linux-kernel - Re: [Nbd] [PATCH v2 4/5]nbd: make nbd device wait for its users.

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Date:	Wed, 15 Jun 2016 14:48:21 +0530
From:	Pranay Srivastava <pranjas@...il.com>
To:	Wouter Verhelst <w@...r.be>
Cc:	Markus Pargmann <mpa@...gutronix.de>,
	nbd-general@...ts.sourceforge.net, linux-kernel@...r.kernel.org
Subject: Re: [Nbd] [PATCH v2 4/5]nbd: make nbd device wait for its users.

On Wed, Jun 15, 2016 at 12:30 PM, Wouter Verhelst <w@...r.be> wrote:
> On Wed, Jun 15, 2016 at 08:30:45AM +0200, Markus Pargmann wrote:
>> Thanks for the explanations. I think my understanding was off by one ;)..
>> I didn't realize that the DO_IT thread from the userspace has the block
>> device open as well.
>
> Obviously, otherwise it couldn't do an ioctl() to it :-)
>
>> I thought a bit about this, does it make sense to delay the essential
>> cleanup steps until really all open file handles were closed? So that
>> even if the DO_IT thread exits, the block device is still there. Only if
>> the file is closed everything is cleaned up. Maybe this makes the code
>> simpler and we can directly use krefs without any strange constructs.
>> What do you think?


>>
>> This would also allow the client to setup a new socket as long as it
>> does not close the nbd file handle.
>
> That sounds like the behaviour that I described earlier about possible
> retries for userspace...
>
>> Could this behavior be potentially problematic for any client
>> implementation?
>
> I don't think it could, but I'm not sure I understand all the details.
> What would happen if:
>
> - nbd is connected from pid X, pid Y does NBD_DISCONNECT, pid X hangs
>   and doesn't exit?

In that case pid X would have an error on recv if I'am correct. Then if no other
users[like mounts or other user space applications] are present for this device
then pidx would exit.

> - nbd is connected from pid X, server disconnects while pid Y is trying
>   to access the device, pid X tries to reconnect but it takes a while?

Not sure what issue you see in the above case but if I understand correctly,

This should again fall in error case [Correct?] as a result of a timeout may be
if any requests were in progress. I don't think reconnect will be
without any error
thrown up user space, not so sure if both sides [server and client]
were sitting idle.
OR
if no timeout was there then things should go OK after a successful reconnect.

Multiple processes can still access the device.

>
>> Does it solve our other issue with setting up a new sockets for an
>> existing nbd blockdevice?
>
> It could, depending.

This should be OK.

>
> --
> < ron> I mean, the main *practical* problem with C++, is there's like a dozen
>        people in the world who think they really understand all of its rules,
>        and pretty much all of them are just lying to themselves too.
>  -- #debian-devel, OFTC, 2016-02-12



-- 
        ---P.K.S