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Fri, 6 Mar 2020 12:38:23 +0000 (UTC) Received: from blackfin.pond.sub.org (ovpn-116-129.ams2.redhat.com [10.36.116.129]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 75722272CC; Fri, 6 Mar 2020 12:38:20 +0000 (UTC) Received: by blackfin.pond.sub.org (Postfix, from userid 1000) id 04EFA11386A6; Fri, 6 Mar 2020 13:38:19 +0100 (CET) From: Markus Armbruster To: Kevin Wolf Subject: Re: [PATCH v4 1/4] qapi: Add a 'coroutine' flag for commands References: <20200121181122.15941-1-kwolf@redhat.com> <20200121181122.15941-2-kwolf@redhat.com> <87lfq0yp9v.fsf@dusky.pond.sub.org> <20200122101021.GB5268@linux.fritz.box> <87h7z2ddjj.fsf@dusky.pond.sub.org> <20200305160654.GC5363@linux.fritz.box> <87pndq7xnj.fsf@dusky.pond.sub.org> <20200306095232.GB7240@linux.fritz.box> Date: Fri, 06 Mar 2020 13:38:18 +0100 In-Reply-To: <20200306095232.GB7240@linux.fritz.box> (Kevin Wolf's message of "Fri, 6 Mar 2020 10:52:32 +0100") Message-ID: <87blp94q11.fsf@dusky.pond.sub.org> User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/26.3 (gnu/linux) MIME-Version: 1.0 X-Scanned-By: MIMEDefang 2.84 on 10.5.11.23 X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: quoted-printable X-detected-operating-system: by eggs.gnu.org: GNU/Linux 2.2.x-3.x [generic] [fuzzy] X-Received-From: 207.211.31.120 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: qemu-block@nongnu.org, Alex =?utf-8?Q?Benn=C3=A9e?= , qemu-devel@nongnu.org, David Alan Gilbert , marcandre.lureau@gmail.com, stefanha@redhat.com, Philippe =?utf-8?Q?Mathieu-Daud=C3=A9?= Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" Kevin Wolf writes: > Am 06.03.2020 um 08:25 hat Markus Armbruster geschrieben: >> Kevin Wolf writes: >>=20 >> > Am 05.03.2020 um 16:30 hat Markus Armbruster geschrieben: >> >> Kevin Wolf writes: >> >>=20 >> >> > Am 22.01.2020 um 07:32 hat Markus Armbruster geschrieben: >> >> >> Kevin Wolf writes: >> >> >>=20 >> >> >> > This patch adds a new 'coroutine' flag to QMP command definition= s that >> >> >> > tells the QMP dispatcher that the command handler is safe to be = run in a >> >> >> > coroutine. >> >> >>=20 >> >> >> I'm afraid I missed this question in my review of v3: when is a ha= ndler >> >> >> *not* safe to be run in a coroutine? >> >> > >> >> > That's a hard one to answer fully. >> >> > >> >> > Basically, I think the biggest problem is with calling functions th= at >> >> > change their behaviour if run in a coroutine compared to running th= em >> >> > outside of coroutine context. In most cases the differences like ha= ving >> >> > a nested event loop instead of yielding are just fine, but they are >> >> > still subtly different. >> >> > >> >> > I know this is vague, but I can assure you that problematic cases e= xist. >> >> > I hit one of them with my initial hack that just moved everything i= nto a >> >> > coroutine. It was related to graph modifications and bdrv_drain and >> >> > resulted in a hang. For the specifics, I would have to try and repr= oduce >> >> > the problem again. >> >>=20 >> >> I'm afraid it's even more complicated. >> >>=20 >> >> Monitors (HMP and QMP) run in the main loop. Before this patch, HMP = and >> >> QMP commands run start to finish, one after the other. >> >>=20 >> >> After this patch, QMP commands may elect to yield. QMP commands stil= l >> >> run one after the other (the shared dispatcher ensures this even when= we >> >> have multiple QMP monitors). >> >>=20 >> >> However, *HMP* commands can now run interleaved with a coroutine-enab= led >> >> QMP command, i.e. between a yield and its re-enter. >> > >> > I guess that's right. :-( >> > >> >> Consider an HMP screendump running in the middle of a coroutine-enabl= ed >> >> QMP screendump, using Marc-Andr=C3=A9's patch. As far as I can tell,= it >> >> works, because: >> >>=20 >> >> 1. HMP does not run in a coroutine. If it did, and both dumps dumped >> >> the same @con, then it would overwrite con->screndump_co. If we ever >> >> decide to make HMP coroutine-capable so it doesn't block the main loo= p, >> >> this will become unsafe in a nasty way. >> > >> > At the same time, switching HMP to coroutines would give us an easy wa= y >> > to fix the problem: Just use a CoMutex so that HMP and QMP commands >> > never run in parallel. Unless we actually _want_ to run both at the sa= me >> > time for some commands, but I don't see why. >>=20 >> As long as running QMP commands from *all* monitors one after the other >> is good enough, I can't see why running HMP commands interleaved is >> worth the risk. > > There is one exception, actually: Obviously, human-monitor-command must > allow HMP commands to run in parallel. Yes. >> > While we don't have this, maybe it's worth considering if there is >> > another simple way to achieve the same thing. Could QMP just suspend a= ll >> > HMP monitors while it's executing a command? At first sight it seems >> > that this would work only for "interactive" monitors. >>=20 >> I believe the non-"interactive" HMP code is used only by gdbstub.c. > > monitor_init_hmp() is called from (based on my block branch): > > * monitor_init(): This is interactive. > * qemu_chr_new_noreplay(): Interactive, too. > * gdbserver_start(): Non-interactive. > > There is also qmp_human_monitor_command(), which manually creates a > MonitorHMP without going through monitor_init_hmp(). It does call > monitor_data_init(), though. There are no additional callers of > monitor_data_init() and I think I would have added it to every creation > of a Monitor object when I did the QMP/HMP split of the struct. > > So GDB and human-monitor-command should be the two non-interactive > cases. Yes. >> I keep forgetting our GDB server stub creates an "HMP" monitor. >> Possibly because I don't understand why. Alex, Philippe, can you >> enlighten me? > > I think you can send HMP commands from within gdb with it: > > (gdb) tar rem:1234 > Remote debugging using :1234 > [...] > (gdb) monitor info block > ide1-cd0: [not inserted] > Attached to: /machine/unattached/device[23] > Removable device: not locked, tray closed > > floppy0: [not inserted] > Attached to: /machine/unattached/device[16] > Removable device: not locked, tray closed > > sd0: [not inserted] > Removable device: not locked, tray closed Argh! Any idea where we define GDB command "monitor"? > So we do want stop it from processing requests while a QMP command is > running. Then a slow QMP command can interfere with debugging. Perhaps we can synchronize just the "monitor" command.