[PATCH bpf-next v6 00/10] bpf: Avoid locks in bpf_timer and bpf_wq

[Mykyta Yatsenko <mykyta.yatsenko5@gmail.com>]

This series reworks implementation of BPF timer and workqueue APIs.
The goal is to make both timers and wq non-blocking, enabling their use
in NMI context.
Today this code relies on a bpf_spin_lock embedded in the map element to
serialize:
 * init of the async object,
 * setting/changing the callback and bpf_prog
 * starting/cancelling the timer/work
 * tearing down when the map element is deleted or the map’s user ref is
 dropped

The basic design approach in this series:
 * Use irq_work to offload all blocking work from NMI
 * Introduce refcount to guarantee lifetime of the bpf_async_cb structs
 deferred to potentially multiple irq_work callbacks
 * Keep objects under RCU protection to make sure they are not freed
 while kfuncs/helpers access them (We can't use refcnt for this, as
 refcnt itself is part of the bpf_async_cb struct)

Signed-off-by: Mykyta Yatsenko <yatsenko@meta.com>
---
Changes in v6:
- Reworked destruction and refcnt use:
  - On cancel_and_free() set last_seq to BPF_ASYNC_DESTROY value, drop
    map's reference
  - In irq work callback, atomically switch DESTROY to DESTROYED, cancel
    timer/wq
  - Free bpf_async_cb on refcnt going to 0.
- Link to v5: https://lore.kernel.org/r/20260115-timer_nolock-v5-0-15e3aef2703d@meta.com

Changes in v5:
- Extracted lock-free algorithm for updating cb->prog and
cb->callback_fn into a function bpf_async_update_prog_callback(),
added a new commit and introduces this function and uses it in
__bpf_async_set_callback(), bpf_timer_cancel() and
bpf_async_cancel_and_free().
This allows to move the change into the separate commit without breaking
correctness.
- Handle NULL prog in bpf_async_update_prog_callback().
- Link to v4: https://lore.kernel.org/r/20260114-timer_nolock-v4-0-fa6355f51fa7@meta.com

Changes in v4:
- Handle irq_work_queue failures in both schedule and cancel_and_free
paths: introduced bpf_async_refcnt_dec_cleanup() that decrements refcnt
and makes sure if last reference is put, there is at least one irq_work
scheduled to execute final cleanup.
- Additional refcnt inc/dec in set_callback() + rcu lock to make sure
cleanup is not running at the same time as set_callback().
- Added READ_ONCE where it was needed.
- Squash 'bpf: Refactor __bpf_async_set_callback()' commit into 'bpf:
Add lock-free cell for NMI-safe
async operations'
- Removed mpmc_cell, use seqcount_latch_t instead.
- Link to v3: https://lore.kernel.org/r/20260107-timer_nolock-v3-0-740d3ec3e5f9@meta.com

Changes in v3:
- Major rework
- Introduce mpmc_cell, allowing concurrent writes and reads
- Implement irq_work deferring
- Adding selftests
- Introduces bpf_timer_cancel_async kfunc
- Link to v2: https://lore.kernel.org/r/20251105-timer_nolock-v2-0-32698db08bfa@meta.com

Changes in v2:
- Move refcnt initialization and put (from cancel_and_free())
from patch 5 into the patch 4, so that patch 4 has more clear and full
implementation and use of refcnt
- Link to v1: https://lore.kernel.org/r/20251031-timer_nolock-v1-0-b064ae403bfb@meta.com

---
Mykyta Yatsenko (10):
      bpf: Factor out timer deletion helper
      bpf: Remove unnecessary arguments from bpf_async_set_callback()
      bpf: Introduce lock-free bpf_async_update_prog_callback()
      bpf: Simplify bpf_timer_cancel()
      bpf: Enable bpf timer and workqueue use in NMI
      bpf: Add verifier support for bpf_timer argument in kfuncs
      bpf: Introduce bpf_timer_cancel_async() kfunc
      selftests/bpf: Refactor timer selftests
      selftests/bpf: Add stress test for timer async cancel
      selftests/bpf: Verify bpf_timer_cancel_async works

 kernel/bpf/helpers.c                           | 526 ++++++++++++++-----------
 kernel/bpf/verifier.c                          |  59 ++-
 tools/testing/selftests/bpf/prog_tests/timer.c |  92 ++++-
 tools/testing/selftests/bpf/progs/timer.c      |  37 +-
 4 files changed, 464 insertions(+), 250 deletions(-)
---
base-commit: efad162f5a840ae178e7761c176c49f433c7bb68
change-id: 20251028-timer_nolock-457f5b9daace

Best regards,
-- 
Mykyta Yatsenko <yatsenko@meta.com>