[bpf-next,v3,0/5] Free htab element out of bucket lock

Message

Hou Tao Jan. 17, 2025, 10:18 a.m. UTC

From: Hou Tao <[e-mail]>

Hi,

The patch set continues the previous work [1] to move all the freeings
of htab elements out of bucket lock. One motivation for the patch set is
the locking problem reported by Sebastian [2]: the freeing of bpf_timer
under PREEMPT_RT may acquire a spin-lock (namely softirq_expiry_lock).
However the freeing procedure for htab element has already held a
raw-spin-lock (namely bucket lock), and it will trigger the warning:
"BUG: scheduling while atomic" as demonstrated by the selftests patch.
Another motivation is to reduce the locked scope of bucket lock.

However, the patch set doesn't move all freeing of htab element out of
bucket lock, it still keep the free of special fields in pre-allocated
hash map under the protect of bucket lock in htab_map_update_elem(). The
patch set is structured as follows:

* Patch #1 moves the element freeing out of bucket lock for
  htab_lru_map_delete_node(). However the freeing is still in the locked
  scope of LRU raw spin lock.
* Patch #2~#3 move the element freeing out of bucket lock for
  __htab_map_lookup_and_delete_elem()
* Patch #4 cancels the bpf_timer in two steps to fix the locking
  problem in htab_map_update_elem() for PREEMPT_PRT.
* Patch #5 adds a selftest for the locking problem

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

---

v3:
 * patch #1: update the commit message to state that the freeing of
   special field is still in the locked scope of LRU raw spin lock
 * patch #4: cancel the bpf_timer in two steps only for PREEMPT_RT
   (suggested by Alexei)

v2: https://lore.kernel.org/bpf/[e-mail]
  * cancels the bpf timer in two steps instead of breaking the reuse
    the refill of per-cpu ->extra_elems into two steps

v1: https://lore.kernel.org/bpf/[e-mail]

[1]: https://lore.kernel.org/bpf/[e-mail]
[2]: https://lore.kernel.org/bpf/[e-mail]

Hou Tao (5):
  bpf: Free special fields after unlock in htab_lru_map_delete_node()
  bpf: Bail out early in __htab_map_lookup_and_delete_elem()
  bpf: Free element after unlock in __htab_map_lookup_and_delete_elem()
  bpf: Cancel the running bpf_timer through kworker for PREEMPT_RT
  selftests/bpf: Add test case for the freeing of bpf_timer

 kernel/bpf/hashtab.c                          |  60 ++++---
 kernel/bpf/helpers.c                          |  18 +-
 .../selftests/bpf/prog_tests/free_timer.c     | 165 ++++++++++++++++++
 .../testing/selftests/bpf/progs/free_timer.c  |  71 ++++++++
 4 files changed, 284 insertions(+), 30 deletions(-)
 create mode 100644 tools/testing/selftests/bpf/prog_tests/free_timer.c
 create mode 100644 tools/testing/selftests/bpf/progs/free_timer.c