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 and 2 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, and patch 2 adds bh context as a valid condition for map lookups. Patch 3 is the main bit that adds the __rcu annotations and updates documentation comments, 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/20210415173551.7ma4slcbqeyiba2r@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxx/ [1] https://lore.kernel.org/bpf/c5192ab3-1c05-8679-79f2-59d98299095b@xxxxxxxxxxxxx/ [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: 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 (1): rcu: Create an unrcu_pointer() to remove __rcu from a pointer Toke Høiland-Jørgensen (15): bpf: allow RCU-protected lookups to happen from bh context xdp: add proper __rcu annotations to redirect map entries 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 drivers/net/ethernet/amazon/ena/ena_netdev.c | 6 +-- drivers/net/ethernet/broadcom/bnxt/bnxt_xdp.c | 5 +- .../net/ethernet/cavium/thunder/nicvf_main.c | 5 +- .../net/ethernet/freescale/dpaa/dpaa_eth.c | 11 ++-- .../net/ethernet/freescale/dpaa2/dpaa2-eth.c | 6 +-- drivers/net/ethernet/intel/i40e/i40e_txrx.c | 5 +- drivers/net/ethernet/intel/i40e/i40e_xsk.c | 11 ++-- 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 | 5 +- drivers/net/ethernet/intel/igc/igc_main.c | 10 ++-- drivers/net/ethernet/intel/ixgbe/ixgbe_main.c | 5 +- drivers/net/ethernet/intel/ixgbe/ixgbe_xsk.c | 9 ++-- .../net/ethernet/intel/ixgbevf/ixgbevf_main.c | 5 +- drivers/net/ethernet/marvell/mvneta.c | 6 ++- .../net/ethernet/marvell/mvpp2/mvpp2_main.c | 8 +-- drivers/net/ethernet/mellanox/mlx4/en_rx.c | 8 +-- .../ethernet/netronome/nfp/nfp_net_common.c | 6 ++- drivers/net/ethernet/qlogic/qede/qede_fp.c | 7 +-- drivers/net/ethernet/sfc/rx.c | 12 ++--- drivers/net/ethernet/socionext/netsec.c | 7 +-- .../net/ethernet/stmicro/stmmac/stmmac_main.c | 13 ++--- drivers/net/ethernet/ti/cpsw_priv.c | 13 ++--- include/linux/rcupdate.h | 14 +++++ include/net/xdp_sock.h | 2 +- kernel/bpf/cpumap.c | 13 +++-- kernel/bpf/devmap.c | 52 ++++++++----------- kernel/bpf/hashtab.c | 21 +++++--- kernel/bpf/helpers.c | 6 +-- kernel/bpf/lpm_trie.c | 6 ++- net/core/filter.c | 28 ++++++++++ net/xdp/xsk.c | 4 +- net/xdp/xsk.h | 4 +- net/xdp/xskmap.c | 29 ++++++----- 34 files changed, 196 insertions(+), 158 deletions(-) -- 2.31.1
