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@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxx/ --- 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(-)