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Miller" , Jakub Kicinski , Paolo Abeni CC: "linux-kernel@vger.kernel.org" , "netdev@vger.kernel.org" , "virtualization@lists.linux-foundation.org" , "kvm@vger.kernel.org" , kernel , Krasnov Arseniy , Arseniy Krasnov Subject: [RFC PATCH v1 0/2] virtio/vsock: fix mutual rx/tx hungup Thread-Topic: [RFC PATCH v1 0/2] virtio/vsock: fix mutual rx/tx hungup Thread-Index: AQHZEk+jG/1mxrs/wUyXm0IW7LxCRw== Date: Sat, 17 Dec 2022 19:42:04 +0000 Message-ID: <39b2e9fd-601b-189d-39a9-914e5574524c@sberdevices.ru> Accept-Language: en-US, ru-RU Content-Language: en-US X-MS-Has-Attach: X-MS-TNEF-Correlator: x-originating-ip: [172.16.1.12] Content-ID: MIME-Version: 1.0 X-KSMG-Rule-ID: 4 X-KSMG-Message-Action: clean X-KSMG-AntiSpam-Status: not scanned, disabled by settings X-KSMG-AntiSpam-Interceptor-Info: not scanned X-KSMG-AntiPhishing: not scanned, disabled by settings X-KSMG-AntiVirus: Kaspersky Secure Mail Gateway, version 1.1.2.30, bases: 2022/12/17 15:49:00 #20678428 X-KSMG-AntiVirus-Status: Clean, skipped X-Spam-Status: No, score=-2.1 required=5.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,DKIM_VALID_EF,SPF_HELO_NONE,SPF_PASS autolearn=ham autolearn_force=no version=3.4.6 X-Spam-Checker-Version: SpamAssassin 3.4.6 (2021-04-09) on lindbergh.monkeyblade.net Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org X-getmail-retrieved-from-mailbox: =?utf-8?q?INBOX?= X-GMAIL-THRID: =?utf-8?q?1752491546387707723?= X-GMAIL-MSGID: =?utf-8?q?1752491546387707723?= Hello, seems I found strange thing(may be a bug) where sender('tx' later) and receiver('rx' later) could stuck forever. Potential fix is in the first patch, second patch contains reproducer, based on vsock test suite. Reproducer is simple: tx just sends data to rx by 'write() syscall, rx dequeues it using 'read()' syscall and uses 'poll()' for waiting. I run server in host and client in guest. rx side params: 1) SO_VM_SOCKETS_BUFFER_SIZE is 256Kb(e.g. default). 2) SO_RCVLOWAT is 128Kb. What happens in the reproducer step by step: 1) tx tries to send 256Kb + 1 byte (in a single 'write()') 2) tx sends 256Kb, data reaches rx (rx_bytes == 256Kb) 3) tx waits for space in 'write()' to send last 1 byte 4) rx does poll(), (rx_bytes >= rcvlowat) 256Kb >= 128Kb, POLLIN is set 5) rx reads 64Kb, credit update is not sent due to * 6) rx does poll(), (rx_bytes >= rcvlowat) 192Kb >= 128Kb, POLLIN is set 7) rx reads 64Kb, credit update is not sent due to * 8) rx does poll(), (rx_bytes >= rcvlowat) 128Kb >= 128Kb, POLLIN is set 9) rx reads 64Kb, credit update is not sent due to * 10) rx does poll(), (rx_bytes < rcvlowat) 64Kb < 128Kb, rx waits in poll() * is optimization in 'virtio_transport_stream_do_dequeue()' which sends OP_CREDIT_UPDATE only when we have not too much space - less than VIRTIO_VSOCK_MAX_PKT_BUF_SIZE. Now tx side waits for space inside write() and rx waits in poll() for 'rx_bytes' to reach SO_RCVLOWAT value. Both sides will wait forever. I think, possible fix is to send credit update not only when we have too small space, but also when number of bytes in receive queue is smaller than SO_RCVLOWAT thus not enough to wake up sleeping reader. I'm not sure about correctness of this idea, but anyway - I think that problem above exists. What do You think? Patchset was rebased and tested on skbuff v7 patch from Bobby Eshleman: https://lore.kernel.org/netdev/20221213192843.421032-1-bobby.eshleman@bytedance.com/ Arseniy Krasnov(2): virtio/vsock: send credit update depending on SO_RCVLOWAT vsock_test: mutual hungup reproducer net/vmw_vsock/virtio_transport_common.c | 9 +++- tools/testing/vsock/vsock_test.c | 78 +++++++++++++++++++++++++++++++++ 2 files changed, 85 insertions(+), 2 deletions(-) --- 2.25.1