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Wed, 22 Jan 2020 12:15:07 +0000 (UTC) Received: by blackfin.pond.sub.org (Postfix, from userid 1000) id 485F81138600; Wed, 22 Jan 2020 13:15:06 +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> Date: Wed, 22 Jan 2020 13:15:06 +0100 In-Reply-To: <20200122101021.GB5268@linux.fritz.box> (Kevin Wolf's message of "Wed, 22 Jan 2020 11:10:21 +0100") Message-ID: <874kwnvgad.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-MC-Unique: 43dpAgUePGqMijbzdiyLrA-1 X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain 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, marcandre.lureau@gmail.com, qemu-devel@nongnu.org, stefanha@redhat.com Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" Kevin Wolf writes: > 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 run in= a >> > coroutine. >>=20 >> I'm afraid I missed this question in my review of v3: when is a handler >> *not* safe to be run in a coroutine? > > That's a hard one to answer fully. You're welcome ;) > Basically, I think the biggest problem is with calling functions that > change their behaviour if run in a coroutine compared to running them > outside of coroutine context. In most cases the differences like having > 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 exist. > I hit one of them with my initial hack that just moved everything into 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 reproduce > the problem again. Interesting. Is coroutine-incompatible command handler code necessary or accidental? By "necessary" I mean there are (and likely always will be) commands that need to do stuff that cannot or should not be done on coroutine context. "Accidental" is the opposite: coroutine-incompatibility can be regarded as a fixable flaw. How widespread is coroutine-incompatibility? Common, or just a few commands? If coroutine-incompatibility is accidental, then your code to drop out of coroutine context can be regarded as a temporary work-around. Such work-arounds receive a modest extra ugliness & complexity budget. If coroutine-incompatibility is rare, we'll eventually want "mark the known-bad ones with 'coroutine': false" instead of "mark the known-good ones with 'coroutine': true". I'm okay with marking the known-good ones initially, and flipping only later. Inability to recognize coroutine-incompatibility by inspection worries me. Can you think of ways to identify causes other than testing things to death?