The bcachefs ktest allocation-leak check writes rcutree.do_rcu_barrier before reading /proc/allocinfo. While testing bcachefs performance changes, small objects released with kfree_rcu() remained visible after repeated writes to the hook and 20 seconds of waiting, causing otherwise clean tests to fail their leak check.
The test assumes a stronger contract than the hook currently documents: rcu_barrier() waits for ordinary callbacks, but does not flush objects still held in kfree_rcu() batching or per-CPU SLUB sheaves. The retained population eventually fell as a sheaf filled; there is no evidence here of unbounded growth or OOM. Changing the hook to drain kvfree_rcu() work let the same unmodified bcachefs workload pass its allocation check. All eight checkpoints in one VM, after 50 through 400 option changes, reported zero retained reconcile_scan objects. This motivated the separate private-cache test used to isolate the incomplete drain from bcachefs. Calling kvfree_rcu_barrier() from rcu_barrier_throttled() was proposed when kvfree_rcu_barrier() was added in 2024, to restore a clean baseline between userspace benchmark runs. The discussion concluded that keeping the existing hook name, adding the second operation and documenting both was the safest compatibility choice, but the follow-up was not added. Add that drain and document the stronger test interface. Always retain the existing start-rate limit and perform the kvfree_rcu() drain: an unrelated ordinary barrier does not establish that this work completed. Retain the entry ordinary-barrier sequence snapshot. After draining, skip the final ordinary barrier only if that snapshot is complete, preserving the memory barrier on the completion path. Otherwise, invoke rcu_barrier() explicitly. This keeps the ordinary-callback guarantee independent of whether kvfree_rcu_barrier() embeds an ordinary barrier. Clarify that the documented completion guarantee covers work queued before the request, without preventing new work from being queued. Earlier validation of the unconditional-drain version used four fresh VM pairs with a private-cache fixture: controls retained the queued object (60 to 60 active objects), and treatments drained it (60 to 59). An ordinary-callback test passed on both kernels. Those runs predated the guarded skip and do not validate that change. No elapsed-time improvement is claimed. Link: https://lore.kernel.org/all/[email protected]/ Signed-off-by: Matthias Goergens <[email protected]> --- .../admin-guide/kernel-parameters.txt | 9 ++++-- kernel/rcu/tree.c | 30 ++++++++++++------- 2 files changed, 26 insertions(+), 13 deletions(-) diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt index 68647ff4bdd2..914b65ae9413 100644 --- a/Documentation/admin-guide/kernel-parameters.txt +++ b/Documentation/admin-guide/kernel-parameters.txt @@ -5699,9 +5699,12 @@ Kernel parameters there is an ongoing too-long CSD-lock wait. rcutree.do_rcu_barrier= [KNL] - Request a call to rcu_barrier(). This is - throttled so that userspace tests can safely - hammer on the sysfs variable if they so choose. + Wait for deferred kfree_rcu() frees and ordinary + call_rcu() callbacks queued before this request to + complete. This does not prevent new work from being + queued concurrently. Requests are throttled so that + userspace tests can safely hammer on the sysfs + variable if they so choose. If triggered before the RCU grace-period machinery is fully active, this will error out with EAGAIN. diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c index 96848fc1f02b..93b71682306c 100644 --- a/kernel/rcu/tree.c +++ b/kernel/rcu/tree.c @@ -3989,12 +3989,12 @@ EXPORT_SYMBOL_GPL(rcu_barrier); static unsigned long rcu_barrier_last_throttle; /** - * rcu_barrier_throttled - Do rcu_barrier(), but limit to one per second + * rcu_barrier_throttled - Drain deferred RCU frees, but rate-limit starts * - * This can be thought of as guard rails around rcu_barrier() that - * permits unrestricted userspace use, at least assuming the hardware's - * try_cmpxchg() is robust. There will be at most one call per second to - * rcu_barrier() system-wide from use of this function, which means that + * This can be thought of as guard rails around the deferred-free barriers + * that permit unrestricted userspace use, at least assuming the hardware's + * try_cmpxchg() is robust. There will be at most one drain operation started + * per sixteenth of a second from use of this function, which means that * callers might needlessly wait a second or three. * * This is intended for use by test suites to avoid OOM by flushing RCU @@ -4016,14 +4016,24 @@ static void rcu_barrier_throttled(void) while (time_in_range(j, old, old + HZ / 16) || !try_cmpxchg(&rcu_barrier_last_throttle, &old, j)) { schedule_timeout_idle(HZ / 16); - if (rcu_seq_done(&rcu_state.barrier_sequence, s)) { - smp_mb(); /* caller's subsequent code after above check. */ - return; - } j = jiffies; old = READ_ONCE(rcu_barrier_last_throttle); } - rcu_barrier(); + /* + * kfree_rcu() can retain objects outside the ordinary callback lists in + * per-CPU SLUB sheaves and kvfree_rcu batches. Always drain those queues: + * an ordinary barrier does not establish that this work was drained. + */ + kvfree_rcu_barrier(); + /* + * A completed barrier can still cover ordinary callbacks queued before + * our entry snapshot. Otherwise, retain an explicit ordinary barrier + * without depending on the implementation of kvfree_rcu_barrier(). + */ + if (rcu_seq_done(&rcu_state.barrier_sequence, s)) + smp_mb(); /* caller's subsequent code after above check. */ + else + rcu_barrier(); } /* -- 2.55.0
