[bpf-next,v4,00/19] Clean up and document RCU-based object protection for XDP and TC BPF

Message

Toke Høiland-Jørgensen June 23, 2021, 11:07 a.m. UTC

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.

It seems[1] that back in the early days of XDP, local_bh_disable() did not
provide RCU protection, which is why the rcu_read_lock() calls were added
to drivers in the first place. But according to Paul[2], in recent kernels
a local_bh_disable()/local_bh_enable() pair functions as one big RCU
read-side section, so no further protection is needed. This even applies to
-rt kernels, which has an explicit rcu_read_lock() in place as part of the
local_bh_disable()[3].

This patch series is mostly a documentation exercise, cleaning up the
description of the lifetime expectations and adding __rcu annotations so
sparse and lockdep can help verify it.

Patches 1-4 are preparatory: Patch 1 adds Paul's unrcu_pointer()
helper (which has already been added to his tree), which we need for some
of the operations in devmap, patches 2 and 3 update the RCU documentation
and patch 4 adds bh context as a valid condition for map lookups. Patch 5
is the main bit that adds the __rcu annotations and updates documentation
comments. Finally, patch 6 removes unneeded rcu_read_lock()s from TC BPF,
and the rest are patches updating the drivers, with one patch per distinct
maintainer.

Unfortunately I don't have any hardware to test any of the driver patches;
Jesper helpfully verified that it doesn't break anything on i40e, but the rest
of the driver patches are only compile-tested.

[0] https://lore.kernel.org/bpf/[e-mail]/
[1] https://lore.kernel.org/bpf/[e-mail]/
[2] https://lore.kernel.org/bpf/20210417002301.GO4212@paulmck-ThinkPad-P17-Gen-1/
[3] https://lore.kernel.org/bpf/20210419165837.GA975577@paulmck-ThinkPad-P17-Gen-1/

Changelog:
v4:
  - Move comment about RCU protection into core instead of leaving it in
    drivers
  - Also remove rcu_read_lock() around TC BPF program execution
  - Fold in a couple of patches from Paul updating the RCU documentation
v3:
  - Remove one other unnecessary change to hlist_for_each_entry_rcu()
  - Carry forward another ACK
v2:
  - Add a comment about RCU protection to the drivers where rcu_read_lock()
    is removed
  - Drop unnecessary patch 3 which changed dev_get_by_index_rcu()
  - Add some more text with the history to cover letter
  - Fix a few places where the wrong RCU checks were used in cpumap and
    xskmap code
  - Carry forward ACKs

Paul E. McKenney (2):
  rcu: Create an unrcu_pointer() to remove __rcu from a pointer
  doc: Clarify and expand RCU updaters and corresponding readers

Toke Høiland-Jørgensen (17):
  doc: Give XDP as example of non-obvious RCU reader/updater pairing
  bpf: allow RCU-protected lookups to happen from bh context
  xdp: add proper __rcu annotations to redirect map entries
  sched: remove unneeded rcu_read_lock() around BPF program invocation
  ena: remove rcu_read_lock() around XDP program invocation
  bnxt: remove rcu_read_lock() around XDP program invocation
  thunderx: remove rcu_read_lock() around XDP program invocation
  freescale: remove rcu_read_lock() around XDP program invocation
  net: intel: remove rcu_read_lock() around XDP program invocation
  marvell: remove rcu_read_lock() around XDP program invocation
  mlx4: remove rcu_read_lock() around XDP program invocation
  nfp: remove rcu_read_lock() around XDP program invocation
  qede: remove rcu_read_lock() around XDP program invocation
  sfc: remove rcu_read_lock() around XDP program invocation
  netsec: remove rcu_read_lock() around XDP program invocation
  stmmac: remove rcu_read_lock() around XDP program invocation
  net: ti: remove rcu_read_lock() around XDP program invocation

 Documentation/RCU/checklist.rst               | 55 ++++++++++++-------
 drivers/net/ethernet/amazon/ena/ena_netdev.c  |  3 -
 drivers/net/ethernet/broadcom/bnxt/bnxt_xdp.c |  2 -
 .../net/ethernet/cavium/thunder/nicvf_main.c  |  2 -
 .../net/ethernet/freescale/dpaa/dpaa_eth.c    |  8 +--
 .../net/ethernet/freescale/dpaa2/dpaa2-eth.c  |  3 -
 drivers/net/ethernet/intel/i40e/i40e_txrx.c   |  2 -
 drivers/net/ethernet/intel/i40e/i40e_xsk.c    |  6 +-
 drivers/net/ethernet/intel/ice/ice_txrx.c     |  6 +-
 drivers/net/ethernet/intel/ice/ice_xsk.c      |  6 +-
 drivers/net/ethernet/intel/igb/igb_main.c     |  2 -
 drivers/net/ethernet/intel/igc/igc_main.c     |  7 +--
 drivers/net/ethernet/intel/ixgbe/ixgbe_main.c |  2 -
 drivers/net/ethernet/intel/ixgbe/ixgbe_xsk.c  |  6 +-
 .../net/ethernet/intel/ixgbevf/ixgbevf_main.c |  2 -
 drivers/net/ethernet/marvell/mvneta.c         |  2 -
 .../net/ethernet/marvell/mvpp2/mvpp2_main.c   |  4 --
 drivers/net/ethernet/mellanox/mlx4/en_rx.c    |  8 +--
 .../ethernet/netronome/nfp/nfp_net_common.c   |  2 -
 drivers/net/ethernet/qlogic/qede/qede_fp.c    |  6 --
 drivers/net/ethernet/sfc/rx.c                 |  9 +--
 drivers/net/ethernet/socionext/netsec.c       |  3 -
 .../net/ethernet/stmicro/stmmac/stmmac_main.c | 10 +---
 drivers/net/ethernet/ti/cpsw_priv.c           | 10 +---
 include/linux/filter.h                        | 10 ++--
 include/linux/rcupdate.h                      | 14 +++++
 include/net/xdp_sock.h                        |  2 +-
 kernel/bpf/cpumap.c                           | 13 +++--
 kernel/bpf/devmap.c                           | 49 +++++++----------
 kernel/bpf/hashtab.c                          | 21 ++++---
 kernel/bpf/helpers.c                          |  6 +-
 kernel/bpf/lpm_trie.c                         |  6 +-
 net/core/filter.c                             | 28 ++++++++++
 net/sched/act_bpf.c                           |  2 -
 net/sched/cls_bpf.c                           |  3 -
 net/xdp/xsk.c                                 |  4 +-
 net/xdp/xsk.h                                 |  4 +-
 net/xdp/xskmap.c                              | 29 ++++++----
 38 files changed, 168 insertions(+), 189 deletions(-)