Re: [PATCH] rcu: Use an intermediate irq_work to start process_srcu()

["Zqiang" <qiang.zhang@linux.dev>]

> 
> Since commit c27cea4416a3 ("rcu: Re-implement RCU Tasks Trace in terms
> of SRCU-fast") we switched to SRCU in BPF. However as BPF instrument can
> happen basically everywhere (including where a scheduler lock is held),
> call_srcu() now needs to avoid acquiring scheduler lock because
> otherwise it could cause deadlock [1]. Fix this by following what the
> previous RCU Tasks Trace did: using an irq_work to delay the queuing of
> the work to start process_srcu().
> 
> [boqun: Apply Joel's feedback]
> 
> Reported-by: Andrea Righi <arighi@nvidia.com>
> Closes: https://lore.kernel.org/all/abjzvz_tL_siV17s@gpd4/
> Fixes: commit c27cea4416a3 ("rcu: Re-implement RCU Tasks Trace in terms of SRCU-fast")
> Link: https://lore.kernel.org/rcu/3c4c5a29-24ea-492d-aeee-e0d9605b4183@nvidia.com/ [1]
> Suggested-by: Zqiang <qiang.zhang@linux.dev>
> Signed-off-by: Boqun Feng <boqun@kernel.org>
> ---
> @Zqiang, I put your name as Suggested-by because you proposed the same
> idea, let me know if you rather not have it.

Thanks Boqun add me to Suggested-by :) .

> 
> @Joel, I did two updates (including your test feedback, other one is
> call irq_work_sync() when we clean the srcu_struct), please give it a
> try.
> 
>  include/linux/srcutree.h | 1 +
>  kernel/rcu/srcutree.c | 29 +++++++++++++++++++++++++++--
>  2 files changed, 28 insertions(+), 2 deletions(-)
> 
> diff --git a/include/linux/srcutree.h b/include/linux/srcutree.h
> index dfb31d11ff05..be76fa4fc170 100644
> --- a/include/linux/srcutree.h
> +++ b/include/linux/srcutree.h
> @@ -95,6 +95,7 @@ struct srcu_usage {
>  unsigned long reschedule_jiffies;
>  unsigned long reschedule_count;
>  struct delayed_work work;
> + struct irq_work irq_work;
>  struct srcu_struct *srcu_ssp;
>  };
>  
> diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c
> index 2328827f8775..73aef361a524 100644
> --- a/kernel/rcu/srcutree.c
> +++ b/kernel/rcu/srcutree.c
> @@ -19,6 +19,7 @@
>  #include <linux/mutex.h>
>  #include <linux/percpu.h>
>  #include <linux/preempt.h>
> +#include <linux/irq_work.h>
>  #include <linux/rcupdate_wait.h>
>  #include <linux/sched.h>
>  #include <linux/smp.h>
> @@ -75,6 +76,7 @@ static bool __read_mostly srcu_init_done;
>  static void srcu_invoke_callbacks(struct work_struct *work);
>  static void srcu_reschedule(struct srcu_struct *ssp, unsigned long delay);
>  static void process_srcu(struct work_struct *work);
> +static void srcu_irq_work(struct irq_work *work);
>  static void srcu_delay_timer(struct timer_list *t);
>  
>  /*
> @@ -216,6 +218,7 @@ static int init_srcu_struct_fields(struct srcu_struct *ssp, bool is_static)
>  mutex_init(&ssp->srcu_sup->srcu_barrier_mutex);
>  atomic_set(&ssp->srcu_sup->srcu_barrier_cpu_cnt, 0);
>  INIT_DELAYED_WORK(&ssp->srcu_sup->work, process_srcu);
> + init_irq_work(&ssp->srcu_sup->irq_work, srcu_irq_work);
>  ssp->srcu_sup->sda_is_static = is_static;
>  if (!is_static) {
>  ssp->sda = alloc_percpu(struct srcu_data);
> @@ -713,6 +716,8 @@ void cleanup_srcu_struct(struct srcu_struct *ssp)
>  return; /* Just leak it! */
>  if (WARN_ON(srcu_readers_active(ssp)))
>  return; /* Just leak it! */
> + /* Wait for irq_work to finish first as it may queue a new work. */
> + irq_work_sync(&sup->irq_work);
>  flush_delayed_work(&sup->work);
>  for_each_possible_cpu(cpu) {
>  struct srcu_data *sdp = per_cpu_ptr(ssp->sda, cpu);
> @@ -1118,9 +1123,13 @@ static void srcu_funnel_gp_start(struct srcu_struct *ssp, struct srcu_data *sdp,


The following should also be replaced, although under normal situation,
we wouldn't go here:

if (snp == snp_leaf && snp_seq != s) {
      srcu_schedule_cbs_sdp(sdp, do_norm ? SRCU_INTERVAL : 0);
      return;
}

Thanks
Zqiang




>  // it isn't. And it does not have to be. After all, it
>  // can only be executed during early boot when there is only
>  // the one boot CPU running with interrupts still disabled.
> + //
> + // Use an irq_work here to avoid acquiring runqueue lock with
> + // srcu rcu_node::lock held. BPF instrument could introduce the
> + // opposite dependency, hence we need to break the possible
> + // locking dependency here.
>  if (likely(srcu_init_done))
> - queue_delayed_work(rcu_gp_wq, &sup->work,
> - !!srcu_get_delay(ssp));
> + irq_work_queue(&sup->irq_work);
>  else if (list_empty(&sup->work.work.entry))
>  list_add(&sup->work.work.entry, &srcu_boot_list);
>  }
> @@ -1979,6 +1988,22 @@ static void process_srcu(struct work_struct *work)
>  srcu_reschedule(ssp, curdelay);
>  }
>  
> +static void srcu_irq_work(struct irq_work *work)
> +{
> + struct srcu_struct *ssp;
> + struct srcu_usage *sup;
> + unsigned long delay;
> +
> + sup = container_of(work, struct srcu_usage, irq_work);
> + ssp = sup->srcu_ssp;
> +
> + raw_spin_lock_irq_rcu_node(ssp->srcu_sup);
> + delay = srcu_get_delay(ssp);
> + raw_spin_unlock_irq_rcu_node(ssp->srcu_sup);
> +
> + queue_delayed_work(rcu_gp_wq, &sup->work, !!delay);
> +}
> +
>  void srcutorture_get_gp_data(struct srcu_struct *ssp, int *flags,
>  unsigned long *gp_seq)
>  {
> -- 
> 2.50.1 (Apple Git-155)
>