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Fri, 16 Apr 2021 08:09:34 +0000 From: Vladimir Sementsov-Ogievskiy To: qemu-block@nongnu.org Cc: qemu-devel@nongnu.org, vsementsov@virtuozzo.com, eblake@redhat.com, mreitz@redhat.com, kwolf@redhat.com, rvkagan@yandex-team.ru, den@openvz.org, Stefan Hajnoczi , Fam Zheng Subject: [PATCH v3 06/33] util/async: aio_co_schedule(): support reschedule in same ctx Date: Fri, 16 Apr 2021 11:08:44 +0300 Message-Id: <20210416080911.83197-7-vsementsov@virtuozzo.com> X-Mailer: git-send-email 2.29.2 In-Reply-To: <20210416080911.83197-1-vsementsov@virtuozzo.com> References: <20210416080911.83197-1-vsementsov@virtuozzo.com> X-Originating-IP: [185.215.60.206] X-ClientProxiedBy: HE1P189CA0030.EURP189.PROD.OUTLOOK.COM (2603:10a6:7:53::43) To AM7PR08MB5494.eurprd08.prod.outlook.com (2603:10a6:20b:dc::15) MIME-Version: 1.0 X-MS-Exchange-MessageSentRepresentingType: 1 Received: from localhost.localdomain (185.215.60.206) by HE1P189CA0030.EURP189.PROD.OUTLOOK.COM (2603:10a6:7:53::43) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.20.4042.16 via Frontend Transport; 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envelope-from=vsementsov@virtuozzo.com; helo=EUR05-DB8-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, 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" With the following patch we want to call wake coroutine from thread. And it doesn't work with aio_co_wake: Assume we have no iothreads. Assume we have a coroutine A, which waits in the yield point for external aio_co_wake(), and no progress can be done until it happen. Main thread is in blocking aio_poll() (for example, in blk_read()). Now, in a separate thread we do aio_co_wake(). It calls aio_co_enter(), which goes through last "else" branch and do aio_context_acquire(ctx). Now we have a deadlock, as aio_poll() will not release the context lock until some progress is done, and progress can't be done until aio_co_wake() wake the coroutine A. And it can't because it wait for aio_context_acquire(). Still, aio_co_schedule() works well in parallel with blocking aio_poll(). So we want use it. Let's add a possibility of rescheduling coroutine in same ctx where it was yield'ed. Fetch co->ctx in same way as it is done in aio_co_wake(). Signed-off-by: Vladimir Sementsov-Ogievskiy --- include/block/aio.h | 2 +- util/async.c | 8 ++++++++ 2 files changed, 9 insertions(+), 1 deletion(-) diff --git a/include/block/aio.h b/include/block/aio.h index 5f342267d5..744b695525 100644 --- a/include/block/aio.h +++ b/include/block/aio.h @@ -643,7 +643,7 @@ static inline bool aio_node_check(AioContext *ctx, bool is_external) /** * aio_co_schedule: - * @ctx: the aio context + * @ctx: the aio context, if NULL, the current ctx of @co will be used. * @co: the coroutine * * Start a coroutine on a remote AioContext. diff --git a/util/async.c b/util/async.c index 674dbefb7c..750be555c6 100644 --- a/util/async.c +++ b/util/async.c @@ -545,6 +545,14 @@ fail: void aio_co_schedule(AioContext *ctx, Coroutine *co) { + if (!ctx) { + /* + * Read coroutine before co->ctx. Matches smp_wmb in + * qemu_coroutine_enter. + */ + smp_read_barrier_depends(); + ctx = qatomic_read(&co->ctx); + } trace_aio_co_schedule(ctx, co); const char *scheduled = qatomic_cmpxchg(&co->scheduled, NULL, __func__);