[PATCH v3 0/1] rcu: make userspace barrier hook drain kvfree_rcu work

[Matthias Goergens]

Extend rcutree.do_rcu_barrier to drain kfree_rcu() batches and per-CPU
SLUB sheaves as well as ordinary callbacks. This strengthens the documented
test interface, implementing the follow-up proposed when
kvfree_rcu_barrier() was added in 2024:

  https://lore.kernel.org/all/20240820155935.1167988-1-urezki@xxxxxxxxx/

While testing bcachefs performance changes, ktest's /proc/allocinfo check
falsely reported leaks despite repeated hook writes and a 20-second wait.
An earlier prototype let the same unmodified workload pass: eight
checkpoints in one VM, after 50 through 400 option changes, found zero
retained reconcile_scan objects. The original kernel's retained count
eventually fell as a sheaf filled; no unbounded growth or OOM was observed.

Following Paul's v2 review, retain the entry sequence snapshot and check
it after the unconditional kvfree_rcu() drain. If complete, retain smp_mb()
and skip the final ordinary barrier; otherwise, call rcu_barrier(). An
unrelated ordinary barrier cannot justify skipping the deferred-free
drain. The start-rate throttle remains unconditional. The documentation
now scopes completion to work queued before the request, without
preventing new concurrent work.

This can avoid an extra barrier operation: v2's trailing rcu_barrier()
takes a fresh snapshot and need not reuse the internal barrier. No
elapsed-time improvement has been measured.

Exact-v3 builds and interface smoke checks passed in four fresh two-vCPU
TREE_RCU VMs: two with kvfree batching and two with SLUB_TINY (unbatched).
Each completed three true requests, remained idle after false, and
rejected invalid input with EINVAL. Full kernel logs showed no WARN,
oops, panic or RCU-stall diagnostics. The v1/v2 private-cache completion
and ordinary-callback regression tests have not been rerun on v3. These
smoke tests neither prove concurrency/weak-memory correctness nor force
the guarded fallback.

v2:
https://lore.kernel.org/all/20260910170040.344864-1-matthias.goergens@xxxxxxxxx/

Matthias Goergens (1):
  rcu: make userspace barrier hook drain kvfree_rcu work

 .../admin-guide/kernel-parameters.txt         |  9 ++++--
 kernel/rcu/tree.c                             | 30 ++++++++++++-------
 2 files changed, 26 insertions(+), 13 deletions(-)

-- 
2.55.0