Re: [RFC PATCH 0/3] fprobe, rcu/tasks, rhashtable: Switch fprobe and BPF kprobe-multi to Tasks-Rude RCU

[Masami Hiramatsu (Google) <mhiramat@kernel.org>]

On Mon, 28 Sep 2026 09:25:02 -0700
"Paul E. McKenney" <paulmck@kernel.org> wrote:

> On Mon, Sep 28, 2026 at 11:29:13PM +0900, Masami Hiramatsu (Google) wrote:
> > Hi,
> > 
> > Here is an RFC patch series which removes standard RCU read lock
> > (guard(rcu) and rcu_read_lock()) from fprobe callback paths
> > by switching fprobe and BPF multi-kprobe to Tasks-Rude RCU.
> > 
> > Motivation & Problem
> > ====================
> > 
> > Currently, fprobe entry and exit callbacks (fprobe_fgraph_entry,
> > fprobe_return, and fprobe_ftrace_entry) execute in the tracing hot
> > path where preemption is disabled.
> > 
> > However, fprobe was forced to wrap hash lookups with guard(rcu)() and
> > rcu_read_lock() because:
> > 
> > - rhashtable defers bucket table deallocation using standard RCU
> > - unregister_fprobe() and BPF waited for a standard RCU grace period
> > 
> > Taking standard RCU read locks in the tracing fast path introduces
> > several drawbacks:
> > 
> > 1. Unnecessary Runtime Overhead:
> >    Every probe hit manipulates current->rcu_read_lock_nesting with
> >    memory barriers, and rcu_read_unlock() adds conditional branches
> >    to check for special quiescent processing.
> >    On debug kernels with CONFIG_PROVE_RCU=y or CONFIG_LOCKDEP=y,
> >    this additionally acquires and releases lockdep maps on every hit,
> >    introducing severe lockdep hashing overhead and tracer recursion risks.
> > 
> > 2. Fragile Dependency on rcu_is_watching():
> >    Standard RCU treats idle CPUs (and user-space on nohz_full CPUs) as
> >    Extended Quiescent States (EQS). If a function is traced while
> >    rcu_is_watching() is false, standard RCU is blind to the read-side
> >    critical section. In such contexts, synchronize_rcu() does not wait
> >    for the reader (risking Use-After-Free), and lockdep emits an
> >    "RCU-illegal: rcu_read_lock() used while not watching!" warning.
> >    To avoid this, ftrace callbacks normally require FTRACE_OPS_FL_RCU,
> >    adding extra trampoline check overhead.
> > 
> > 3. Asymmetric Synchronization with fprobe_return():
> >    fprobe_return() executes under preempt_disable_notrace() without
> >    holding rcu_read_lock(). Prior to this series, unregister_fprobe()
> >    only waited on synchronize_rcu(), which does NOT wait for pure
> >    preempt-disabled sections, leaving a potential Use-After-Free window
> >    during probe unregistration.
> > 
> > I've tried to fix the last UAF with simply introducing guard(rcu)()[1]
> > but Sashiko found the 2nd problem [2]. So I decided to implement this
> > series.
> 
> Unless I am missing something subtle, Sashiko needs to be taught a
> little bit more about RCU.  Preemption-disabled regions of code really
> are valid RCU readers.  If you have a reproducer showing that this is
> not the case in some situation, that would be a bug in RCU.

Thanks for the clarification!
So I think we can replace rcu_read_lock() with rcu_read_lock_sched_notrace()?
(also, maybe we need to add rcu_is_watching() check)
Current code may involves lockdep in fprobe, that is not good.

BTW, if we want to trace some code in extended quiescent state, do we
still need this Tasks-Rude RCU?

Thank you,

> 
> Adding Roman Gushchin and Chris Mason on CC for their thoughts.
> 
> 							Thanx, Paul
> 
> > [1] https://lore.kernel.org/all/179055575009.241711.6358052647499787191.stgit@devnote2/
> > [2] https://lore.kernel.org/all/20260928005114.9C9FC1F000FF@smtp.kernel.org/
> > 
> > 
> > Solution: Tasks-Rude RCU
> > ========================
> > 
> > Because fprobe callbacks already run strictly within preempt-disabled
> > contexts, we can transition fprobe and its deferred table reclamation
> > to Tasks-Rude RCU:
> > 
> > - Tasks-Rude RCU detects grace periods via schedule_on_each_cpu(),
> >   forcing a schedule on every online CPU. This guarantees that all
> >   preempt-disabled sections that began prior to the grace period have
> >   completed before memory is reclaimed.
> > - Unlike standard RCU, Tasks-Rude RCU does not rely on dyntick-idle /
> >   EQS tracking. It does NOT require rcu_is_watching() to be true and
> >   does not trigger lockdep warnings in pre-RCU/idle execution paths.
> > - Within fprobe, guard(rcu)() is replaced with guard(rcu_sched_notrace)(),
> >   reducing the lookup lock to pure, non-tracing preempt counter
> >   increments without lockdep or RCU state manipulation.
> > 
> > Feedback and suggestions from RCU, BPF, and tracing maintainers are welcome!
> > 
> > Thank you,
> > 
> > ---
> > base-commit: 5bfa9f1a9dcb6ecb607adbc1c0226605c972935b
> > 
> > Masami Hiramatsu (Google) (3):
> >       rcu/tasks: Export call_rcu_tasks_rude()
> >       rhashtable: Add use_tasks_rude parameter to defer bucket table free
> >       fprobe: Switch fprobe and BPF kprobe-multi to Tasks-Rude RCU
> > 
> > 
> >  include/linux/rcupdate.h         |    6 +++
> >  include/linux/rhashtable-types.h |    2 +
> >  kernel/bpf/syscall.c             |    2 +
> >  kernel/rcu/tasks.h               |    8 ++---
> >  kernel/trace/fprobe.c            |   66 +++++++++++++++++++++-----------------
> >  lib/rhashtable.c                 |    5 ++-
> >  6 files changed, 54 insertions(+), 35 deletions(-)
> > 
> > -- 
> > Masami Hiramatsu (Google) <mhiramat@kernel.org>


-- 
Masami Hiramatsu (Google) <mhiramat@kernel.org>