Re: [PATCH v2] md/cluster: fix deadlock when doing reshape job

["heming.zhao@suse.com" <heming.zhao@suse.com>]

Hello,

On 11/12/20 1:08 PM, Xiao Ni wrote:
> 
> 
> On 11/11/2020 11:51 PM, Zhao Heming wrote:
>> There is a similar deadlock in commit 0ba959774e93
>> ("md-cluster: use sync way to handle METADATA_UPDATED msg")
>> My patch fixed issue is very like commit 0ba959774e93, except <c>.
>> 0ba959774e93 step <c> is "update sb", my fix is "mdadm --remove"
>>
>> ... ...
>> +               !test_and_set_bit(MD_CLUSTER_SEND_LOCK, &cinfo->state),
>> +               msecs_to_jiffies(5000));
>> +    if (rv)
>> +        return lock_token(cinfo, mddev_locked);
>> +    else
>> +        return -1;
>>   }
> Hi Heming
> 
> Can we handle this problem like metadata_update_start? lock_comm and metadata_update_start are two users that
> want to get token lock. lock_comm can do the same thing as metadata_update_start does. If so, process_recvd_msg
> can call md_reload_sb without waiting. All threads can work well when the initiated node release token lock. Resync
> can send message and clear MD_CLUSTER_SEND_LOCK, then lock_comm can go on working. In this way, all threads
> work successfully without failure.
> 

Currently MD_CLUSTER_SEND_LOCKED_ALREADY only for adding a new disk.
(please refer Documentation/driver-api/md/md-cluster.rst section: 5. Adding a new Device")
During ADD_NEW_DISK process, md-cluster will block all other msg sending until metadata_update_finish calls
unlock_comm.

With your idea, md-cluster will allow to concurrently send msg. I'm not very familiar with all raid1 scenarios.
But at least, you break the rule of handling ADD_NEW_DISK. it will allow send other msg during ADD_NEW_DISK.

>>   static void unlock_comm(struct md_cluster_info *cinfo)
>> @@ -784,9 +789,11 @@ static int sendmsg(struct md_cluster_info *cinfo, struct cluster_msg *cmsg,
>>   {
>>       int ret;
>> -    lock_comm(cinfo, mddev_locked);
>> ... ...
>> +    if (mddev_is_clustered(mddev)) {
>> +        if (md_cluster_ops->remove_disk(mddev, rdev))
>> +            goto busy;
>> +    }
>>       md_kick_rdev_from_array(rdev);
>>       set_bit(MD_SB_CHANGE_DEVS, &mddev->sb_flags);
> These codes are not related with this deadlock problem. It's better to file a new patch
> to fix checking the return value problem.
> 

In my opinion: we should include these code in this patch. 
For totally fix the deadlock, md layer should return error to userspace.

But with your comments, I found other potential issues of md-cluster. 
There still have some cluster_ops APIs, which caller doesn't care error return:
.resync_finish - may cause other nodes never clean MD_RESYNCING_REMOTE.
.resync_info_update - this error could be safely ignore
.metadata_update_finish - may cause other nodes kernel md info is inconsistent with disk metadata.

maybe I or other guys fix them in the future.

> Best Regards
> Xiao
>