[PATCH v3] btrfs: fix mount failure caused by race with umount

[Boris Burkov <boris@bur.io>]

It is possible to cause a btrfs mount to fail by racing it with a slow
umount. The crux of the sequence is generic_shutdown_super not yet
calling sop->put_super before btrfs_mount_root calls btrfs_open_devices.
If that occurs, btrfs_open_devices will decide the opened counter is
non-zero, increment it, and skip resetting fs_devices->total_rw_bytes to
0. From here, mount will call sget which will result in grab_super
trying to take the super block umount semaphore. That semaphore will be
held by the slow umount, so mount will block. Before up-ing the
semaphore, umount will delete the super block, resulting in mount's sget
reliably allocating a new one, which causes the mount path to dutifully
fill it out, and increment total_rw_bytes a second time, which causes
the mount to fail, as we see double the expected bytes.

Here is the sequence laid out in greater detail:

CPU0                                                    CPU1
down_write sb->s_umount
btrfs_kill_super
  kill_anon_super(sb)
    generic_shutdown_super(sb);
      shrink_dcache_for_umount(sb);
      sync_filesystem(sb);
      evict_inodes(sb); // SLOW

                                              btrfs_mount_root
                                                btrfs_scan_one_device
                                                fs_devices = device->fs_devices
                                                fs_info->fs_devices = fs_devices
                                                // fs_devices-opened makes this a no-op
                                                btrfs_open_devices(fs_devices, mode, fs_type)
                                                s = sget(fs_type, test, set, flags, fs_info);
                                                  find sb in s_instances
                                                  grab_super(sb);
                                                    down_write(&s->s_umount); // blocks

      sop->put_super(sb)
        // sb->fs_devices->opened == 2; no-op
      spin_lock(&sb_lock);
      hlist_del_init(&sb->s_instances);
      spin_unlock(&sb_lock);
      up_write(&sb->s_umount);
                                                    return 0;
                                                  retry lookup
                                                  don't find sb in s_instances (deleted by CPU0)
                                                  s = alloc_super
                                                  return s;
                                                btrfs_fill_super(s, fs_devices, data)
                                                  open_ctree // fs_devices total_rw_bytes improperly set!
                                                    btrfs_read_chunk_tree
                                                      read_one_dev // increment total_rw_bytes again!!
                                                      super_total_bytes < fs_devices->total_rw_bytes // ERROR!!!

To fix this, we clear total_rw_bytes from within btrfs_read_chunk_tree
before the calls to read_one_dev, while holding the sb umount semaphore
and the uuid mutex.

To reproduce, it is sufficient to dirty a decent number of inodes, then
quickly umount and mount.

for i in $(seq 0 500)
do
  dd if=/dev/zero of="/mnt/foo/$i" bs=1M count=1
done
umount /mnt/foo&
mount /mnt/foo

does the trick for me.

Signed-off-by: Boris Burkov <boris@bur.io>
---
 fs/btrfs/volumes.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c
index c7a3d4d730a3..26b9bcb00c2b 100644
--- a/fs/btrfs/volumes.c
+++ b/fs/btrfs/volumes.c
@@ -7035,6 +7035,14 @@ int btrfs_read_chunk_tree(struct btrfs_fs_info *fs_info)
 	mutex_lock(&uuid_mutex);
 	mutex_lock(&fs_info->chunk_mutex);
 
+	/*
+	 * It is possible for mount and umount to race in such a way that
+	 * we execute this code path, but open_fs_devices failed to clear
+	 * total_rw_bytes. We certainly want it cleared before reading the
+	 * device items, so clear it here.
+	 */
+	fs_info->fs_devices->total_rw_bytes = 0;
+
 	/*
 	 * Read all device items, and then all the chunk items. All
 	 * device items are found before any chunk item (their object id
-- 
2.24.1