Re: [PATCH 1/6] rcu: Make call_rcu() safe to call from any context

["Zqiang" <qiang.zhang@linux.dev>]

> 
> On Thu, Jul 30, 2026 at 12:20:54PM +0000, Zqiang wrote:
> 
> > 
> > call_rcu() touches its per-CPU callback list only with interrupts
> >  disabled: the enqueue runs under local_irq_save() (plus the nocb locks
> >  when offloaded), as do callback invocation and grace-period work. A
> >  call_rcu() that arrives with interrupts already disabled may be
> >  interrupting one of those, so enqueuing directly could corrupt the list
> >  or deadlock -- as an NMI or re-entrant instrumentation can.
> >  
> >  Defer in that case: stage the callback on a per-CPU llist and kick an
> >  irq_work that re-issues it once interrupts are back on, going straight to
> >  the enqueue so it cannot defer again. Callers that merely hold interrupts
> >  off are deferred too, which is harmless. Skip the gate while the
> >  scheduler is down (RCU_SCHEDULER_INACTIVE): irq_work is not usable until
> >  init_IRQ(), yet rcu_init() already calls call_rcu().
> >  
> >  rcu_barrier() flushes pending deferred callbacks first: it waits out each
> >  online CPU's irq_work and drains an offline CPU's list directly, as that
> >  irq_work may never run again. rcutree_migrate_callbacks() also drains an
> >  outgoing CPU's ->defer_head, so a callback deferred late in the offline
> >  path runs even without an rcu_barrier(). The irq_work is
> >  IRQ_WORK_INIT_HARD so the re-issue runs promptly and callbacks do not back
> >  up; on PREEMPT_RT a non-HARD irq_work would run in a kthread that can be
> >  delayed under load. A new hidden CONFIG_RCU_DEFER gates the code and the
> >  IRQ_WORK dependency; without it call_rcu() enqueues directly as before.
> >  
> >  Under CONFIG_PROVE_RCU, warn if the direct path is reached from NMI, which
> >  would mean interrupts were enabled on NMI entry.
> >  
> >  Suggested-by: Paul E. McKenney <paulmck@kernel.org>
> >  Signed-off-by: Puranjay Mohan <puranjay@kernel.org>
> >  ---
> >  kernel/rcu/Kconfig | 6 +++
> >  kernel/rcu/rcu.h | 15 ++++++
> >  kernel/rcu/tree.c | 120 +++++++++++++++++++++++++++++++++++++++++----
> >  kernel/rcu/tree.h | 5 ++
> >  4 files changed, 136 insertions(+), 10 deletions(-)
> >  
> >  diff --git a/kernel/rcu/Kconfig b/kernel/rcu/Kconfig
> >  index f15da8038d0ba..1a5fb3156c062 100644
> >  --- a/kernel/rcu/Kconfig
> >  +++ b/kernel/rcu/Kconfig
> >  @@ -175,6 +175,12 @@ config RCU_STALL_COMMON
> >  config RCU_NEED_SEGCBLIST
> >  def_bool ( TREE_RCU || TREE_SRCU || TASKS_RCU_GENERIC )
> >  
> >  +# The deferral (and the IRQ_WORK it uses) is only needed where call_rcu() /
> >  +# call_srcu() can be invoked while a callback-list operation is in flight.
> >  +config RCU_DEFER
> >  + def_bool HAVE_NMI || KPROBES || FUNCTION_TRACER || TRACEPOINTS
> >  + select IRQ_WORK
> >  +
> >  config RCU_FANOUT
> >  int "Tree-based hierarchical RCU fanout value"
> >  range 2 64 if 64BIT
> >  diff --git a/kernel/rcu/rcu.h b/kernel/rcu/rcu.h
> >  index 39a9f6fa9a7b2..ed6604445f2ac 100644
> >  --- a/kernel/rcu/rcu.h
> >  +++ b/kernel/rcu/rcu.h
> >  @@ -572,6 +572,21 @@ static inline void tasks_cblist_init_generic(void) { }
> >  #define RCU_SCHEDULER_INIT 1
> >  #define RCU_SCHEDULER_RUNNING 2
> >  
> >  +/*
> >  + * Should a call_rcu()/call_srcu() callback be deferred rather than enqueued
> >  + * now? Defer whenever interrupts are disabled: a callback-list operation may
> >  + * be in flight on this CPU, so enqueuing now could corrupt it. But not while
> >  + * the scheduler is down -- early boot is single-threaded and can call this
> >  + * before init_IRQ() makes irq_work usable (e.g. rcu_init()'s self-tests).
> >  + */
> >  +static inline bool should_rcu_defer(void)
> >  +{
> >  + if (!IS_ENABLED(CONFIG_RCU_DEFER))
> >  + return false;
> >  +
> >  + return irqs_disabled() && rcu_scheduler_active != RCU_SCHEDULER_INACTIVE;
> >  +}
> >  +
> >  enum rcutorture_type {
> >  RCU_FLAVOR,
> >  RCU_TASKS_FLAVOR,
> >  diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> >  index 21b6ce1dffb63..93c4242345e67 100644
> >  --- a/kernel/rcu/tree.c
> >  +++ b/kernel/rcu/tree.c
> >  @@ -24,6 +24,7 @@
> >  #include <linux/smp.h>
> >  #include <linux/rcupdate_wait.h>
> >  #include <linux/interrupt.h>
> >  +#include <linux/llist.h>
> >  #include <linux/sched.h>
> >  #include <linux/sched/debug.h>
> >  #include <linux/nmi.h>
> >  @@ -3148,21 +3149,27 @@ static void check_cb_ovld(struct rcu_data *rdp)
> >  raw_spin_unlock_rcu_node(rnp);
> >  }
> >  
> >  -static void
> >  -__call_rcu_common(struct rcu_head *head, rcu_callback_t func, bool lazy_in)
> >  +/*
> >  + * The callback list is only ever accessed with interrupts disabled (enqueue,
> >  + * callback invocation, grace-period work). A call_rcu() with interrupts already
> >  + * disabled may interrupt one of those, so __call_rcu_common() defers: the
> >  + * callback is staged on a per-CPU llist that an irq_work re-issues once
> >  + * interrupts are on. Only CONFIG_RCU_DEFER kernels can hit this.
> >  + */
> >  +static void rcu_defer_drain(struct irq_work *iw);
> >  +
> >  +/*
> >  + * Enqueue @head on this CPU's rcu_segcblist. Also called by rcu_defer_drain()
> >  + * to re-issue a deferred callback, so it must not re-check the deferral
> >  + * condition. Either caller may have interrupts already disabled.
> >  + */
> >  +static void rcu_do_enqueue(struct rcu_head *head, rcu_callback_t func, bool lazy_in)
> >  {
> >  static atomic_t doublefrees;
> >  unsigned long flags;
> >  bool lazy;
> >  struct rcu_data *rdp;
> >  
> >  - /* Misaligned rcu_head! */
> >  - WARN_ON_ONCE((unsigned long)head & (sizeof(void *) - 1));
> >  -
> >  - /* Avoid NULL dereference if callback is NULL. */
> >  - if (WARN_ON_ONCE(!func))
> >  - return;
> >  -
> >  if (debug_rcu_head_queue(head)) {
> >  /*
> >  * Probable double call_rcu(), so leak the callback.
> >  @@ -3206,6 +3213,83 @@ __call_rcu_common(struct rcu_head *head, rcu_callback_t func, bool lazy_in)
> >  local_irq_restore(flags);
> >  }
> >  
> >  +/*
> >  + * Re-issue deferred callbacks, going straight to the enqueue so they cannot
> >  + * defer again. defer_lock is held across llist_del_all() and the re-issue so
> >  + * the drainers -- this CPU's irq_work, rcu_defer_flush() and
> >  + * rcutree_migrate_callbacks() -- serialize and never leave a callback off
> >  + * ->defer_head yet not on a callback list.
> >  + */
> >  +static void rcu_defer_drain(struct irq_work *iw)
> >  +{
> >  + struct rcu_data *rdp = container_of(iw, struct rcu_data, defer_work);
> >  + struct llist_node *node, *next;
> >  + unsigned long flags;
> >  +
> >  + raw_spin_lock_irqsave(&rdp->defer_lock, flags);
> >  + llist_for_each_safe(node, next, llist_del_all(&rdp->defer_head)) {
> >  + struct rcu_head *head = (struct rcu_head *)node;
> >  +
> >  + rcu_do_enqueue(head, head->func, false);
> >  + }
> >  + raw_spin_unlock_irqrestore(&rdp->defer_lock, flags);
> >  +}
> >  +
> >  +/* Stage @head for this CPU's irq_work when call_rcu() cannot enqueue now. */
> >  +static void call_rcu_defer(struct rcu_head *head, rcu_callback_t func)
> >  +{
> >  + struct rcu_data *rdp = this_cpu_ptr(&rcu_data);
> >  +
> >  + head->func = func;
> >  + if (llist_add((struct llist_node *)head, &rdp->defer_head))
> >  + irq_work_queue(&rdp->defer_work);
> >  
> >  If current CPU is isolation and nozh_full, maybe we should avoid to
> >  queue irq_work to current CPUs(include srcu, tasks-rcu) ?
> >  
> >  Maybe can wrap a helper function:
> >  
> >  bool rcu_irq_work_queue(struct irq_work *iw) 
> >  {
> >  if (in_nmi())
> >  return irq_work_queue(iw);
> >  
> >  int cpu = smp_processor_id();
> >  if (!housekeeping_test_cpu(cpu, HK_TYPE_KERNEL_NOISE))
> >  cpu = housekeeping_any_cpu(HK_TYPE_KERNEL_NOISE);
> >  
> >  return irq_work_queue_on(iw, cpu);
> >  }
> > 
> You lost me on this one. We cannot invoke call_rcu() and friends unless
> we are running in kernel context, so usermode operation has already
> been interrupted, perhaps due to a configuration error that failed to
> direct interrupts away from the current CPU. In that case, is the added
> disruption of the irq-work really worth worrying about?

The irqs_disabled() can return true, even if we not in hardirq context,
for example, before this we invoke local_irq_save() to disable local
CPU's irq.
If a userspace application bound to isolate and nohz_full CPUS, and
cycle enters the kernelspace do wakeup. we use ebpf hook tracepoint in 
try_to_wake_up(), in ebpf code, the call_rcu() or call_srcu() check
irq_disabled() return true, enter defer path, trigger irq_work to current
CPUs, can make some noise.

Did I miss something?

Thanks
Zqiang


> 
> What am I missing here?
> 
>  Thanx, Paul
> 
> > 
> > Thanks
> >  Zqiang
> >  
> >  
> >  
> >  +}
> >  +
> >  +/*
> >  + * Register pending deferred callbacks into the callback lists so a following
> >  + * rcu_barrier() waits for them. This runs before rcu_barrier() scans the
> >  + * lists. An online CPU's own irq_work re-issues its callbacks, so wait it out;
> >  + * an offline CPU's irq_work may never run again, so drain its list directly
> >  + * onto this CPU instead.
> >  + */
> >  +static void rcu_defer_flush(void)
> >  +{
> >  + int cpu;
> >  +
> >  + if (!IS_ENABLED(CONFIG_RCU_DEFER))
> >  + return;
> >  +
> >  + for_each_possible_cpu(cpu) {
> >  + struct rcu_data *rdp = per_cpu_ptr(&rcu_data, cpu);
> >  +
> >  + if (cpu_online(cpu))
> >  + irq_work_sync(&rdp->defer_work);
> >  + else
> >  + rcu_defer_drain(&rdp->defer_work);
> >  + }
> >  +}
> >  +
> >  +static void
> >  +__call_rcu_common(struct rcu_head *head, rcu_callback_t func, bool lazy_in)
> >  +{
> >  + /* Misaligned rcu_head! */
> >  + WARN_ON_ONCE((unsigned long)head & (sizeof(void *) - 1));
> >  +
> >  + /* Avoid NULL dereference if callback is NULL. */
> >  + if (WARN_ON_ONCE(!func))
> >  + return;
> >  +
> >  + if (should_rcu_defer()) {
> >  + call_rcu_defer(head, func);
> >  + return;
> >  + }
> >  +
> >  + /* An NMI reaching here entered with irqs enabled, so the enqueue can race. */
> >  + WARN_ON_ONCE(IS_ENABLED(CONFIG_PROVE_RCU) && in_nmi());
> >  +
> >  + rcu_do_enqueue(head, func, lazy_in);
> >  +}
> >  +
> >  #ifdef CONFIG_RCU_LAZY
> >  static bool enable_rcu_lazy __read_mostly = !IS_ENABLED(CONFIG_RCU_LAZY_DEFAULT_OFF);
> >  module_param(enable_rcu_lazy, bool, 0444);
> >  @@ -3896,8 +3980,12 @@ void rcu_barrier(void)
> >  unsigned long flags;
> >  unsigned long gseq;
> >  struct rcu_data *rdp;
> >  - unsigned long s = rcu_seq_snap(&rcu_state.barrier_sequence);
> >  + unsigned long s;
> >  +
> >  + /* Register any deferred callbacks before snapshotting the sequence. */
> >  + rcu_defer_flush();
> >  
> >  + s = rcu_seq_snap(&rcu_state.barrier_sequence);
> >  rcu_barrier_trace(TPS("Begin"), -1, s);
> >  
> >  /* Take mutex to serialize concurrent rcu_barrier() requests. */
> >  @@ -4231,6 +4319,10 @@ rcu_boot_init_percpu_data(int cpu)
> >  rdp->rcu_onl_gp_state = RCU_GP_CLEANED;
> >  rdp->last_sched_clock = jiffies;
> >  rdp->cpu = cpu;
> >  + init_llist_head(&rdp->defer_head);
> >  + raw_spin_lock_init(&rdp->defer_lock);
> >  + /* Hard irq_work so the re-issue runs promptly. */
> >  + rdp->defer_work = IRQ_WORK_INIT_HARD(rcu_defer_drain);
> >  rcu_boot_init_nocb_percpu_data(rdp);
> >  }
> >  
> >  @@ -4528,6 +4620,14 @@ void rcutree_migrate_callbacks(int cpu)
> >  struct rcu_data *rdp = per_cpu_ptr(&rcu_data, cpu);
> >  bool needwake;
> >  
> >  + /*
> >  + * Callbacks the outgoing CPU deferred late in the offline path (past the
> >  + * point its irq_work can run) sit on ->defer_head, which the ->cblist
> >  + * migration below does not cover. Drain them here, before the early
> >  + * returns; the re-issue lands on this CPU.
> >  + */
> >  + rcu_defer_drain(&rdp->defer_work);
> >  +
> >  if (rcu_rdp_is_offloaded(rdp))
> >  return;
> >  
> >  diff --git a/kernel/rcu/tree.h b/kernel/rcu/tree.h
> >  index eedfa43059e80..3a8e17136c5a7 100644
> >  --- a/kernel/rcu/tree.h
> >  +++ b/kernel/rcu/tree.h
> >  @@ -229,6 +229,11 @@ struct rcu_data {
> >  struct rcu_head barrier_head;
> >  int exp_watching_snap; /* Double-check need for IPI. */
> >  
> >  + /* Deferral of an NMI/reentrant call_rcu(); see __call_rcu_common(). */
> >  + struct llist_head defer_head;
> >  + struct irq_work defer_work;
> >  + raw_spinlock_t defer_lock;
> >  +
> >  /* 5) Callback offloading. */
> >  #ifdef CONFIG_RCU_NOCB_CPU
> >  struct swait_queue_head nocb_cb_wq; /* For nocb kthreads to sleep on. */
> >  -- 
> >  2.53.0-Meta
> >
>