linux-kernel - Re: [PATCH 2/3] nbd: notify userland even if nbd has already disconnected

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Date:   Tue, 28 May 2019 11:36:21 -0500
From:   Mike Christie <mchristi@...hat.com>
To:     Yao Liu <yotta.liu@...oud.cn>, Josef Bacik <josef@...icpanda.com>
Cc:     Jens Axboe <axboe@...nel.dk>, linux-block@...r.kernel.org,
        nbd@...er.debian.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/3] nbd: notify userland even if nbd has already
 disconnected

On 05/27/2019 01:23 PM, Yao Liu wrote:
> On Fri, May 24, 2019 at 09:08:58AM -0400, Josef Bacik wrote:
>> On Fri, May 24, 2019 at 05:43:55PM +0800, Yao Liu wrote:
>>> Some nbd client implementations have a userland's daemon, so we should
>>> inform client daemon to clean up and exit.
>>>
>>> Signed-off-by: Yao Liu <yotta.liu@...oud.cn>
>>
>> Except the nbd_disconnected() check is for the case that the client told us
>> specifically to disconnect, so we don't want to send the notification to
>> re-connect because we've already been told we want to tear everything down.
>> Nack to this as well.  Thanks,
>>
>> Josef
>>
> 
> But in userland, client daemon process and process which send disconnect
> command are not same process, so they are not clear to each other, so
> client daemon expect driver inform it to exit.
> In addition, client daemon will get nbd status with nbd_genl_status interface
> after it get notified and it should not re-connect if status connected == 0
> 

When using the netlink interface you get the NBD_CMD_LINK_DEAD first
then the configs_refs goes to zero right?

nbd_disconnect_and_put -> sock_shutdown -> nbd_mark_nsock_dead

then later we do the final nbd_config_put?

Maybe it would be best to add a new netlink event to signal what has
happened, because the above nl and stat algorithm seems like a pain. The
NBD_CMD_LINK_DEAD will be sent, then userspace has to possibly poll the
status to check if this was caused due to nbd_genl_disconnect instead of
a downed link due to something like a command timeout, because the
refcount may not be down when userspace gets the NL event.

Or, I guess the admin/tool process could just send a msg to the daemon
process to tell it to do the netlink disconnect request.