From: Boris Burkov boris@bur.io
commit 48cfa61b58a1fee0bc49eef04f8ccf31493b7cdd upstream.
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.
CC: stable@vger.kernel.org # 4.4+ Signed-off-by: Boris Burkov boris@bur.io Reviewed-by: David Sterba dsterba@suse.com Signed-off-by: David Sterba dsterba@suse.com Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org
--- fs/btrfs/volumes.c | 8 ++++++++ 1 file changed, 8 insertions(+)
--- a/fs/btrfs/volumes.c +++ b/fs/btrfs/volumes.c @@ -6902,6 +6902,14 @@ int btrfs_read_chunk_tree(struct btrfs_f 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 * is smaller than the lowest possible object id for a chunk