[bpf-next,0/3] bpf: Overwrite the htab element atomically

Message

Hou Tao Feb. 4, 2025, 8:28 a.m. UTC

Hi,

The motivation for the patch set comes from the question raised by Cody
Haas [1]. He asked whether or not the update of htab of map is atomic in
the perspective of lookup operation. Currently, the update is not atomic
because the overwrite of existing element happens in a two-steps way,
but the support of atomic update is feasible. Initiallly, I only plan to
support atomic update for htab of map because array of map has already
supported that. Afterwards, I think it may be reasonable to support
atomic update for all kinds of hash map. However, for the BPF_F_LOCK
case, although the update is protected by a spin-lock, the update is
still not atomic in the perspective of the lookup operation.

Please see individual patches for details. Comments are always welcome.

---

[1] : https://lore.kernel.org/xdp-newbies/[e-mail]/T/#m06fcd687c6cfdbd0f9b643b227e69b479fc8c2f6

Hou Tao (3):
  rculist: add hlist_nulls_replace_rcu() helper
  bpf: Overwrite the element in hash map atomically
  selftests/bpf: Add test case for atomic htab update

 include/linux/rculist_nulls.h                 |  42 ++++++
 kernel/bpf/hashtab.c                          |  14 +-
 .../selftests/bpf/prog_tests/htab_lookup.c    | 130 ++++++++++++++++++
 .../testing/selftests/bpf/progs/htab_lookup.c |  13 ++
 4 files changed, 193 insertions(+), 6 deletions(-)
 create mode 100644 tools/testing/selftests/bpf/prog_tests/htab_lookup.c
 create mode 100644 tools/testing/selftests/bpf/progs/htab_lookup.c