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

On Thu, Jul 30, 2026 at 11:05:35PM +0000, Zqiang wrote:
> > 
> > On Thu, Jul 30, 2026 at 10:40:21PM +0000, Zqiang wrote:
> > 
> > > 
> > > 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 <[email protected]>
> > >  > Signed-off-by: Puranjay Mohan <[email protected]>
> > >  > ---
> > >  > 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?
> > > 
> > No, what you described really can happen, but only if the application
> > does a system call that does a fair amount of work. In that case,
> > is the overhead of the irq-work measureable compared to the total
> > system-call overhead?
> > 
> > Don't get me wrong, it might well be measureable. But I do need to see
> > hard numbers to justify the added complexity.
> 
> Ok, I see.  
> 
> We have similar scenarios on our product, I'm trying to measure it.

Looking forward to seeing what you come up with!

							Thanx, Paul