Messages in this thread | | | From | Arseniy Krasnov <> | Subject | [RFC PATCH v1 0/3] virtio/vsock: use SO_RCVLOWAT to set POLLIN/POLLRDNORM | Date | Mon, 18 Jul 2022 08:12:52 +0000 |
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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 https://www.open-std.org/jtc1/sc22/open/n4217.pdf, 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've added some fixes to af_vsock.c and virtio_transport_common.c, test is also implemented.
What do You think guys?
Thank You
Arseniy Krasnov(3): vsock_test: POLLIN + SO_RCVLOWAT test. virtio/vsock: use 'target' in notify_poll_in callback. vsock: use sk_skrcvlowat to set POLLIN,POLLRDNORM bits.
net/vmw_vsock/af_vsock.c | 2 +- net/vmw_vsock/virtio_transport_common.c | 2 +- tools/testing/vsock/vsock_test.c | 90 +++++++++++++++++++++++++++++++++ 3 files changed, 92 insertions(+), 2 deletions(-)
-- 2.25.1
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