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d="scan'208";a="5489132" From: Paul Durrant To: Anthony Perard Thread-Topic: [PATCH v2 3/3] xen: perform XenDevice clean-up in XenBus watch handler Thread-Index: AQHVaWP1a6uo9FDbNkeviSOLMWhOMKcoAmKAgAAj/tA= Date: Thu, 12 Sep 2019 15:15:30 +0000 Message-ID: References: <20190912121646.29148-1-paul.durrant@citrix.com> <20190912121646.29148-4-paul.durrant@citrix.com> <20190912150417.GD1308@perard.uk.xensource.com> In-Reply-To: <20190912150417.GD1308@perard.uk.xensource.com> Accept-Language: en-GB, en-US Content-Language: en-US X-MS-Has-Attach: X-MS-TNEF-Correlator: x-ms-exchange-transport-fromentityheader: Hosted Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: quoted-printable MIME-Version: 1.0 X-detected-operating-system: by eggs.gnu.org: FreeBSD 9.x X-Received-From: 216.71.145.153 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" > -----Original Message----- > From: Anthony PERARD > Sent: 12 September 2019 16:04 > To: Paul Durrant > Cc: qemu-devel@nongnu.org; xen-devel@lists.xenproject.org; Stefano Stabel= lini > Subject: Re: [PATCH v2 3/3] xen: perform XenDevice clean-up in XenBus wat= ch handler >=20 > On Thu, Sep 12, 2019 at 01:16:46PM +0100, Paul Durrant wrote: > > Cleaning up offine XenDevice objects directly in > ^ offline >=20 > > xen_device_backend_changed() is dangerous as xen_device_unrealize() wil= l > > 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 renderi= ng > > 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 wh= ich > > 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 wi= ll > > walk 'offline_devices' and perform the necessary actions. > > For safety a an extra 'online' check is also added to > ^ one 'a' too many? >=20 > > 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 fronten= d. > > - 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 bu= t, > > - * if a backend is not 'online', then the device should be destroy= ed > > - * 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 clean= ed > > + * by xen_bus_cleanup(). > > */ > > - if (!xendev->backend_online && > > - (xendev->backend_state =3D=3D XenbusStateClosed || > > - xendev->backend_state =3D=3D XenbusStateInitialising || > > - xendev->backend_state =3D=3D XenbusStateInitWait || > > - xendev->backend_state =3D=3D XenbusStateUnknown)) { > > - Error *local_err =3D NULL; > > + if (!online && > > + (state =3D=3D XenbusStateClosed || state =3D=3D XenbusStateIn= itialising || > > + state =3D=3D XenbusStateInitWait || state =3D=3D XenbusStateU= nknown) && > > + !QLIST_NEXT(xendev, list)) { >=20 > 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.) >=20 Sure, it's worth a comment. > > + XenBus *xenbus =3D 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 notifica= tion, > > + * resulting in a call to xen_bus_cleanup(). > > + */ > > + xen_device_backend_printf(xendev, "state", "%u", state); >=20 > 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. >=20 I had originally considered setting up an event notifier but that seemed li= ke more fds than were strictly necessary ;-) > With a note about the use of QLIST_NEXT and the few typo fixed, the > patch will be good to go. >=20 Cool. I'll tidy up and send a v3. Paul > Thanks, >=20 > -- > Anthony PERARD