On Thu, Dec 13, 2018 at 09:17:25PM +0000, fdmanana@xxxxxxxxxx wrote:
> - if (list_empty(&fs_devices->resized_devices))
> - return;
> -
> - mutex_lock(&fs_devices->device_list_mutex);
> mutex_lock(&fs_info->chunk_mutex);
> list_for_each_entry_safe(curr, next, &fs_devices->resized_devices,
> resized_list) {
> @@ -7309,7 +7306,6 @@ void btrfs_update_commit_device_size(struct btrfs_fs_info *fs_info)
> curr->commit_total_bytes = curr->disk_total_bytes;
I'm not sure about removing the device_list_mutex that's said to protect
the commit_total_bytes (comment in struct btrfs_device).
Otherwise the logic is ok, the double lock could happen as you describe.
btrfs_update_commit_device_size is called from btrfs_commit_transaction,
at the same time as commit_bytes_used. The latter is handled in a
similar way in btrfs_update_commit_device_bytes_used, but does not take
the device_list_mutex.
commit_total_bytes is checked several times (eg. in write_dev_supers) to
see if writing the superblock copy is still within the device range.
So, without the protected change, it's theoretically possible that a
stale value is used for the test and the superblock is either written
though it should not, and the other way around.
This would require a resize racing at the time of the check. Grow and
shrink seem to take chunk_mutex while adjusting all the total/size
values, but it's not actually easy to follow as sometimes there are
helpers like btrfs_device_set_total_bytes used and sometimes it's direct
access.
That the device_list_mutex can be safely dropped probably follows from
the simple fact that btrfs_update_commit_device_bytes_used is called
before write_dev_supers in the same context.
But this sounds too simple, given that there are locks taken and
released and btrfs_write_and_wait_transaction called between.
Referencing this code:
2201 btrfs_update_commit_device_size(fs_info);
2202 btrfs_update_commit_device_bytes_used(cur_trans);
2203
2204 clear_bit(BTRFS_FS_LOG1_ERR, &fs_info->flags);
2205 clear_bit(BTRFS_FS_LOG2_ERR, &fs_info->flags);
2206
2207 btrfs_trans_release_chunk_metadata(trans);
2208
2209 spin_lock(&fs_info->trans_lock);
2210 cur_trans->state = TRANS_STATE_UNBLOCKED;
2211 fs_info->running_transaction = NULL;
2212 spin_unlock(&fs_info->trans_lock);
2213 mutex_unlock(&fs_info->reloc_mutex);
2214
2215 wake_up(&fs_info->transaction_wait);
2216
2217 ret = btrfs_write_and_wait_transaction(trans);
2218 if (ret) {
2219 btrfs_handle_fs_error(fs_info, ret,
2220 "Error while writing out transaction");
2221 mutex_unlock(&fs_info->tree_log_mutex);
2222 goto scrub_continue;
2223 }
2224
2225 ret = write_all_supers(fs_info, 0);
