[PATCH 2/2] rcu: Fix lockup when RCU reader used while IRQ exiting

[Joel Fernandes <joelagnelf@nvidia.com>]

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
using context-tracking to tell us if we're still in an IRQ.
context-tracking keeps track of the IRQ until after the tracepoint, so
it cures the issues.

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();
 +

While at it, add some comments to this code.

Reported-by: Xiongfeng Wang <wangxiongfeng2@huawei.com>
Closes: https://lore.kernel.org/all/9acd5f9f-6732-7701-6880-4b51190aa070@huawei.com/
Tested-by: Xiongfeng Wang <wangxiongfeng2@huawei.com>
Signed-off-by: Joel Fernandes <joelagnelf@nvidia.com>
---
 kernel/rcu/tree_plugin.h | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index 3c0bbbbb686f..53d8b3415776 100644
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -653,6 +653,9 @@ static void rcu_read_unlock_special(struct task_struct *t)
 		struct rcu_data *rdp = this_cpu_ptr(&rcu_data);
 		struct rcu_node *rnp = rdp->mynode;
 
+		// In cases where the RCU-reader is boosted, we'd attempt deboost sooner than
+		// later to prevent inducing latency to other RT tasks. Also, expedited GPs
+		// should not be delayed too much. Track both these needs in expboost.
 		expboost = (t->rcu_blocked_node && READ_ONCE(t->rcu_blocked_node->exp_tasks)) ||
 			   (rdp->grpmask & READ_ONCE(rnp->expmask)) ||
 			   (IS_ENABLED(CONFIG_RCU_STRICT_GRACE_PERIOD) &&
@@ -670,10 +673,15 @@ static void rcu_read_unlock_special(struct task_struct *t)
 			// Also if no expediting and no possible deboosting,
 			// slow is OK.  Plus nohz_full CPUs eventually get
 			// tick enabled.
+			//
+			// Also prevent doing this if context-tracking thinks
+			// we're handling an IRQ (including when we're exiting
+			// one -- required to prevent self-IPI deadloops).
 			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 && cpu_online(rdp->cpu) &&
+			    !ct_in_irq()) {
 				// Get scheduler to re-evaluate and call hooks.
 				// If !IRQ_WORK, FQS scan will eventually IPI.
 				if (IS_ENABLED(CONFIG_RCU_STRICT_GRACE_PERIOD) &&
-- 
2.34.1