Re: [PATCH 1/1] rcu: drain kfree_rcu sheaves from the userspace barrier hook

[Harry Yoo <harry@kernel.org>]

On Thu, Sep 10, 2026 at 06:11:12PM +0800, Matthias Goergens wrote:
> The rcutree.do_rcu_barrier test hook is intended to prevent deferred RCU
> callbacks from one stress test spilling into the next. Since kfree_rcu()
> sheaves were added, an object can remain deferred without appearing on an
> ordinary RCU callback list. rcu_barrier() therefore no longer fulfils the
> hook's stated purpose by itself.
>
> Drain kfree_rcu sheaves and kvfree_rcu batches before completing the
> ordinary RCU barrier.

I'm convinced that the behavior of "do rcu_barrier()" knob
to imply a kvfree_rcu_barrier() is the right fix.

And also I wonder what's the user-facing problem you are trying to fix.
You can't pile up unbounded amount of objects via kvfree_rcu() to cause
an OOM during the userspace tests?

How did you discover the problem?

> Keep the explicit rcu_barrier() because the hook's
> original ordinary-callback contract should not depend on the current,
> undocumented fact that kvfree_rcu_barrier() includes one internally.
> 
> Keep the existing throttling because this remains a deliberately expensive
> test-only action. Do not coalesce requests based on the ordinary
> rcu_barrier() sequence: an unrelated ordinary barrier does not prove that
> sheaves were drained.
> 
> A reproducer creates a private SLAB_NO_MERGE cache whose first allocation
> populates a 60-object slab. It queues that object with kfree_rcu(), invokes
> the hook, and reads the active-object count from /proc/slabinfo. In four
> fresh VM pairs, the parent retained the object (60 to 60). The patched hook
> drained it (60 to 59).
> 
> Fixes: ec66e0d59952 ("slab: add sheaf support for batching kfree_rcu() operations")

This Fixes: commit is incorrect because the problem was introduced
by kvfree_rcu(), not sheaves.

> Signed-off-by: Matthias Goergens <matthias.goergens@gmail.com>
> ---
>  .../admin-guide/kernel-parameters.txt         |  7 ++---
>  kernel/rcu/tree.c                             | 27 ++++++++++++-------
>  2 files changed, 21 insertions(+), 13 deletions(-)
> 

-- 
Cheers,
Harry / Hyeonggon