From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:41644) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1f5nVw-0003Ax-59 for qemu-devel@nongnu.org; Tue, 10 Apr 2018 03:16:26 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1f5nVs-0004eP-T0 for qemu-devel@nongnu.org; Tue, 10 Apr 2018 03:16:24 -0400 Received: from mx3-rdu2.redhat.com ([66.187.233.73]:52960 helo=mx1.redhat.com) by eggs.gnu.org with esmtps (TLS1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1f5nVs-0004e5-M0 for qemu-devel@nongnu.org; Tue, 10 Apr 2018 03:16:20 -0400 Date: Tue, 10 Apr 2018 15:15:57 +0800 From: Peter Xu Message-ID: <20180410071557.GE27604@xz-mi> References: <20180309090006.10018-15-peterx@redhat.com> <20180326080738.GA17789@xz-mi> <20180326090822.GC17789@xz-mi> <20180328040209.GA29554@xz-mi> <20180408030236.GC10540@xz-mi> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: Content-Transfer-Encoding: quoted-printable Subject: Re: [Qemu-devel] [PATCH v8 14/23] monitor: separate QMP parser and dispatcher List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: =?utf-8?Q?Marc-Andr=C3=A9?= Lureau Cc: QEMU , Laurent Vivier , Fam Zheng , Juan Quintela , Michael Roth , Markus Armbruster , Stefan Hajnoczi , Paolo Bonzini , "Dr . David Alan Gilbert" On Mon, Apr 09, 2018 at 11:19:43AM +0200, Marc-Andr=C3=A9 Lureau wrote: > Hi >=20 > On Sun, Apr 8, 2018 at 5:02 AM, Peter Xu wrote: > > On Wed, Apr 04, 2018 at 03:58:56PM +0200, Marc-Andr=C3=A9 Lureau wrot= e: > >> Hi Peter > >> > >> On Wed, Mar 28, 2018 at 6:02 AM, Peter Xu wrote: > >> > On Mon, Mar 26, 2018 at 11:46:13AM +0200, Marc-Andr=C3=A9 Lureau w= rote: > >> >> Hi > >> >> > >> >> On Mon, Mar 26, 2018 at 11:08 AM, Peter Xu wr= ote: > >> >> > On Mon, Mar 26, 2018 at 10:33:27AM +0200, Marc-Andr=C3=A9 Lurea= u wrote: > >> >> >> Hi > >> >> >> > >> >> >> On Mon, Mar 26, 2018 at 10:07 AM, Peter Xu = wrote: > >> >> >> > On Fri, Mar 23, 2018 at 05:18:53PM +0100, Marc-Andr=C3=A9 Lu= reau wrote: > >> >> >> > > >> >> >> > [...] > >> >> >> > > >> >> >> >> > +/* > >> >> >> >> > + * Dispatch one single QMP request. The function will fr= ee the req_obj > >> >> >> >> > + * and objects inside it before return. > >> >> >> >> > + */ > >> >> >> >> > +static void monitor_qmp_dispatch_one(QMPRequest *req_obj= ) > >> >> >> >> > { > >> >> >> >> > - QObject *req, *rsp =3D NULL, *id =3D NULL; > >> >> >> >> > + Monitor *mon, *old_mon; > >> >> >> >> > + QObject *req, *rsp =3D NULL, *id; > >> >> >> >> > QDict *qdict =3D NULL; > >> >> >> >> > - MonitorQMP *mon_qmp =3D container_of(parser, Monitor= QMP, parser); > >> >> >> >> > - Monitor *old_mon, *mon =3D container_of(mon_qmp, Mon= itor, qmp); > >> >> >> >> > - > >> >> >> >> > - Error *err =3D NULL; > >> >> >> >> > + bool need_resume; > >> >> >> >> > > >> >> >> >> > - req =3D json_parser_parse_err(tokens, NULL, &err); > >> >> >> >> > - if (!req && !err) { > >> >> >> >> > - /* json_parser_parse_err() sucks: can fail witho= ut setting @err */ > >> >> >> >> > - error_setg(&err, QERR_JSON_PARSING); > >> >> >> >> > - } > >> >> >> >> > - if (err) { > >> >> >> >> > - goto err_out; > >> >> >> >> > - } > >> >> >> >> > + req =3D req_obj->req; > >> >> >> >> > + mon =3D req_obj->mon; > >> >> >> >> > + id =3D req_obj->id; > >> >> >> >> > + need_resume =3D req_obj->need_resume; > >> >> >> >> > > >> >> >> >> > - qdict =3D qobject_to_qdict(req); > >> >> >> >> > - if (qdict) { > >> >> >> >> > - id =3D qdict_get(qdict, "id"); > >> >> >> >> > - qobject_incref(id); > >> >> >> >> > - qdict_del(qdict, "id"); > >> >> >> >> > - } /* else will fail qmp_dispatch() */ > >> >> >> >> > + g_free(req_obj); > >> >> >> >> > > >> >> >> >> > if (trace_event_get_state_backends(TRACE_HANDLE_QMP_= COMMAND)) { > >> >> >> >> > QString *req_json =3D qobject_to_json(req); > >> >> >> >> > @@ -3900,7 +3932,7 @@ static void handle_qmp_command(JSON= MessageParser *parser, GQueue *tokens) > >> >> >> >> > old_mon =3D cur_mon; > >> >> >> >> > cur_mon =3D mon; > >> >> >> >> > >> >> >> >> There is another issue with this series, since cur_mon is g= lobal (and > >> >> >> >> not protected), an oob command may change the cur_mon while= another > >> >> >> >> command is running in the main thread with unexpected conse= quences. I > >> >> >> >> don't have a clear idea what is the best way to solve it. M= aking the > >> >> >> >> variable per-thread, or going all the way to get rid of cur= _mon (my > >> >> >> >> preference, but much harder) > >> >> >> > > >> >> >> > IMHO it is fine too. > >> >> >> > > >> >> >> > Note that this cur_mon operation is in monitor_qmp_dispatch_= one() now, > >> >> >> > which is still running in main thread. So AFAICT all the cu= r_mon > >> >> >> > references are in main thread, and monitor IOThread does not= modify > >> >> >> > that variable at all. Then we should probably be safe. > >> >> >> > >> >> >> But monitor_qmp_dispatch_one() is called from iothread if the = command > >> >> >> is oob, so cur_mon may be updated while another command is run= ning in > >> >> >> main thread, or am I wrong? > >> >> > > >> >> > You are right. I missed that, sorry... > >> >> > > >> >> > Would this be a simple workaround (but hopefully efficient) sol= ution? > >> >> > > >> >> > diff --git a/monitor.c b/monitor.c > >> >> > index 77f4c41cfa..99641c0c6d 100644 > >> >> > --- a/monitor.c > >> >> > +++ b/monitor.c > >> >> > @@ -4023,7 +4023,7 @@ typedef struct QMPRequest QMPRequest; > >> >> > * Dispatch one single QMP request. The function will free the= req_obj > >> >> > * and objects inside it before return. > >> >> > */ > >> >> > -static void monitor_qmp_dispatch_one(QMPRequest *req_obj) > >> >> > +static void monitor_qmp_dispatch_one(QMPRequest *req_obj, bool= hack_curmon) > >> >> > { > >> >> > Monitor *mon, *old_mon; > >> >> > QObject *req, *rsp =3D NULL, *id; > >> >> > @@ -4043,12 +4043,16 @@ static void monitor_qmp_dispatch_one(QM= PRequest *req_obj) > >> >> > QDECREF(req_json); > >> >> > } > >> >> > > >> >> > - old_mon =3D cur_mon; > >> >> > - cur_mon =3D mon; > >> >> > + if (hack_curmon) { > >> >> > + old_mon =3D cur_mon; > >> >> > + cur_mon =3D mon; > >> >> > + } > >> >> > > >> >> > rsp =3D qmp_dispatch(mon->qmp.commands, req); > >> >> > > >> >> > - cur_mon =3D old_mon; > >> >> > + if (hack_curmon) { > >> >> > + cur_mon =3D old_mon; > >> >> > + } > >> >> > > >> >> > if (mon->qmp.commands =3D=3D &qmp_cap_negotiation_commands= ) { > >> >> > qdict =3D qdict_get_qdict(qobject_to(QDict, rsp), "err= or"); > >> >> > @@ -4116,7 +4120,7 @@ static void monitor_qmp_bh_dispatcher(voi= d *data) > >> >> > > >> >> > if (req_obj) { > >> >> > trace_monitor_qmp_cmd_in_band(qobject_get_try_str(req_= obj->id) ?: ""); > >> >> > - monitor_qmp_dispatch_one(req_obj); > >> >> > + monitor_qmp_dispatch_one(req_obj, true); > >> >> > /* Reschedule instead of looping so the main loop stay= s responsive */ > >> >> > qemu_bh_schedule(mon_global.qmp_dispatcher_bh); > >> >> > } > >> >> > @@ -4175,7 +4179,7 @@ static void handle_qmp_command(JSONMessag= eParser *parser, GQueue *tokens) > >> >> > /* Out-Of-Band (OOB) requests are executed directly in= parser. */ > >> >> > trace_monitor_qmp_cmd_out_of_band(qobject_get_try_str(= req_obj->id) > >> >> > ?: ""); > >> >> > - monitor_qmp_dispatch_one(req_obj); > >> >> > + monitor_qmp_dispatch_one(req_obj, false); > >> >> > return; > >> >> > } > >> >> > > >> >> > Then we forbit touching that evil cur_mon in OOB-capable comman= d > >> >> > handlers. Thanks, > >> >> > >> >> That's not easy to enforce though, afaict it is being used for: > >> >> - error reporting decision > >> > > >> > IMO this should not be a problem, since any QMP handler (including > >> > OOB-capable ones) will be with an Error** there, so logically spea= king > >> > people should never call things like error_report() in that. > >> > > >> >> - file & socket lookup (fd: & /dev/fdset etc) > >> > > >> > I suppose only very rare commands will use it? It'll be a big pro= blem > >> > to solve when we want to completely remove cur_mon though. > >> > > >> >> - the current state of the monitor / list of commands, cpu_path, = capabilities.. > >> > > >> > This is very rare to be used too? Most commands should not use th= em AFAIU. > >> > > >> >> > >> >> Wouldn't it be simpler to make it per-thread? I think it could al= so > >> >> use helpers to push/pop the current monitor. > >> > > >> > Anyway I think yes this is still a good option (though the cur_mon > >> > logic will be a bit more complicated). > >> > > >> > Do you plan to post some patch about this, or do you want me to do > >> > this? I suppose we'll change the qemu_thread_create() a bit to pa= ss > >> > the cur_mon inside, and I suppose this might be better material af= ter > >> > 2.12 release if OOB is off now. > >> > >> Have you looked at making cur_mon per-thread? > > > > Above was my idea, nothing else has been done. > > > > Please feel free to post a patch for this, or I'll do this after 2.12 > > release. >=20 > If it's fixed after 2.12, I think we should document the race as a know= n issue. Do you mean this page? https://wiki.qemu.org/Planning/2.12 To be simpler, I'll see whether I can post the patches soon, and whether that can be accepted as 2.12 material. --=20 Peter Xu