Btrfs: add missing spin_lock for insertion into tree mod log

tree_mod_alloc calls __get_tree_mod_seq and must acquire a spinlock before
doing so.

Signed-off-by: Jan Schmidt <list.btrfs@jan-o-sch.net>

1 files changed, 18 insertions, 5 deletions

diff --git a/fs/btrfs/ctree.c b/fs/btrfs/ctree.c
index 0954f1770fd0..26e8dc1681b0 100644
--- a/fs/btrfs/ctree.c
+++ b/fs/btrfs/ctree.c
@@ -455,11 +455,11 @@ unlock:
         return ret;
 }
 
-int tree_mod_alloc(struct btrfs_fs_info *fs_info, gfp_t flags,
-                   struct tree_mod_elem **tm_ret)
+static inline int tree_mod_alloc(struct btrfs_fs_info *fs_info, gfp_t flags,
+                                 struct tree_mod_elem **tm_ret)
 {
         struct tree_mod_elem *tm;
-        u64 seq = 0;
+        int seq;
 
         smp_mb();
         if (list_empty(&fs_info->tree_mod_seq_list))
@@ -469,9 +469,22 @@ int tree_mod_alloc(struct btrfs_fs_info *fs_info, gfp_t flags,
         if (!tm)
                 return -ENOMEM;
 
-        __get_tree_mod_seq(fs_info, &tm->elem);
-        seq = tm->elem.seq;
         tm->elem.flags = 0;
+        spin_lock(&fs_info->tree_mod_seq_lock);
+        if (list_empty(&fs_info->tree_mod_seq_list)) {
+                /*
+                 * someone emptied the list while we were waiting for the lock.
+                 * we must not add to the list, because no blocker exists. items
+                 * are removed from the list only when the existing blocker is
+                 * removed from the list.
+                 */
+                kfree(tm);
+                seq = 0;
+        } else {
+                __get_tree_mod_seq(fs_info, &tm->elem);
+                seq = tm->elem.seq;
+        }
+        spin_unlock(&fs_info->tree_mod_seq_lock);
 
         return seq;
 }