On 9.01.19 г. 20:26 ч., David Sterba wrote:
> 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.
But can it really, btrfs_[grow|shrink]_device happen under transaction
and their modification of the device_disk_total_bytes (the value
assigned to commit_total_bytes) always happen under chunk_mutex. Also
the updates to both values are really owned by the transaction, so even
if grow/shrink modify those value they will queue those changes in a new
transaction, hence write_dev_super will see consistent value in the
current transaction.
I have a patch from Jeff (which is part of a bigger work) that actually
unifies the resize/device size change lists into a single single and
makes the code a bit easier to grok, nevertheless the above explanation
is still correct even without this patch.
>
> 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);
>
