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Arseniy Krasnov says:
====================
vsock: updates for SO_RCVLOWAT handling
This patchset includes some updates for SO_RCVLOWAT:
1) af_vsock:
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.
2) virtio/vsock:
It adds some optimization to wake ups, when new data arrived. Now,
SO_RCVLOWAT is considered before wake up sleepers who wait new data.
There is no sense, to kick waiter, when number of available bytes
in socket's queue < SO_RCVLOWAT, because if we wake up reader in
this case, it will wait for SO_RCVLOWAT data anyway during dequeue,
or in poll() case, POLLIN/POLLRDNORM bits won't be set, so such
exit from poll() will be "spurious". This logic is also used in TCP
sockets.
3) vmci/vsock:
Same as 2), but i'm not sure about this changes. Will be very good,
to get comments from someone who knows this code.
4) Hyper-V:
As Dexuan Cui mentioned, for Hyper-V transport it is difficult to
support SO_RCVLOWAT, so he suggested to disable this feature for
Hyper-V.
====================
Link: https://lore.kernel.org/r/de41de4c-0345-34d7-7c36-4345258b7ba8@sberdevices.ru
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Linux kernel
============
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Please read the Documentation/process/changes.rst file, as it contains the
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