empty(&splice)) {
@@ -8495,6 +8496,7 @@ static int __start_delalloc_inodes(struct btrfs_root *root, int delay_iput,
 		list_splice_tail(&splice, &root->delalloc_inodes);
 		spin_unlock(&root->delalloc_lock);
 	}
+	mutex_unlock(&root->delalloc_mutex);
 	return ret;
 }
 
@@ -8536,6 +8538,7 @@ int btrfs_start_delalloc_roots(struct btrfs_fs_info *fs_info, int delay_iput,
 
 	INIT_LIST_HEAD(&splice);
 
+	mutex_lock(&fs_info->delalloc_root_mutex);
 	spin_lock(&fs_info->delalloc_root_lock);
 	list_splice_init(&fs_info->delalloc_roots, &splice);
 	while (!list_empty(&splice) && nr) {
@@ -8575,6 +8578,7 @@ int btrfs_start_delalloc_roots(struct btrfs_fs_info *fs_info, int delay_iput,
 		list_splice_tail(&splice, &fs_info->delalloc_roots);
 		spin_unlock(&fs_info->delalloc_root_lock);
 	}
+	mutex_unlock(&fs_info->delalloc_root_mutex);
 	return ret;
 }