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

[qiang.zhang@linux.dev]

> 
> 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) {


For Preempt-RT kernels, the rcu_preempt_deferred_qs_handler() be invoked
in per-cpu irq_work kthreads, the return value of rcu_preempt_depth()
may always be 0, should we use IRQ_WORK_INIT_HARD() to initialize defer_qs_iw?

Thanks
Zqiang



> 
> + 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);
> 
>  }
> 
>  }
> 
> -- 
> 
> 2.43.0
>