A Netlink socket that receives asynchronous notifications (e.g. from a
multicast group) cannot be used for transactions or dumps, because those
operations would discard asynchronous messages that arrive while waiting
for replies.
This commit documents this issue in a comment on nl_sock_join_mcgroup().
It also removes an internal attempt to avoid mixing multicast reception
with other operations. The attempt was incomplete, because it only
handled dumps even though ordinary transactions are also problematic. It
seems better to remove it than to fix it because, first, all of the
existing users in OVS already separate multicast reception from other
operations and, second, an upcoming commit will start using unicast
Netlink for asynchronous notifications, which has the same issues but
doesn't use nl_sock_join_mcgroup().
The nl_lookup_genl_mcgroup() function can fail on older kernels
which do not support the required netlink interface. Before this
patch, dpif-linux would refuse to create a datapath when this
happened. With this patch, it attempts to use a workaround. If
the workaround fails it simply disables the affected features
without completely disabling the dpif.
Commit e408762f "netlink-socket: New function nl_lookup_genl_mcgroup()"
modified do_lookup_genl_family() to return the Netlink attributes to the
caller, but it still freed the Netlink message itself, which meant that
the attributes pointed into freed memory. This commit fixes the problem.
This commit is not a minimal fix. It refactors do_lookup_genl_family(),
changing the return value from "negative errno value or positive genl
family id" to the more common "zero or positive errno value".
Found by valgrind.
Until now, each attempt to receive a message from a Netlink socket has
taken at least two system calls, one to check the size of the message to
be received and a second one to delete the message from the socket buffer.
This commit switches to a new strategy that requires only one system call
per message received.
In my testing this increases the maximum flow setups per second by a little
over 10%.
The ids for Generic Netlink family names aren't very helpful because they
can vary from machine to machine and even from one boot to the next. So
this change logs their names too.
This only affects logging at DBG level.
Reviewed by Ethan Jackson <ethan@nicira.com>.
nlmsghdr_to_string() wrote sequence numbers in hex, but nl_sock_transact()
wrote them in decimal. This consistently switches to hexadecimal.
Reviewed by Ethan Jackson <ethan@nicira.com>.
It's not safe to use a single Netlink fd to do multiple operations in an
synchronous way. Some of the limitations are fundamental; for example, the
kernel only supports a single "dump" operation at a time. Others are
limitations imposed by the OVS coding style; for example, our Netlink
library is not callback based, so nothing can be done about incoming
messages that can't be handled immediately. Regardless, in OVS multicast
groups, transactions, and dumps cannot coexist on a single nl_sock.
This is only mildly irritating at the moment, but it will become much worse
later on, when dpif-linux shifts to using Netlink dumps for listing various
kinds of datapath entities. When that happens, a dump will be in progress
in situations where the dpif-linux client might want to do other
operations. For example, it is reasonable for the client to list flows
and, in the middle, look up information on vports mentioned in those flows.
It might be possible to simply ban and avoid such nested operations--I have
not even audited the source tree to find out whether we do anything like
that already--but that seems like an unnecessary cramp on our coding style.
Furthermore, it's difficult to explain and justify without understanding
the implementation.
This patch takes another approach, by improving the Netlink socket library
to avoid artificial constraints. When an operation, or a dump, or joining
a multicast group would cause a problem, this patch makes the library
transparently create a separate Netlink socket. This solves the problem
without putting any onerous restrictions on use.
This commit also slightly simplifies netdev_vport_reset_names(). It had
been written to destroy the dump object before the Netlink socket that it
used, but this is no longer necessary and doing it in the opposite order
saved a few lines of code.
Reviewed by Ethan Jackson <ethan@nicira.com>.
When this library was originally implemented, support for Linux 2.4 was
important. The Netlink implementation in Linux only added support for
joining and leaving multicast groups after a socket is bound as of Linux
2.6.14, so the library did not support it either. But the current version
of Open vSwitch targets Linux 2.6.18 and over, so it's fine to add this
support now, and this commit does so.
This will be used more extensively in upcoming commits.
Reviewed by Justin Pettit.
The parts of the netlink module that are related to sockets are
Linux-specific, since only Linux has AF_NETLINK sockets. The rest can be
built anywhere. This commit breaks them into two modules, and builds the
generic one on all platforms.
Acked-by: Jesse Gross <jesse@nicira.com>
