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bh=/5Q+q6yNqAxBk6g6i9jyTzPE/hWWcqQrxs+KLHIhU8E=; b=ax5EJdDe1MPsmkCD7YpiXYnFm3vVOQIA0nWgFGCGy149f6U2vVNte6LYFd86hSaBX+gSBA gpjwlFOm7dxfRetqiJJt8HxRQ9ZECHk6rkasXmP1FngqmsJm7Q4GDxG3hCdCB0TtrdWNlt 2gcWik6rn13rgK59GPimlwgrBdMcXks= X-MC-Unique: iwzTqUN_NG2GJF4aKSkC1g-1 From: Stefan Hajnoczi To: qemu-devel@nongnu.org Cc: "Dr. David Alan Gilbert" , Eduardo Habkost , pbonzini@redhat.com, Markus Armbruster , Eric Blake , kwolf@redhat.com, Maxim Levitsky , =?UTF-8?q?Daniel=20P=2E=20Berrang=C3=A9?= , Stefan Hajnoczi Subject: [RFC 3/3] qmp: make qmp_device_add() a coroutine Date: Wed, 6 Sep 2023 15:01:41 -0400 Message-ID: <20230906190141.1286893-4-stefanha@redhat.com> In-Reply-To: <20230906190141.1286893-1-stefanha@redhat.com> References: <20230906190141.1286893-1-stefanha@redhat.com> MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable X-Scanned-By: MIMEDefang 3.1 on 10.11.54.6 Received-SPF: pass (zohomail.com: domain of gnu.org designates 209.51.188.17 as permitted sender) client-ip=209.51.188.17; envelope-from=qemu-devel-bounces+importer=patchew.org@nongnu.org; helo=lists.gnu.org; Received-SPF: pass client-ip=170.10.133.124; envelope-from=stefanha@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -20 X-Spam_score: -2.1 X-Spam_bar: -- X-Spam_report: (-2.1 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.001, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_NONE=-0.0001, RCVD_IN_MSPIKE_H4=0.001, RCVD_IN_MSPIKE_WL=0.001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: qemu-devel-bounces+importer=patchew.org@nongnu.org Sender: qemu-devel-bounces+importer=patchew.org@nongnu.org X-ZohoMail-DKIM: pass (identity @redhat.com) X-ZM-MESSAGEID: 1694026961211100003 Content-Type: text/plain; charset="utf-8" It is not safe to call drain_call_rcu() from qmp_device_add() because some call stacks are not prepared for drain_call_rcu() to drop the Big QEMU Lock (BQL). For example, device emulation code is protected by the BQL but when it calls aio_poll() -> ... -> qmp_device_add() -> drain_call_rcu() then the BQL is dropped. See bz#2215192 below for a concrete bug of this type. Another limitation of drain_call_rcu() is that it cannot be invoked within an RCU read-side critical section since the reclamation phase cannot complete until the end of the critical section. Unfortunately, call stacks have been seen where this happens (see bz#2214985 below). Switch to call_drain_rcu_co() to avoid these problems. This requires making qmp_device_add() a coroutine. qdev_device_add() is not designed to be called from coroutines, so it must be invoked from a BH and then switch back to the coroutine. Fixes: 7bed89958bfbf40df9ca681cefbdca63abdde39d ("device_core: use drain_ca= ll_rcu in in qmp_device_add") Buglink: https://bugzilla.redhat.com/show_bug.cgi?id=3D2215192 Buglink: https://bugzilla.redhat.com/show_bug.cgi?id=3D2214985 Signed-off-by: Stefan Hajnoczi --- qapi/qdev.json | 1 + include/monitor/qdev.h | 3 ++- monitor/qmp-cmds.c | 2 +- softmmu/qdev-monitor.c | 34 ++++++++++++++++++++++++++++++---- hmp-commands.hx | 1 + 5 files changed, 35 insertions(+), 6 deletions(-) diff --git a/qapi/qdev.json b/qapi/qdev.json index 6bc5a733b8..78e9d7f7b8 100644 --- a/qapi/qdev.json +++ b/qapi/qdev.json @@ -79,6 +79,7 @@ ## { 'command': 'device_add', 'data': {'driver': 'str', '*bus': 'str', '*id': 'str'}, + 'coroutine': true, 'gen': false, # so we can get the additional arguments 'features': ['json-cli', 'json-cli-hotplug'] } =20 diff --git a/include/monitor/qdev.h b/include/monitor/qdev.h index 1d57bf6577..1fed9eb9ea 100644 --- a/include/monitor/qdev.h +++ b/include/monitor/qdev.h @@ -5,7 +5,8 @@ =20 void hmp_info_qtree(Monitor *mon, const QDict *qdict); void hmp_info_qdm(Monitor *mon, const QDict *qdict); -void qmp_device_add(QDict *qdict, QObject **ret_data, Error **errp); +void coroutine_fn +qmp_device_add(QDict *qdict, QObject **ret_data, Error **errp); =20 int qdev_device_help(QemuOpts *opts); DeviceState *qdev_device_add(QemuOpts *opts, Error **errp); diff --git a/monitor/qmp-cmds.c b/monitor/qmp-cmds.c index b0f948d337..a7419226fe 100644 --- a/monitor/qmp-cmds.c +++ b/monitor/qmp-cmds.c @@ -202,7 +202,7 @@ static void __attribute__((__constructor__)) monitor_in= it_qmp_commands(void) qmp_init_marshal(&qmp_commands); =20 qmp_register_command(&qmp_commands, "device_add", - qmp_device_add, 0, 0); + qmp_device_add, QCO_COROUTINE, 0); =20 QTAILQ_INIT(&qmp_cap_negotiation_commands); qmp_register_command(&qmp_cap_negotiation_commands, "qmp_capabilities", diff --git a/softmmu/qdev-monitor.c b/softmmu/qdev-monitor.c index 74f4e41338..85ae62f7cf 100644 --- a/softmmu/qdev-monitor.c +++ b/softmmu/qdev-monitor.c @@ -839,8 +839,28 @@ void hmp_info_qdm(Monitor *mon, const QDict *qdict) qdev_print_devinfos(true); } =20 -void qmp_device_add(QDict *qdict, QObject **ret_data, Error **errp) +typedef struct { + Coroutine *co; + QemuOpts *opts; + Error **errp; + DeviceState *dev; +} QmpDeviceAdd; + +static void qmp_device_add_bh(void *opaque) { + QmpDeviceAdd *data =3D opaque; + + data->dev =3D qdev_device_add(data->opts, data->errp); + aio_co_wake(data->co); +} + +void coroutine_fn +qmp_device_add(QDict *qdict, QObject **ret_data, Error **errp) +{ + QmpDeviceAdd data =3D { + .co =3D qemu_coroutine_self(), + .errp =3D errp, + }; QemuOpts *opts; DeviceState *dev; =20 @@ -852,7 +872,13 @@ void qmp_device_add(QDict *qdict, QObject **ret_data, = Error **errp) qemu_opts_del(opts); return; } - dev =3D qdev_device_add(opts, errp); + + /* Perform qdev_device_add() call outside coroutine context */ + data.opts =3D opts; + aio_bh_schedule_oneshot(qemu_coroutine_get_aio_context(data.co), + qmp_device_add_bh, &data); + qemu_coroutine_yield(); + dev =3D data.dev; =20 /* * Drain all pending RCU callbacks. This is done because @@ -863,7 +889,7 @@ void qmp_device_add(QDict *qdict, QObject **ret_data, E= rror **errp) * will finish its job completely once qmp command returns result * to the user */ - drain_call_rcu(); + drain_call_rcu_co(); =20 if (!dev) { qemu_opts_del(opts); @@ -956,7 +982,7 @@ void qmp_device_del(const char *id, Error **errp) } } =20 -void hmp_device_add(Monitor *mon, const QDict *qdict) +void coroutine_fn hmp_device_add(Monitor *mon, const QDict *qdict) { Error *err =3D NULL; =20 diff --git a/hmp-commands.hx b/hmp-commands.hx index 2cbd0f77a0..c737d1fd64 100644 --- a/hmp-commands.hx +++ b/hmp-commands.hx @@ -695,6 +695,7 @@ ERST .params =3D "driver[,prop=3Dvalue][,...]", .help =3D "add device, like -device on the command line", .cmd =3D hmp_device_add, + .coroutine =3D true, .command_completion =3D device_add_completion, }, =20 --=20 2.41.0