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rcu: Fix rcu_read_unlock() deadloop due to IRQ work

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During rcu_read_unlock_special(), if this happens during irq_exit(), we
can lockup if an IPI is issued. This is because the IPI itself triggers
the irq_exit() path causing a recursive lock up.

This is precisely what Xiongfeng found when invoking a BPF program on
the trace_tick_stop() tracepoint As shown in the trace below. Fix by
managing the irq_work state correctly.

irq_exit()
  __irq_exit_rcu()
    /* in_hardirq() returns false after this */
    preempt_count_sub(HARDIRQ_OFFSET)
    tick_irq_exit()
      tick_nohz_irq_exit()
	    tick_nohz_stop_sched_tick()
	      trace_tick_stop()  /* a bpf prog is hooked on this trace point */
		   __bpf_trace_tick_stop()
		      bpf_trace_run2()
			    rcu_read_unlock_special()
                              /* will send a IPI to itself */
			      irq_work_queue_on(&rdp->defer_qs_iw, rdp->cpu);

A simple reproducer can also be obtained by doing the following in
tick_irq_exit(). It will hang on boot without the patch:

  static inline void tick_irq_exit(void)
  {
 +	rcu_read_lock();
 +	WRITE_ONCE(current->rcu_read_unlock_special.b.need_qs, true);
 +	rcu_read_unlock();
 +

Reported-by: Xiongfeng Wang <wangxiongfeng2@huawei.com>
Closes: https://lore.kernel.org/all/9acd5f9f-6732-7701-6880-4b51190aa070@huawei.com/
Tested-by: Qi Xi <xiqi2@huawei.com>
Signed-off-by: Joel Fernandes <joelagnelf@nvidia.com>
Reviewed-by: "Paul E. McKenney" <paulmck@kernel.org>
Reported-by: Linux Kernel Functional Testing <lkft@linaro.org>
[neeraj: Apply Frederic's suggested fix for PREEMPT_RT]
Signed-off-by: Neeraj Upadhyay (AMD) <neeraj.upadhyay@kernel.org>
This commit is contained in:
Joel Fernandes 2025-07-08 10:22:19 -04:00 • committed by Neeraj Upadhyay (AMD)
parent d827673d8a
commit b41642c877
2 changed files with 38 additions and 12 deletions
Download patch file Download diff file Expand all files Collapse all files

13
kernel/rcu/tree.h
View file

@ -174,6 +174,17 @@ struct rcu_snap_record {
unsigned long jiffies; /* Track jiffies value */
};
/*
* An IRQ work (deferred_qs_iw) is used by RCU to get the scheduler's attention.
* to report quiescent states at the soonest possible time.
* The request can be in one of the following states:
* - DEFER_QS_IDLE: An IRQ work is yet to be scheduled.
* - DEFER_QS_PENDING: An IRQ work was scheduled but either not yet run, or it
* ran and we still haven't reported a quiescent state.
*/
#define DEFER_QS_IDLE 0
#define DEFER_QS_PENDING 1
/* Per-CPU data for read-copy update. */
struct rcu_data {
/* 1) quiescent-state and grace-period handling : */
@ -192,7 +203,7 @@ struct rcu_data {
/* during and after the last grace */
/* period it is aware of. */
struct irq_work defer_qs_iw; /* Obtain later scheduler attention. */
bool defer_qs_iw_pending; /* Scheduler attention pending? */
int defer_qs_iw_pending; /* Scheduler attention pending? */
struct work_struct strict_work; /* Schedule readers for strict GPs. */
/* 2) batch handling */

37
kernel/rcu/tree_plugin.h
View file

@ -486,13 +486,16 @@ rcu_preempt_deferred_qs_irqrestore(struct task_struct *t, unsigned long flags)
struct rcu_node *rnp;
union rcu_special special;
rdp = this_cpu_ptr(&rcu_data);
if (rdp->defer_qs_iw_pending == DEFER_QS_PENDING)
rdp->defer_qs_iw_pending = DEFER_QS_IDLE;
/*
* If RCU core is waiting for this CPU to exit its critical section,
* report the fact that it has exited. Because irqs are disabled,
* t->rcu_read_unlock_special cannot change.
*/
special = t->rcu_read_unlock_special;
rdp = this_cpu_ptr(&rcu_data);
if (!special.s && !rdp->cpu_no_qs.b.exp) {
local_irq_restore(flags);
return;
@ -629,7 +632,23 @@ static void rcu_preempt_deferred_qs_handler(struct irq_work *iwp)
rdp = container_of(iwp, struct rcu_data, defer_qs_iw);
local_irq_save(flags);
rdp->defer_qs_iw_pending = false;
/*
* If the IRQ work handler happens to run in the middle of RCU read-side
* critical section, it could be ineffective in getting the scheduler's
* attention to report a deferred quiescent state (the whole point of the
* IRQ work). For this reason, requeue the IRQ work.
*
* Basically, we want to avoid following situation:
* 1. rcu_read_unlock() queues IRQ work (state -> DEFER_QS_PENDING)
* 2. CPU enters new rcu_read_lock()
* 3. IRQ work runs but cannot report QS due to rcu_preempt_depth() > 0
* 4. rcu_read_unlock() does not re-queue work (state still PENDING)
* 5. Deferred QS reporting does not happen.
*/
if (rcu_preempt_depth() > 0)
WRITE_ONCE(rdp->defer_qs_iw_pending, DEFER_QS_IDLE);
local_irq_restore(flags);
}
@ -676,17 +695,13 @@ static void rcu_read_unlock_special(struct task_struct *t)
set_tsk_need_resched(current);
set_preempt_need_resched();
if (IS_ENABLED(CONFIG_IRQ_WORK) && irqs_were_disabled &&
expboost && !rdp->defer_qs_iw_pending && cpu_online(rdp->cpu)) {
expboost && rdp->defer_qs_iw_pending != DEFER_QS_PENDING &&
cpu_online(rdp->cpu)) {
// Get scheduler to re-evaluate and call hooks.
// If !IRQ_WORK, FQS scan will eventually IPI.
if (IS_ENABLED(CONFIG_RCU_STRICT_GRACE_PERIOD) &&
IS_ENABLED(CONFIG_PREEMPT_RT))
rdp->defer_qs_iw = IRQ_WORK_INIT_HARD(
rcu_preempt_deferred_qs_handler);
else
init_irq_work(&rdp->defer_qs_iw,
rcu_preempt_deferred_qs_handler);
rdp->defer_qs_iw_pending = true;
rdp->defer_qs_iw =
IRQ_WORK_INIT_HARD(rcu_preempt_deferred_qs_handler);
rdp->defer_qs_iw_pending = DEFER_QS_PENDING;
irq_work_queue_on(&rdp->defer_qs_iw, rdp->cpu);
}
}