Re: [PATCH RFC 1/3] rcu: Fix rcu_read_unlock() deadloop due to IRQ work

[Joel Fernandes <joelagnelf@nvidia.com>]

Bleh, my commit message got lost, but hey this is an RFC ;-)

For context, it repairs this issue:
https://lore.kernel.org/all/9acd5f9f-6732-7701-6880-4b51190aa070@huawei.com/

Will add commit message when sending non-RFC later.

thanks,

 - Joel

On 7/5/2025 4:39 PM, Joel Fernandes wrote:
> Signed-off-by: Joel Fernandes <joelagnelf@nvidia.com>
> ---
>  kernel/rcu/tree.h        | 11 ++++++++++-
>  kernel/rcu/tree_plugin.h | 29 ++++++++++++++++++++++-------
>  2 files changed, 32 insertions(+), 8 deletions(-)
> 
> diff --git a/kernel/rcu/tree.h b/kernel/rcu/tree.h
> index 3830c19cf2f6..f8f612269e6e 100644
> --- a/kernel/rcu/tree.h
> +++ b/kernel/rcu/tree.h
> @@ -174,6 +174,15 @@ struct rcu_snap_record {
>  	unsigned long   jiffies;	/* Track jiffies value */
>  };
>  
> +/*
> + * The IRQ work (deferred_qs_iw) is used by RCU to get scheduler's attention.
> + * It can be in one of the following states:
> + * - DEFER_QS_IDLE: An IRQ work was never scheduled.
> + * - DEFER_QS_PENDING: An IRQ work was scheduler but never run.
> + */
> +#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 +201,7 @@ 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 */
> diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
> index dd1c156c1759..baf57745b42f 100644
> --- a/kernel/rcu/tree_plugin.h
> +++ b/kernel/rcu/tree_plugin.h
> @@ -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;
> @@ -623,12 +626,24 @@ notrace void rcu_preempt_deferred_qs(struct task_struct *t)
>   */
>  static void rcu_preempt_deferred_qs_handler(struct irq_work *iwp)
>  {
> -	unsigned long flags;
> -	struct rcu_data *rdp;
> +	volatile unsigned long flags;
> +	struct rcu_data *rdp = this_cpu_ptr(&rcu_data);
>  
> -	rdp = container_of(iwp, struct rcu_data, defer_qs_iw);
>  	local_irq_save(flags);
> -	rdp->defer_qs_iw_pending = false;
> +
> +	/*
> +	 * Requeue the IRQ work on next unlock in 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);
> +		return;
> +	}
>  	local_irq_restore(flags);
>  }
>  
> @@ -675,7 +690,7 @@ 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) &&
> @@ -685,7 +700,7 @@ static void rcu_read_unlock_special(struct task_struct *t)
>  				else
>  					init_irq_work(&rdp->defer_qs_iw,
>  						      rcu_preempt_deferred_qs_handler);
> -				rdp->defer_qs_iw_pending = true;
> +				rdp->defer_qs_iw_pending = DEFER_QS_PENDING;
>  				irq_work_queue_on(&rdp->defer_qs_iw, rdp->cpu);
>  			}
>  		}