On Thu, Jun 07, 2018 at 06:39:32PM +0200, David Sterba wrote: > On Mon, Jun 04, 2018 at 11:00:30PM +0800, Anand Jain wrote: > > In an instrumented testing it is possible that the mount and > > a newer mkfs.btrfs thread on the same device can race and if the new > > mkfs.btrfs wins it will free the older fs_devices, then the mount thread > > will lead to oops. > > > > Thread1 Thread2 > > ------- ------- > > mkfs.btrfs -fq /dev/sdb > > mount /dev/sdb /btrfs > > |_btrfs_mount_root() > > |_btrfs_scan_one_device(... &fs_devices) > > > > mkfs.btrfs -fq /dev/sdb > > |_btrfs_contol_ioctl() > > |_btrfs_scan_one_device(... > > &fs_devices) > > |_:: > > > > |_btrfs_free_stale_devices() > > > > |_btrfs_open_devices(fs_devices ..) <-- stale fs_devices. > > > > Fix this with a mutually exclusive flag BTRFS_VOL_FLAG_EXCL_OPS. > > I'm not sure this is the right way to fix it, adding another bit to the > uuid_mutex and device_list_mutex combo. > > To fix the race between mount and mkfs we can add a bit of logic to the > device scanning so that mount calling scan will track the purpose and > mkfs scan will not be allowed to free that device.
Last version of the proposed fix is to extend the uuid_mutex over the whole mount callback and use it around calls to btrfs_scan_one_device. That way we'll be sure the mount will always get to the device it scanned and that will not be freed by the parallel scan. -- To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
