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d="scan'208";a="5543206" Date: Thu, 12 Sep 2019 16:04:17 +0100 From: Anthony PERARD To: Paul Durrant Message-ID: <20190912150417.GD1308@perard.uk.xensource.com> References: <20190912121646.29148-1-paul.durrant@citrix.com> <20190912121646.29148-4-paul.durrant@citrix.com> MIME-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Disposition: inline In-Reply-To: <20190912121646.29148-4-paul.durrant@citrix.com> User-Agent: Mutt/1.12.1 (2019-06-15) X-detected-operating-system: by eggs.gnu.org: FreeBSD 9.x X-Received-From: 216.71.145.142 Subject: Re: [Qemu-devel] [PATCH v2 3/3] xen: perform XenDevice clean-up in XenBus watch handler X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.23 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: xen-devel@lists.xenproject.org, Stefano Stabellini , qemu-devel@nongnu.org Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On Thu, Sep 12, 2019 at 01:16:46PM +0100, Paul Durrant wrote: > Cleaning up offine XenDevice objects directly in ^ offline > xen_device_backend_changed() is dangerous as xen_device_unrealize() will > modify the watch list that is being walked. Even the QLIST_FOREACH_SAFE() > used in notifier_list_notify() is insufficient as *two* notifiers (for > the frontend and backend watches) are removed, thus potentially rendering > the 'next' pointer unsafe. > > The solution is to use the XenBus backend_watch handler to do the clean-up > instead, as it is invoked whilst walking a separate watch list. > > This patch therefore adds a new 'offline_devices' list to XenBus, to which > offline devices are added by xen_device_backend_changed(). The XenBus > backend_watch registration is also changed to not only invoke > xen_bus_enumerate() but also a new xen_bus_cleanup() function, which will > walk 'offline_devices' and perform the necessary actions. > For safety a an extra 'online' check is also added to ^ one 'a' too many? > xen_bus_type_enumerate() to make sure that no attempt is made to create a > new XenDevice object for a backend that is offline. > > NOTE: This patch also include some cosmetic changes: > - substitute the local variable name 'backend_state' > in xen_bus_type_enumerate() with 'state', since there > is no ambiguity with any other state in that context. > - change xen_device_state_is_active() to > xen_device_frontend_is_active() (and pass a XenDevice directly) > since the state tests contained therein only apply to a frontend. > - use 'state' rather then 'xendev->backend_state' in > xen_device_backend_changed() to shorten the code. > > Signed-off-by: Paul Durrant > --- > > v2: > - Make sure we don't try to add a XenDevice to 'offline_devices' more than > once > --- > > /* > * If a backend is still 'online' then we should leave it alone but, > - * if a backend is not 'online', then the device should be destroyed > - * once the state is Closed. > + * if a backend is not 'online', then the device is a candidate > + * for destruction. Hence add it to the 'offline' list to be cleaned > + * by xen_bus_cleanup(). > */ > - if (!xendev->backend_online && > - (xendev->backend_state == XenbusStateClosed || > - xendev->backend_state == XenbusStateInitialising || > - xendev->backend_state == XenbusStateInitWait || > - xendev->backend_state == XenbusStateUnknown)) { > - Error *local_err = NULL; > + if (!online && > + (state == XenbusStateClosed || state == XenbusStateInitialising || > + state == XenbusStateInitWait || state == XenbusStateUnknown) && > + !QLIST_NEXT(xendev, list)) { Could you add a note about this QLIST_NEXT? I don't think it's going to be obvious enough why we check that there are no next item. I might only understand it just because of your reply to the v1 of the patch. (Well the changelog of the patch also point out what it's for.) > + XenBus *xenbus = XEN_BUS(qdev_get_parent_bus(DEVICE(xendev))); > > - if (!xen_backend_try_device_destroy(xendev, &local_err)) { > - object_unparent(OBJECT(xendev)); > - } > + QLIST_INSERT_HEAD(&xenbus->offline_devices, xendev, list); > > - if (local_err) { > - error_report_err(local_err); > - } > + /* > + * Re-write the state to cause a XenBus backend_watch notification, > + * resulting in a call to xen_bus_cleanup(). > + */ > + xen_device_backend_printf(xendev, "state", "%u", state); It kind of feels wrong to rely on xenstore to notify QEMU's xenbus driver that a device needs cleanup. But that does the job. With a note about the use of QLIST_NEXT and the few typo fixed, the patch will be good to go. Thanks, -- Anthony PERARD