Re: [RFC PATCH v1 0/2] virtio/vsock: fix mutual rx/tx hungup

[Arseniy Krasnov]

On 19.12.2022 18:41, Stefano Garzarella wrote:

Hello!

> Hi Arseniy,
> 
> On Sat, Dec 17, 2022 at 8:42 PM Arseniy Krasnov <AVKrasnov@xxxxxxxxxxxxxx> 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.
> 
>>
>> * 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.

> 
> 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

Thanks, Arseniy
> 
> Thanks,
> Stefano
>