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Date:   Tue, 6 Jun 2023 13:31:25 -0400
From:   Demi Marie Obenour <demi@invisiblethingslab.com>
To:     Roger Pau =?utf-8?B?TW9ubsOp?= <roger.pau@citrix.com>
Cc:     Jens Axboe <axboe@kernel.dk>, Alasdair Kergon <agk@redhat.com>,
        Mike Snitzer <snitzer@kernel.org>, dm-devel@redhat.com,
        Marek =?utf-8?Q?Marczykowski-G=C3=B3recki?= 
        <marmarek@invisiblethingslab.com>, linux-block@vger.kernel.org,
        linux-kernel@vger.kernel.org, xen-devel@lists.xenproject.org
Subject: Re: [PATCH v2 16/16] xen-blkback: Inform userspace that device has
 been opened
Message-ID: <ZH9tcGh0N2fEcwjH@itl-email>
References: <20230530203116.2008-1-demi@invisiblethingslab.com>
 <20230530203116.2008-17-demi@invisiblethingslab.com>
 <ZH75OTMA6N3zYrH2@Air-de-Roger>
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Date: Tue, 6 Jun 2023 13:31:25 -0400
From: Demi Marie Obenour <demi@invisiblethingslab.com>
To: Roger Pau =?utf-8?B?TW9ubsOp?= <roger.pau@citrix.com>
Cc: Jens Axboe <axboe@kernel.dk>, Alasdair Kergon <agk@redhat.com>,
	Mike Snitzer <snitzer@kernel.org>, dm-devel@redhat.com,
	Marek =?utf-8?Q?Marczykowski-G=C3=B3recki?= <marmarek@invisiblethingslab.com>,
	linux-block@vger.kernel.org, linux-kernel@vger.kernel.org,
	xen-devel@lists.xenproject.org
Subject: Re: [PATCH v2 16/16] xen-blkback: Inform userspace that device has
 been opened

On Tue, Jun 06, 2023 at 11:15:37AM +0200, Roger Pau Monn=C3=A9 wrote:
> On Tue, May 30, 2023 at 04:31:16PM -0400, Demi Marie Obenour wrote:
> > Set "opened" to "0" before the hotplug script is called.  Once the
> > device node has been opened, set "opened" to "1".
> >=20
> > "opened" is used exclusively by userspace.  It serves two purposes:
> >=20
> > 1. It tells userspace that the diskseq Xenstore entry is supported.
> >=20
> > 2. It tells userspace that it can wait for "opened" to be set to 1.
> >    Once "opened" is 1, blkback has a reference to the device, so
> >    userspace doesn't need to keep one.
> >=20
> > Together, these changes allow userspace to use block devices with
> > delete-on-close behavior, such as loop devices with the autoclear flag
> > set or device-mapper devices with the deferred-remove flag set.
>=20
> There was some work in the past to allow reloading blkback as a
> module, it's clear that using delete-on-close won't work if attempting
> to reload blkback.

Should blkback stop itself from being unloaded if delete-on-close is in
use?

> Isn't there some existing way to check whether a device is opened?
> (stat syscall maybe?).

Knowing that the device has been opened isn=E2=80=99t enough.  The block sc=
ript
needs to be able to wait for blkback (and not something else) to open
the device.  Otherwise it will be confused if the device is opened by
e.g. udev.

> I would like to avoid adding more xenstore blkback state if such
> information can be fetched from other methods.

I don=E2=80=99t think it can be, unless the information is passed via a
completely different method.  Maybe netlink(7) or ioctl(2)?  Arguably
this information should not be stored in Xenstore at all, as it exposes
backend implementation details to the frontend.

> > diff --git a/drivers/block/xen-blkback/xenbus.c b/drivers/block/xen-blk=
back/xenbus.c
> > index 9c3eb148fbd802c74e626c3d7bcd69dcb09bd921..519a78aa9073d1faa1dce5c=
1b36e95ae58da534b 100644
> > --- a/drivers/block/xen-blkback/xenbus.c
> > +++ b/drivers/block/xen-blkback/xenbus.c
> > @@ -699,6 +713,14 @@ static int xen_blkbk_probe(struct xenbus_device *d=
ev,
> >  	if (err)
> >  		pr_warn("%s write out 'max-ring-page-order' failed\n", __func__);
> > =20
> > +	/*
> > +	 * This informs userspace that the "opened" node will be set to "1" w=
hen
> > +	 * the device has been opened successfully.
> > +	 */
> > +	err =3D xenbus_write(XBT_NIL, dev->nodename, "opened", "0");
> > +	if (err)
> > +		goto fail;
> > +
>=20
> You would need to set "opened" before registering the xenstore backend
> watch AFAICT, or else it could be racy.

Will fix in the next version.
--=20
Sincerely,
Demi Marie Obenour (she/her/hers)
Invisible Things Lab

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