On Thu, Jul 16, 2020 at 01:29:46PM -0700, Boris Burkov wrote: > 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@xxxxxx> Added to misc-next, thanks. > --- For patch iterations, please put a short list of changes description under the "---" marker. This does not get applied to the patch and is intended to help people reviewing the patches to see only what's new.
