netdev - [PATCH RFC bpf-next 0/4] Clean up and document RCU-based object protection for XDP

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Message-ID: <161917591559.102337.3558507780042453425.stgit@toke.dk>
Date:   Fri, 23 Apr 2021 13:05:15 +0200
From:   Toke Høiland-Jørgensen <toke@...hat.com>
To:     bpf@...r.kernel.org
Cc:     netdev@...r.kernel.org, Martin KaFai Lau <kafai@...com>,
        Hangbin Liu <liuhangbin@...il.com>,
        Jesper Dangaard Brouer <hawk@...nel.org>,
        Magnus Karlsson <magnus.karlsson@...il.com>,
        "Paul E. McKenney" <paulmck@...nel.org>
Subject: [PATCH RFC bpf-next 0/4] Clean up and document RCU-based object
 protection for XDP_REDIRECT

During the discussion[0] of Hangbin's multicast patch series, Martin pointed out
that the lifetime of the RCU-protected  map entries used by XDP_REDIRECT is by
no means obvious. I promised to look into cleaning this up, and Paul helpfully
provided some hints and a new unrcu_pointer() helper to aid in this.

This is mostly a documentation exercise, clearing up the description of the
lifetime expectations and adding __rcu annotations so sparse and lockdep can
help verify it. I'm sending this as RFC since I don't have any i40e hardware to
test on. A complete submission would also involve going through all the drivers,
of course, but I wanted to get some feedback onthis first. I did test on mlx5,
but that uses an rhashtable in the driver code, so we can't actually remove the
top-level rcu_read_lock() from that without getting lockdep splats.

Patches 1-2 are prepatory: Patch 1 adds Paul's unrcu_pointer() helper and patch
2 is a small fix for dev_get_by_index_rcu() so lockdep understands _bh-disabled
access to it. Patch 3 is the main bit that adds the __rcu annotations and
updates documentation comments, and patch 4 is an example of driver changes,
removing the rcu_read_lock() from i40e.

Please take a look, and let me know if you think this is the right direction for
clarifying the usage.

Thanks,
-Toke

[0] https://lore.kernel.org/bpf/20210415173551.7ma4slcbqeyiba2r@kafai-mbp.dhcp.thefacebook.com/

---

Paul E. McKenney (1):
      rcu: Create an unrcu_pointer() to remove __rcu from a pointer

Toke Høiland-Jørgensen (3):
      dev: add rcu_read_lock_bh_held() as a valid check when getting a RCU dev ref
      xdp: add proper __rcu annotations to redirect map entries
      i40e: remove rcu_read_lock() around XDP program invocation

 drivers/net/ethernet/intel/i40e/i40e_txrx.c |  2 -
 drivers/net/ethernet/intel/i40e/i40e_xsk.c  |  6 +--
 include/net/xdp_sock.h                      |  2 +-
 kernel/bpf/cpumap.c                         | 14 ++++--
 kernel/bpf/devmap.c                         | 52 +++++++++------------
 net/core/dev.c                              |  2 +-
 net/core/filter.c                           | 28 +++++++++++
 net/xdp/xsk.c                               |  4 +-
 net/xdp/xsk.h                               |  4 +-
 net/xdp/xskmap.c                            | 29 +++++++-----
 10 files changed, 85 insertions(+), 58 deletions(-)