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dkim=none (message not signed) header.d=none;nongnu.org; dmarc=none action=none header.from=virtuozzo.com; Received: from AM7PR08MB5494.eurprd08.prod.outlook.com (2603:10a6:20b:dc::15) by AM6PR08MB3624.eurprd08.prod.outlook.com (2603:10a6:20b:51::17) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.20.3933.32; Tue, 16 Mar 2021 18:37:15 +0000 Received: from AM7PR08MB5494.eurprd08.prod.outlook.com ([fe80::f1f0:6610:11f5:5e4a]) by AM7PR08MB5494.eurprd08.prod.outlook.com ([fe80::f1f0:6610:11f5:5e4a%8]) with mapi id 15.20.3933.032; Tue, 16 Mar 2021 18:37:15 +0000 Subject: Re: [PATCH 7/7] block/nbd: stop manipulating in_flight counter To: Roman Kagan , qemu-devel@nongnu.org, yc-core@yandex-team.ru, Eric Blake , Max Reitz , Kevin Wolf , qemu-block@nongnu.org References: <20210315060611.2989049-1-rvkagan@yandex-team.ru> <20210315060611.2989049-8-rvkagan@yandex-team.ru> From: Vladimir Sementsov-Ogievskiy Message-ID: <37c7fd4a-fa68-22b0-6d4c-4a161116515a@virtuozzo.com> Date: Tue, 16 Mar 2021 21:37:13 +0300 User-Agent: Mozilla/5.0 (X11; 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envelope-from=vsementsov@virtuozzo.com; helo=EUR03-DB5-obe.outbound.protection.outlook.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, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, MSGID_FROM_MTA_HEADER=0.001, NICE_REPLY_A=-0.001, RCVD_IN_DNSWL_NONE=-0.0001, RCVD_IN_MSPIKE_H2=-0.001, SPF_HELO_PASS=-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: 2.1.23 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" 16.03.2021 19:08, Roman Kagan wrote: > On Mon, Mar 15, 2021 at 11:15:44PM +0300, Vladimir Sementsov-Ogievskiy wrote: >> 15.03.2021 09:06, Roman Kagan wrote: >>> As the reconnect logic no longer interferes with drained sections, it >>> appears unnecessary to explicitly manipulate the in_flight counter. >>> >>> Fixes: 5ad81b4946 ("nbd: Restrict connection_co reentrance") >> >> And here you actually allow qemu_aio_coroutine_enter() call in >> nbd_client_attach_aio_context_bh() to enter connection_co in any yield >> point which is possible during drained section. The analysis should be >> done to be sure that all these yield points are safe for reentering by >> external qemu_aio_coroutine_enter(). (By external I mean not by the >> actual enter() we are waiting for at the yield() point. For example >> qemu_channel_yield() supports reentering.. And therefore (as I >> understand after fast looking through) nbd_read() should support >> reentering too.. > > I'll do a more thorough analysis of how safe it is. > > FWIW this hasn't triggered any test failures yet, but that assert in > patch 3 didn't ever go off either so I'm not sure I can trust the tests > on this. > Hmm. First, we should consider qemu_coroutine_yield() in nbd_co_establish_connection(). Most of nbd_co_establish_connection_cancel() purpose is to avoid reentering this yield().. And I don't know, how to make it safely reenterable: keep in mind bh that may be already scheduled by connect_thread_func(). And if bh is already scheduled, can we cancel it? I'm not sure. We have qemu_bh_delete(). But is it possible, that BH is near to be executed and already cannot be removed by qemu_bh_delete()? I don't know. And if we can't safely drop the bh at any moment, we should wait in nbd_client_detach_aio_context until the scheduled bh enters the connection_co.. Or something like this -- Best regards, Vladimir