Merge branch 'rtnetlink-per-netns-rtnl'

Kuniyuki Iwashima says:

====================
rtnetlink: Per-netns RTNL.

rtnl_lock() is a "Big Kernel Lock" in the networking slow path and
serialised all rtnetlink requests until 4.13.

Since RTNL_FLAG_DOIT_UNLOCKED and RTNL_FLAG_DUMP_UNLOCKED have been
introduced in 4.14 and 6.9, respectively, rtnetlink message handlers
are ready to be converted to RTNL-less/free.

15 out of 44 dumpit()s have been converted to RCU so far, and the
progress is pretty good.  We can now dump various major network
resources without RTNL.

12 out of 87 doit()s have been converted, but most of the converted
doit()s are also on the reader side of RTNL; their message types are
RTM_GET*.

So, most of RTM_(NEW|DEL|SET)* operations are still serialised by RTNL.

For example, one of our services creates 2K netns and a small number
of network interfaces in each netns that require too many writer-side
rtnetlink requests, and setting up a single host takes 10+ minutes.

RTNL is still a huge pain for network configuration paths, and we need
more granular locking, given converting all doit()s would be unfeasible.

Actually, most RTNL users do not need to freeze multiple netns, and such
users can be protected by per-netns RTNL mutex.  The exceptions would be
RTM_NEWLINK, RTM_DELLINK, and RTM_SETLINK.  (See [0] and [1])

This series is the first step of the per-netns RTNL conversion that
gradually replaces rtnl_lock() with rtnl_net_lock(net) under
CONFIG_DEBUG_NET_SMALL_RTNL.

[0]: https://netdev.bots.linux.dev/netconf/2024/index.html
[1]: https://lpc.events/event/18/contributions/1959/
====================

Link: https://patch.msgid.link/20241004221031.77743-1-kuniyu@amazon.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

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