Btrfs: fix unprotected deletion from pending_chunks list

On block group remove if the corresponding extent map was on the
transaction->pending_chunks list, we were deleting the extent map
from that list, through remove_extent_mapping(), without any
synchronization with chunk allocation (which iterates that list
and adds new elements to it). Fix this by ensure that this is done
while the chunk mutex is held, since that's the mutex that protects
the list in the chunk allocation code path.

This applies on top (depends on) of my previous patch titled:
"Btrfs: fix race between fs trimming and block group remove/allocation"

But the issue in fact was already present before that change, it only
became easier to hit after Josef's 3.18 patch that added automatic
removal of empty block groups.

Signed-off-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: Chris Mason <clm@fb.com>
(cherry picked from commit 8dbcd10f6978ca3ccee2f43288d16b7b9da2fb2b)

diff --git a/fs/btrfs/extent-tree.c b/fs/btrfs/extent-tree.c
index a719a54995ac94f71d99fa5408f345c6bb9ca284..efb5c8145f2bedae7d46576de848011a96e99079 100644 (file)
--- a/fs/btrfs/extent-tree.c
+++ b/fs/btrfs/extent-tree.c
@@ -9510,19 +9510,25 @@ int btrfs_remove_block_group(struct btrfs_trans_handle *trans,
                list_move_tail(&em->list, &root->fs_info->pinned_chunks);
        }
        spin_unlock(&block_group->lock);
-       unlock_chunks(root);
 
        if (remove_em) {
                struct extent_map_tree *em_tree;
 
                em_tree = &root->fs_info->mapping_tree.map_tree;
                write_lock(&em_tree->lock);
+               /*
+                * The em might be in the pending_chunks list, so make sure the
+                * chunk mutex is locked, since remove_extent_mapping() will
+                * delete us from that list.
+                */
                remove_extent_mapping(em_tree, em);
                write_unlock(&em_tree->lock);
                /* once for the tree */
                free_extent_map(em);
        }
 
+       unlock_chunks(root);
+
        btrfs_put_block_group(block_group);
        btrfs_put_block_group(block_group);