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Name=false; Custom Display Name List=false; Reply-to Address Mismatch=false; Targeted Threat Dictionary=false; Mimecast Threat Dictionary=false; Custom Threat Dictionary=false X-Scanned-By: MIMEDefang 3.1 on 10.11.54.3 X-Mailman-Approved-At: Wed, 07 Jun 2023 06:48:52 +0000 Subject: Re: [dm-devel] [PATCH v2 16/16] xen-blkback: Inform userspace that device has been opened X-BeenThere: dm-devel@redhat.com X-Mailman-Version: 2.1.29 Precedence: list List-Id: device-mapper development List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Jens Axboe , Mike Snitzer , Marek =?utf-8?Q?Marczykowski-G=C3=B3recki?= , linux-kernel@vger.kernel.org, linux-block@vger.kernel.org, dm-devel@redhat.com, xen-devel@lists.xenproject.org, Alasdair Kergon Errors-To: dm-devel-bounces@redhat.com Sender: "dm-devel" X-Scanned-By: MIMEDefang 3.1 on 10.11.54.7 X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: citrix.com Content-Disposition: inline Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit 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". > > "opened" is used exclusively by userspace. It serves two purposes: > > 1. It tells userspace that the diskseq Xenstore entry is supported. > > 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. > > 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. 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. Isn't there some existing way to check whether a device is opened? (stat syscall maybe?). I would like to avoid adding more xenstore blkback state if such information can be fetched from other methods. > Signed-off-by: Demi Marie Obenour > --- > drivers/block/xen-blkback/xenbus.c | 35 ++++++++++++++++++++++++++++++ > 1 file changed, 35 insertions(+) > > diff --git a/drivers/block/xen-blkback/xenbus.c b/drivers/block/xen-blkback/xenbus.c > index 9c3eb148fbd802c74e626c3d7bcd69dcb09bd921..519a78aa9073d1faa1dce5c1b36e95ae58da534b 100644 > --- a/drivers/block/xen-blkback/xenbus.c > +++ b/drivers/block/xen-blkback/xenbus.c > @@ -3,6 +3,20 @@ > Copyright (C) 2005 Rusty Russell > Copyright (C) 2005 XenSource Ltd > > +In addition to the Xenstore nodes required by the Xen block device > +specification, this implementation of blkback uses a new Xenstore > +node: "opened". blkback sets "opened" to "0" before the hotplug script > +is called. Once the device node has been opened, blkback sets "opened" > +to "1". > + > +"opened" is read exclusively by userspace. It serves two purposes: > + > +1. It tells userspace that diskseq@major:minor syntax for "physical-device" is > + supported. > + > +2. It tells userspace that it can wait for "opened" to be set to 1 after writing > + "physical-device". Once "opened" is 1, blkback has a reference to the > + device, so userspace doesn't need to keep one. > > */ > > @@ -699,6 +713,14 @@ static int xen_blkbk_probe(struct xenbus_device *dev, > if (err) > pr_warn("%s write out 'max-ring-page-order' failed\n", __func__); > > + /* > + * This informs userspace that the "opened" node will be set to "1" when > + * the device has been opened successfully. > + */ > + err = xenbus_write(XBT_NIL, dev->nodename, "opened", "0"); > + if (err) > + goto fail; > + You would need to set "opened" before registering the xenstore backend watch AFAICT, or else it could be racy. Thanks, Roger. -- dm-devel mailing list dm-devel@redhat.com https://listman.redhat.com/mailman/listinfo/dm-devel