linux-kernel - Re: [PATCH] nbd: correct disconnect behavior

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Message-Id: <20130627152800.494a6043607a62feb35e9739@linux-foundation.org>
Date:	Thu, 27 Jun 2013 15:28:00 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Paul Clements <paul.clements@...sios.com>
Cc:	kernel list <linux-kernel@...r.kernel.org>,
	"nbd-general@...ts.sourceforge.net" 
	<nbd-general@...ts.sourceforge.net>
Subject: Re: [PATCH] nbd: correct disconnect behavior

On Thu, 27 Jun 2013 18:20:37 -0400 Paul Clements <paul.clements@...sios.com> wrote:

> > --- a/drivers/block/nbd.c
> > > +++ b/drivers/block/nbd.c
> > > @@ -623,6 +623,8 @@ static int __nbd_ioctl(struct block_device *bdev,
> > struct nbd_device *nbd,
> > >               if (!nbd->sock)
> > >                       return -EINVAL;
> > >
> > > +             nbd->disconnect = 1;
> > > +
> > >               nbd_send_req(nbd, &sreq);
> > >                  return 0;
> > >       }
> > > @@ -654,6 +656,7 @@ static int __nbd_ioctl(struct block_device *bdev,
> > struct nbd_device *nbd,
> > >                               nbd->sock = SOCKET_I(inode);
> > >                               if (max_part > 0)
> > >                                       bdev->bd_invalidated = 1;
> > > +                             nbd->disconnect = 0; /* we're connected
> > now */
> > >                               return 0;
> > >                       } else {
> > >                               fput(file);
> > > @@ -742,6 +745,8 @@ static int __nbd_ioctl(struct block_device *bdev,
> > struct nbd_device *nbd,
> > >               set_capacity(nbd->disk, 0);
> > >               if (max_part > 0)
> > >                       ioctl_by_bdev(bdev, BLKRRPART, 0);
> > > +             if (nbd->disconnect) /* user requested, ignore socket
> > errors */
> > > +                     return 0;
> > >               return nbd->harderror;
> > >       }
> >
> > hm, how does nbd work...  Hard to tell as nothing seems to be documented
> > anywhere :(
> >
> 
> I think most people look at the nbd-client to see. :)
> 
> But I will work on some docs for the ioctls...
> 
> Briefly, though, it goes something like this:
> 
> nbd-client and nbd-server negotiate parameters, including the server
> telling the client how big the export is
> 
> Then the nbd-client does a series of ioctls to set up the device:
> 
> NBD_SET_SIZE - set the size of the device
> NBD_SET_SOCK - tell the kernel which socket to use
> NBD_DO_IT - this ioctl lasts for the life of the device and causes
> nbd-client to go into a receive loop, handling I/O replies from the
> nbd-server
> 
> NBD_DISCONNECT, meanwhile, can be called from a separate thread (usually
> "nbd-client -d" calls it). This requests a disconnect of the device.

OK, but.  "Would it be safer to clear ->disconnect in NBD_DO_IT?"

If not safer, would it be cleaner?

> > The cool kids are using bool lately ;)
> >
> 
> Hey, maybe I want to be able to compile with gcc 2.7.2 ? :)

Sob, I miss 2.7.2.  It was a good 50% faster than the new improved
models.  But I don't think this yearning makes us cool.
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