On Tue, Sep 29, 2026 at 08:28:28AM +0900, Masami Hiramatsu wrote:
> On Mon, 28 Sep 2026 09:25:02 -0700
> "Paul E. McKenney" <[email protected]> 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)
That would work, and yes, you are quite right that if !rcu_is_watching(),
RCU is studiously ignoring you completely, no matter what your preemption
state is.
> Current code may involves lockdep in fprobe, that is not good.
Agreed, that could be problematic in any number of contexts. ;-)
> BTW, if we want to trace some code in extended quiescent state, do we
> still need this Tasks-Rude RCU?
Well, you need something other than vanilla RCU, anyway. And Tasks Rude
RCU is one candidate, though it is also a candidate that might annoy
people wishing to minimize IPIs.
Thanx, Paul
> 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/[email protected]/
> > >
> > >
> > > 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) <[email protected]>
>
>
> --
> Masami Hiramatsu (Google) <[email protected]>
