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Fri, 6 Mar 2020 09:52:38 +0000 (UTC) Received: from linux.fritz.box (unknown [10.36.118.33]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 36B48272D3; Fri, 6 Mar 2020 09:52:34 +0000 (UTC) Date: Fri, 6 Mar 2020 10:52:32 +0100 From: Kevin Wolf To: Markus Armbruster Subject: Re: [PATCH v4 1/4] qapi: Add a 'coroutine' flag for commands Message-ID: <20200306095232.GB7240@linux.fritz.box> 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> MIME-Version: 1.0 In-Reply-To: <87pndq7xnj.fsf@dusky.pond.sub.org> User-Agent: Mutt/1.12.1 (2019-06-15) 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=iso-8859-1 Content-Transfer-Encoding: quoted-printable Content-Disposition: inline X-detected-operating-system: by eggs.gnu.org: GNU/Linux 2.2.x-3.x [generic] [fuzzy] X-Received-From: 207.211.31.81 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, Philippe =?iso-8859-1?Q?Mathieu-Daud=E9?= , qemu-devel@nongnu.org, David Alan Gilbert , marcandre.lureau@gmail.com, stefanha@redhat.com, Alex =?iso-8859-1?Q?Benn=E9e?= Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" 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 definitions= that > >> >> > tells the QMP dispatcher that the command handler is safe to be r= un in a > >> >> > coroutine. > >> >>=20 > >> >> I'm afraid I missed this question in my review of v3: when is a han= dler > >> >> *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 tha= t > >> > change their behaviour if run in a coroutine compared to running the= m > >> > outside of coroutine context. In most cases the differences like hav= ing > >> > 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 ex= ist. > >> > I hit one of them with my initial hack that just moved everything in= to 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 repro= duce > >> > 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 a= nd > >> QMP commands run start to finish, one after the other. > >>=20 > >> After this patch, QMP commands may elect to yield. QMP commands still > >> 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-enabl= ed > >> 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-enable= d > >> QMP screendump, using Marc-Andr=E9'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 loop= , > >> this will become unsafe in a nasty way. > > > > At the same time, switching HMP to coroutines would give us an easy way > > 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 sam= e > > 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. > > 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 al= l > > 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. > 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 So we do want stop it from processing requests while a QMP command is running. Kevin