Stefano Garzarella
2022-Dec-20 10:43 UTC
[RFC PATCH v1 0/2] virtio/vsock: fix mutual rx/tx hungup
On Tue, Dec 20, 2022 at 09:23:17AM +0000, Arseniy Krasnov wrote:>On 20.12.2022 11:33, Stefano Garzarella wrote: >> On Tue, Dec 20, 2022 at 07:14:27AM +0000, Arseniy Krasnov wrote: >>> On 19.12.2022 18:41, Stefano Garzarella wrote: >>> >>> Hello! >>> >>>> Hi Arseniy, >>>> >>>> On Sat, Dec 17, 2022 at 8:42 PM Arseniy Krasnov <AVKrasnov at sberdevices.ru> wrote: >>>>> >>>>> 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: >>>>> >>>> >>>> I put the values of the variables involved to facilitate understanding: >>>> >>>> RX: buf_alloc = 256 KB; fwd_cnt = 0; last_fwd_cnt = 0; >>>> ??? free_space = buf_alloc - (fwd_cnt - last_fwd_cnt) = 256 KB >>>> >>>> The credit update is sent if >>>> free_space < VIRTIO_VSOCK_MAX_PKT_BUF_SIZE [64 KB] >>>> >>>>> 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 * >>>> >>>> RX: buf_alloc = 256 KB; fwd_cnt = 64 KB; last_fwd_cnt = 0; >>>> ??? free_space = 192 KB >>>> >>>>> 6) rx does poll(), (rx_bytes >= rcvlowat) 192Kb >= 128Kb, POLLIN is set >>>>> 7) rx reads 64Kb, credit update is not sent due to * >>>> >>>> RX: buf_alloc = 256 KB; fwd_cnt = 128 KB; last_fwd_cnt = 0; >>>> ??? free_space = 128 KB >>>> >>>>> 8) rx does poll(), (rx_bytes >= rcvlowat) 128Kb >= 128Kb, POLLIN is set >>>>> 9) rx reads 64Kb, credit update is not sent due to * >>>> >>>> Right, (free_space < VIRTIO_VSOCK_MAX_PKT_BUF_SIZE) is still false. >>>> >>>> RX: buf_alloc = 256 KB; fwd_cnt = 196 KB; last_fwd_cnt = 0; >>>> ??? free_space = 64 KB >>>> >>>>> 10) rx does poll(), (rx_bytes < rcvlowat) 64Kb < 128Kb, rx waits in poll() >>>> >>>> I agree that the TX is stuck because we are not sending the credit >>>> update, but also if RX sends the credit update at step 9, RX won't be >>>> woken up at step 10, right? >>> >>> Yes, RX will sleep, but TX will wake up and as we inform TX how much >>> free space we have, now there are two cases for TX: >>> 1) send "small" rest of data(e.g. without blocking again), leave 'write()' >>> ? and continue execution. RX still waits in 'poll()'. Later TX will >>> ? send enough data to wake up RX. >>> 2) send "big" rest of data - if rest is too big to leave 'write()' and TX >>> ? will wait again for the free space - it will be able to send enough data >>> ? to wake up RX as we compared 'rx_bytes' with rcvlowat value in RX. >> >> Right, so I'd update the test to behave like this. >Sorry, You mean vsock_test? To cover TX waiting for free space at RX, thus checking >this kernel patch logic?Yep, I mean the test that you added in this series.>> And I'd explain better the problem we are going to fix in the commit message. >Ok >> >>>> >>>>> >>>>> * 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? >>>> >>>> I'm not sure, I have to think more about it, but if RX reads less than >>>> SO_RCVLOWAT, I expect it's normal to get to a case of stuck. >>>> >>>> In this case we are only unstucking TX, but even if it sends that single >>>> byte, RX is still stuck and not consuming it, so it was useless to wake >>>> up TX if RX won't consume it anyway, right? >>> >>> 1) I think it is not useless, because we inform(not just wake up) TX that >>> there is free space at RX side - as i mentioned above. >>> 2) Anyway i think that this situation is a little bit strange: TX thinks that >>> there is no free space at RX and waits for it, but there is free space at RX! >>> At the same time, RX waits in poll() forever - it is ready to get new portion >>> of data to return POLLIN, but TX "thinks" exactly opposite thing - RX is full >>> of data. Of course, if there will be just stalls in TX data handling - it will >>> be ok - just performance degradation, but TX stucks forever. >> >> We did it to avoid a lot of credit update messages. >Yes, i see >> Anyway I think here the main point is why RX is setting SO_RCVLOWAT to 128 KB and then reads only half of it? >> >> So I think if the users set SO_RCVLOWAT to a value and then RX reads less then it, is expected to get stuck. >That a really interesting question, I've found nothing about this case in Google(not sure for 100%) or POSIX. But, >i can modify reproducer: it sets SO_RCVLOWAT to 128Kb BEFORE entering its last poll where it will stuck. In this >case behaviour looks more legal: it uses default SO_RCVLOWAT of 1, read 64Kb each time. Finally it sets SO_RCVLOWAT >to 128Kb(and imagine that it prepares 128Kb 'read()' buffer) and enters poll() - we will get same effect: TX will wait >for space, RX waits in 'poll()'.Good point!>> >> Anyway, since the change will not impact the default behaviour (SO_RCVLOWAT = 1) we can merge this patch, but IMHO we need to explain the case better and improve the test. >I see, of course I'm not sure about this change, just want to ask >someone who knows this code betterYes, it's an RFC, so you did well! :-)>> >>> >>>> >>>> If RX woke up (e.g. SO_RCVLOWAT = 64KB) and read the remaining 64KB, >>>> then it would still send the credit update even without this patch and >>>> TX will send the 1 byte. >>> >>> But how RX will wake up in this case? E.g. it calls poll() without timeout, >>> connection is established, RX ignores signal >> >> RX will wake up because SO_RCVLOWAT is 64KB and there are 64 KB in the buffer. Then RX will read it and send the credit update to TX because >> free_space is 0. >IIUC, i'm talking about 10 steps above, e.g. RX will never wake up, >because TX is waiting for space.Yep, but if RX uses SO_RCVLOWAT = 64 KB instead of 128 KB (I mean if RX reads all the bytes that it's waiting as it specified in SO_RCVLOWAT), then RX will send the credit message. But there is the case that you mentioned, when SO_RCVLOWAT is chagend while executing. Thanks, Stefano