2 files changed, 14 insertions, 0 deletions

diff --git a/fs/btrfs/ctree.h b/fs/btrfs/ctree.h
index 169baa817d96..d8e539fe5544 100644
--- a/fs/btrfs/ctree.h
+++ b/fs/btrfs/ctree.h
@@ -1304,6 +1304,16 @@ struct btrfs_fs_info {
          * so it is also safe.
          */
         u64 max_inline;
+        /*
+         * Protected by ->chunk_mutex and sb->s_umount.
+         *
+         * The reason that we use two lock to protect it is because only
+         * remount and mount operations can change it and these two operations
+         * are under sb->s_umount, but the read side (chunk allocation) can not
+         * acquire sb->s_umount or the deadlock would happen. So we use two
+         * locks to protect it. On the write side, we must acquire two locks,
+         * and on the read side, we just need acquire one of them.
+         */
         u64 alloc_start;
         struct btrfs_transaction *running_transaction;
         wait_queue_head_t transaction_throttle;
diff --git a/fs/btrfs/super.c b/fs/btrfs/super.c
index 6846ededfe95..eda330df45a4 100644
--- a/fs/btrfs/super.c
+++ b/fs/btrfs/super.c
@@ -519,7 +519,9 @@ int btrfs_parse_options(struct btrfs_root *root, char *options)
                 case Opt_alloc_start:
                         num = match_strdup(&args[0]);
                         if (num) {
+                                mutex_lock(&info->chunk_mutex);
                                 info->alloc_start = memparse(num, NULL);
+                                mutex_unlock(&info->chunk_mutex);
                                 kfree(num);
                                 printk(KERN_INFO
                                         "btrfs: allocations start at %llu\n",
@@ -1289,7 +1291,9 @@ restore:
         fs_info->mount_opt = old_opts;
         fs_info->compress_type = old_compress_type;
         fs_info->max_inline = old_max_inline;
+        mutex_lock(&fs_info->chunk_mutex);
         fs_info->alloc_start = old_alloc_start;
+        mutex_unlock(&fs_info->chunk_mutex);
         btrfs_resize_thread_pool(fs_info,
                 old_thread_pool_size, fs_info->thread_pool_size);
         fs_info->metadata_ratio = old_metadata_ratio;