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Date:   Wed, 20 Jul 2022 06:07:47 +0000
From:   Arseniy Krasnov <>
To:     Stefano Garzarella <>,
        "David S. Miller" <>,
        Jakub Kicinski <>,
        Paolo Abeni <>
CC:     Stefan Hajnoczi <>,
        "Michael S. Tsirkin" <>,
        Krasnov Arseniy <>,
        "" <>,
        "" <>,
        "" <>,
        kernel <>
Subject: Re: [RFC PATCH v1 0/3] virtio/vsock: use SO_RCVLOWAT to set

On 19.07.2022 15:58, Stefano Garzarella wrote:
> On Mon, Jul 18, 2022 at 08:12:52AM +0000, Arseniy Krasnov wrote:
>> Hello,
>> during my experiments with zerocopy receive, i found, that in some
>> cases, poll() implementation violates POSIX: when socket has non-
>> default SO_RCVLOWAT(e.g. not 1), poll() will always set POLLIN and
>> POLLRDNORM bits in 'revents' even number of bytes available to read
>> on socket is smaller than SO_RCVLOWAT value. In this case,user sees
>> POLLIN flag and then tries to read data(for example using  'read()'
>> call), but read call will be blocked, because  SO_RCVLOWAT logic is
>> supported in dequeue loop in af_vsock.c. But the same time,  POSIX
>> requires that:
>> "POLLIN     Data other than high-priority data may be read without
>>            blocking.
>> POLLRDNORM Normal data may be read without blocking."
>> See, page 293.
>> So, we have, that poll() syscall returns POLLIN, but read call will
>> be blocked.
>> Also in man page socket(7) i found that:
>> "Since Linux 2.6.28, select(2), poll(2), and epoll(7) indicate a
>> socket as readable only if at least SO_RCVLOWAT bytes are available."
>> I checked TCP callback for poll()(net/ipv4/tcp.c, tcp_poll()), it
>> uses SO_RCVLOWAT value to set POLLIN bit, also i've tested TCP with
>> this case for TCP socket, it works as POSIX required.
> I tried to look at the code and it seems that only TCP complies with it or am I wrong?
Yes, i checked AF_UNIX, it also don't care about that. It calls skb_queue_empty() that of
course ignores SO_RCVLOWAT.
>> I've added some fixes to af_vsock.c and virtio_transport_common.c,
>> test is also implemented.
>> What do You think guys?
> Nice, thanks for fixing this and for the test!
> I left some comments, but I think the series is fine if we will support it in all transports.
> I'd just like to understand if it's just TCP complying with it or I'm missing some check included in the socket layer that we could reuse.
Seems sock_poll() which is socket layer entry point for poll() doesn't contain any such checks
> @David, @Jakub, @Paolo, any advice?
> Thanks,
> Stefano

PS: moreover, i found one more interesting thing with TCP and poll: TCP receive logic wakes up poll waiter
only when number of available bytes > SO_RCVLOWAT. E.g. it prevents "spurious" wake ups, when poll will be
woken up because new data arrived, but POLLIN to allow user dequeue this data won't be set(as amount of data
is too small).
See tcp_data_ready() in net/ipv4/tcp_input.c


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