Messages in this thread | | | Subject | Re: [PATCH v2] btrfs: fix mount failure caused by race with umount | From | Josef Bacik <> | Date | Fri, 10 Jul 2020 13:51:48 -0400 |
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On 7/10/20 1:23 PM, 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 observe that if we have already filled the device, the > state bit BTRFS_DEV_STATE_IN_FS_METADATA will be set on it, and we can > use that to avoid filling it a second time for no reason and, > critically, avoid double counting in total_rw_bytes. One gotcha is that > read_one_chunk also sets this bit, which happens before read_one_dev (in > read_sys_array), so we must remove that setting of the bit as well, for > the state bit to truly correspond to the device struct being filled from > disk. > > 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. > > A final note is that this fix actually breaks the fstest btrfs/163, but > having investigated it, I believe that is due to a subtle flaw in how > btrfs replace works when used on a seed device. The replace target device > never gets a correct dev_item with the sprout fsid written out. This > causes several problems, but for the sake of btrfs/163, read_one_chunk > marking the device with IN_FS_METADATA was wallpapering over it, which > this patch breaks. I will be sending a subsequent fix for the seed replace > issue which will also fix btrfs/163. > > Signed-off-by: Boris Burkov <boris@bur.io>
Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Thanks,
Josef
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