Btrfs: fix ->iterate_shared() by upgrading i_rwsem for delayed nodes

Commit fe742fd4f90f ("Revert "btrfs: switch to ->iterate_shared()"")
backed out the conversion to ->iterate_shared() for Btrfs because the
delayed inode handling in btrfs_real_readdir() is racy. However, we can
still do readdir in parallel if there are no delayed nodes.

This is a temporary fix which upgrades the shared inode lock to an
exclusive lock only when we have delayed items until we come up with a
more complete solution. While we're here, rename the
btrfs_{get,put}_delayed_items functions to make it very clear that
they're just for readdir.

Tested with xfstests and by doing a parallel kernel build:

	while make tinyconfig && make -j4 && git clean dqfx; do
		:
	done

along with a bunch of parallel finds in another shell:

	while true; do
		for ((i=0; i<4; i++)); do
			find . >/dev/null &
		done
		wait
	done

Signed-off-by: Omar Sandoval <osandov@fb.com>
Signed-off-by: David Sterba <dsterba@suse.com>
Signed-off-by: Chris Mason <clm@fb.com>

3 files changed, 34 insertions, 13 deletions

diff --git a/fs/btrfs/delayed-inode.c b/fs/btrfs/delayed-inode.c
index 61561c2a3f96..d3aaabbfada0 100644
--- a/fs/btrfs/delayed-inode.c
+++ b/fs/btrfs/delayed-inode.c
@@ -1606,15 +1606,23 @@ int btrfs_inode_delayed_dir_index_count(struct inode *inode)
         return 0;
 }
 
-void btrfs_get_delayed_items(struct inode *inode, struct list_head *ins_list,
-                             struct list_head *del_list)
+bool btrfs_readdir_get_delayed_items(struct inode *inode,
+                                     struct list_head *ins_list,
+                                     struct list_head *del_list)
 {
         struct btrfs_delayed_node *delayed_node;
         struct btrfs_delayed_item *item;
 
         delayed_node = btrfs_get_delayed_node(inode);
         if (!delayed_node)
-                return;
+                return false;
+
+        /*
+         * We can only do one readdir with delayed items at a time because of
+         * item->readdir_list.
+         */
+        inode_unlock_shared(inode);
+        inode_lock(inode);
 
         mutex_lock(&delayed_node->mutex);
         item = __btrfs_first_delayed_insertion_item(delayed_node);
@@ -1641,10 +1649,13 @@ void btrfs_get_delayed_items(struct inode *inode, struct list_head *ins_list,
          * requeue or dequeue this delayed node.
          */
         atomic_dec(&delayed_node->refs);
+
+        return true;
 }
 
-void btrfs_put_delayed_items(struct list_head *ins_list,
-                             struct list_head *del_list)
+void btrfs_readdir_put_delayed_items(struct inode *inode,
+                                     struct list_head *ins_list,
+                                     struct list_head *del_list)
 {
         struct btrfs_delayed_item *curr, *next;
 
@@ -1659,6 +1670,12 @@ void btrfs_put_delayed_items(struct list_head *ins_list,
                 if (atomic_dec_and_test(&curr->refs))
                         kfree(curr);
         }
+
+        /*
+         * The VFS is going to do up_read(), so we need to downgrade back to a
+         * read lock.
+         */
+        downgrade_write(&inode->i_rwsem);
 }
 
 int btrfs_should_delete_dir_index(struct list_head *del_list,
diff --git a/fs/btrfs/delayed-inode.h b/fs/btrfs/delayed-inode.h
index 0167853c84ae..2495b3d4075f 100644
--- a/fs/btrfs/delayed-inode.h
+++ b/fs/btrfs/delayed-inode.h
@@ -137,10 +137,12 @@ void btrfs_kill_all_delayed_nodes(struct btrfs_root *root);
 void btrfs_destroy_delayed_inodes(struct btrfs_root *root);
 
 /* Used for readdir() */
-void btrfs_get_delayed_items(struct inode *inode, struct list_head *ins_list,
-                             struct list_head *del_list);
-void btrfs_put_delayed_items(struct list_head *ins_list,
-                             struct list_head *del_list);
+bool btrfs_readdir_get_delayed_items(struct inode *inode,
+                                     struct list_head *ins_list,
+                                     struct list_head *del_list);
+void btrfs_readdir_put_delayed_items(struct inode *inode,
+                                     struct list_head *ins_list,
+                                     struct list_head *del_list);
 int btrfs_should_delete_dir_index(struct list_head *del_list,
                                   u64 index);
 int btrfs_readdir_delayed_dir_index(struct dir_context *ctx,
diff --git a/fs/btrfs/inode.c b/fs/btrfs/inode.c
index d2be95cfb6d1..969a25c5abcb 100644
--- a/fs/btrfs/inode.c
+++ b/fs/btrfs/inode.c
@@ -5757,6 +5757,7 @@ static int btrfs_real_readdir(struct file *file, struct dir_context *ctx)
         int name_len;
         int is_curr = 0;        /* ctx->pos points to the current index? */
         bool emitted;
+        bool put = false;
 
         /* FIXME, use a real flag for deciding about the key type */
         if (root->fs_info->tree_root == root)
@@ -5774,7 +5775,8 @@ static int btrfs_real_readdir(struct file *file, struct dir_context *ctx)
         if (key_type == BTRFS_DIR_INDEX_KEY) {
                 INIT_LIST_HEAD(&ins_list);
                 INIT_LIST_HEAD(&del_list);
-                btrfs_get_delayed_items(inode, &ins_list, &del_list);
+                put = btrfs_readdir_get_delayed_items(inode, &ins_list,
+                                                      &del_list);
         }
 
         key.type = key_type;
@@ -5921,8 +5923,8 @@ next:
 nopos:
         ret = 0;
 err:
-        if (key_type == BTRFS_DIR_INDEX_KEY)
-                btrfs_put_delayed_items(&ins_list, &del_list);
+        if (put)
+                btrfs_readdir_put_delayed_items(inode, &ins_list, &del_list);
         btrfs_free_path(path);
         return ret;
 }
@@ -10534,7 +10536,7 @@ static const struct inode_operations btrfs_dir_ro_inode_operations = {
 static const struct file_operations btrfs_dir_file_operations = {
         .llseek         = generic_file_llseek,
         .read           = generic_read_dir,
-        .iterate        = btrfs_real_readdir,
+        .iterate_shared = btrfs_real_readdir,
         .unlocked_ioctl = btrfs_ioctl,
 #ifdef CONFIG_COMPAT
         .compat_ioctl   = btrfs_compat_ioctl,