Re: [PATCH -next v1 01/12] rcutorture: Fully test lazy RCU

[Uladzislau Rezki]

On Mon, May 18, 2026 at 05:38:48PM -0700, Paul E. McKenney wrote:
> On Mon, May 18, 2026 at 05:36:08PM +0200, Uladzislau Rezki wrote:
> > On Sun, May 17, 2026 at 11:30:14AM -0700, Paul E. McKenney wrote:
> > > On Sun, May 17, 2026 at 02:40:42PM +0200, Uladzislau Rezki wrote:
> > > > On Thu, May 14, 2026 at 09:33:10PM +0800, Z qiang wrote:
> > > > > >
> > > > > > From: "Paul E. McKenney" <paulmck@xxxxxxxxxx>
> > > > > >
> > > > > > Currently, rcutorture bypasses lazy RCU by using call_rcu_hurry().
> > > > > > This works, avoiding the dreaded rtort_pipe_count WARN(), but fails to
> > > > > > fully test lazy RCU.  The rtort_pipe_count WARN() splats because lazy RCU
> > > > > > could delay the start of an RCU grace period for a full stutter period,
> > > > > > which defaults to only three seconds.
> > > > > >
> > > > > > This commit therefore reverts the call_rcu_hurry() instances
> > > > > > back to call_rcu(), but, in kernels built with CONFIG_RCU_LAZY=y,
> > > > > > queues a workqueue handler just before the call to stutter_wait() in
> > > > > > rcu_torture_writer().  This workqueue handler invokes rcu_barrier(),
> > > > > > which motivates any lingering lazy callbacks, thus avoiding the splat.
> > > > > >
> > > > > > Questions for review:
> > > > > >
> > > > > > 1.      Should we avoid queueing work for RCU implementations not
> > > > > >         supporting lazy callbacks?
> > > > > 
> > > > > Hello, Paul
> > > > > 
> > > > > maybe we can do this:
> > > > > 
> > > > > rcu_ops = {
> > > > >          ...
> > > > >          .support_lazy = IS_ENABLED(CONFIG_RCU_LAZY),
> > > > > };
> > > > > 
> > > > > and
> > > > > 
> > > > > if (cur_ops->support_lazy )
> > > > >         queue_work(..., &lazy_work);
> > > > > 
> > > > > >
> > > > > > 2.      Should we avoid queueing work in kernels built with
> > > > > >         CONFIG_RCU_LAZY=y, but that were not booted with the
> > > > > >         rcutree.enable_rcu_lazy kernel boot parameter set?  (Note that
> > > > > >         this requires some ugliness to access this parameter, and must
> > > > > >         also handle Tiny RCU.)
> > > > > >
> > > > > > 3.      Does the rcu_torture_ops structure need a ->call_hurry() field,
> > > > > >         and if so, why?  If not, why not?
> > > > > >
> > > > > > 4.      Your additional questions here!
> > > > > >
> > > > > > Reported-by: Saravana Kannan <saravanak@xxxxxxxxxx>
> > > > > > Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
> > > > > > Signed-off-by: Uladzislau Rezki (Sony) <urezki@xxxxxxxxx>
> > > > > > ---
> > > > > >  kernel/rcu/rcutorture.c | 21 ++++++++++++++++++---
> > > > > >  1 file changed, 18 insertions(+), 3 deletions(-)
> > > > > >
> > > > > > diff --git a/kernel/rcu/rcutorture.c b/kernel/rcu/rcutorture.c
> > > > > > index 5f2848b828dc..91ba3160ba6a 100644
> > > > > > --- a/kernel/rcu/rcutorture.c
> > > > > > +++ b/kernel/rcu/rcutorture.c
> > > > > > @@ -572,7 +572,7 @@ static unsigned long rcu_no_completed(void)
> > > > > >
> > > > > >  static void rcu_torture_deferred_free(struct rcu_torture *p)
> > > > > >  {
> > > > > > -       call_rcu_hurry(&p->rtort_rcu, rcu_torture_cb);
> > > > > > +       call_rcu(&p->rtort_rcu, rcu_torture_cb);
> > > > > >  }
> > > > > >
> > > > > >  static void rcu_sync_torture_init(void)
> > > > > > @@ -619,7 +619,7 @@ static struct rcu_torture_ops rcu_ops = {
> > > > > >         .poll_gp_state_exp      = poll_state_synchronize_rcu,
> > > > > >         .cond_sync_exp          = cond_synchronize_rcu_expedited,
> > > > > >         .cond_sync_exp_full     = cond_synchronize_rcu_expedited_full,
> > > > > > -       .call                   = call_rcu_hurry,
> > > > > > +       .call                   = call_rcu,
> > > > > >         .cb_barrier             = rcu_barrier,
> > > > > >         .fqs                    = rcu_force_quiescent_state,
> > > > > >         .gp_kthread_dbg         = show_rcu_gp_kthreads,
> > > > > > @@ -1145,7 +1145,7 @@ static void rcu_tasks_torture_deferred_free(struct rcu_torture *p)
> > > > > >
> > > > > >  static void synchronize_rcu_mult_test(void)
> > > > > >  {
> > > > > > -       synchronize_rcu_mult(call_rcu_tasks, call_rcu_hurry);
> > > > > > +       synchronize_rcu_mult(call_rcu_tasks, call_rcu);
> > > > > >  }
> > > > > >
> > > > > >  static struct rcu_torture_ops tasks_ops = {
> > > > > > @@ -1631,6 +1631,17 @@ static void do_rtws_sync(struct torture_random_state *trsp, void (*sync)(void))
> > > > > >                 cpus_read_unlock();
> > > > > >  }
> > > > > >
> > > > > > +/*
> > > > > > + * Do an rcu_barrier() to motivate lazy callbacks during a stutter
> > > > > > + * pause.  Without this, we can get false-positives rtort_pipe_count
> > > > > > + * splats.
> > > > > > + */
> > > > > > +static void rcu_torture_writer_work(struct work_struct *work)
> > > > > > +{
> > > > > > +       if (cur_ops->cb_barrier)
> > > > > > +               cur_ops->cb_barrier();
> > > > > > +}
> > > > > > +
> > > > > >  /*
> > > > > >   * RCU torture writer kthread.  Repeatedly substitutes a new structure
> > > > > >   * for that pointed to by rcu_torture_current, freeing the old structure
> > > > > > @@ -1651,6 +1662,7 @@ rcu_torture_writer(void *arg)
> > > > > >         int i;
> > > > > >         int idx;
> > > > > >         unsigned long j;
> > > > > > +       struct work_struct lazy_work;
> > > > > >         int oldnice = task_nice(current);
> > > > > >         struct rcu_gp_oldstate *rgo = NULL;
> > > > > >         int rgo_size = 0;
> > > > > > @@ -1667,6 +1679,7 @@ rcu_torture_writer(void *arg)
> > > > > >                 stallsdone += (stall_cpu_holdoff + stall_gp_kthread + stall_cpu + 60) *
> > > > > >                               HZ * (stall_cpu_repeat + 1);
> > > > > >         VERBOSE_TOROUT_STRING("rcu_torture_writer task started");
> > > > > > +       INIT_WORK_ONSTACK(&lazy_work, rcu_torture_writer_work);
> > > > > >         if (!can_expedite)
> > > > > >                 pr_alert("%s" TORTURE_FLAG
> > > > > >                          " GP expediting controlled from boot/sysfs for %s.\n",
> > > > > > @@ -1895,6 +1908,8 @@ rcu_torture_writer(void *arg)
> > > > > >                                        !rcu_gp_is_normal();
> > > > > >                 }
> > > > > >                 rcu_torture_writer_state = RTWS_STUTTER;
> > > > > > +               if (IS_ENABLED(CONFIG_RCU_LAZY))
> > > > > > +                       queue_work(system_percpu_wq, &lazy_work);
> > > > > 
> > > > > 
> > > > > When the task ends, the lazy_work should be cancel and destroy:
> > > > > 
> > > > > diff --git a/kernel/rcu/rcutorture.c b/kernel/rcu/rcutorture.c
> > > > > index f593f8b794dd..5adf537ab410 100644
> > > > > --- a/kernel/rcu/rcutorture.c
> > > > > +++ b/kernel/rcu/rcutorture.c
> > > > > @@ -1682,7 +1682,6 @@ rcu_torture_writer(void *arg)
> > > > > stallsdone += (stall_cpu_holdoff + stall_gp_kthread + stall_cpu + 60) *
> > > > > HZ * (stall_cpu_repeat + 1);
> > > > > VERBOSE_TOROUT_STRING("rcu_torture_writer task started");
> > > > > - INIT_WORK_ONSTACK(&lazy_work, rcu_torture_writer_work);
> > > > > if (!can_expedite)
> > > > > pr_alert("%s" TORTURE_FLAG
> > > > > " GP expediting controlled from boot/sysfs for %s.\n",
> > > > > @@ -1719,6 +1718,8 @@ rcu_torture_writer(void *arg)
> > > > > pr_alert("%s" TORTURE_FLAG " Waited %lu jiffies for boot to complete.\n",
> > > > > torture_type, jiffies - j);
> > > > > 
> > > > > + INIT_WORK_ONSTACK(&lazy_work, rcu_torture_writer_work);
> > > > > +
> > > > > do {
> > > > > rcu_torture_writer_state = RTWS_FIXED_DELAY;
> > > > > torture_hrtimeout_us(500, 1000, &rand);
> > > > > @@ -1943,6 +1944,9 @@ rcu_torture_writer(void *arg)
> > > > > pr_alert("%s" TORTURE_FLAG
> > > > > " Dynamic grace-period expediting was disabled.\n",
> > > > > torture_type);
> > > > > + if (IS_ENABLED(CONFIG_RCU_LAZY))
> > > > > + cancel_work_sync(&lazy_work);
> > > > > + destroy_work_on_stack(&lazy_work);
> > > > > kfree(ulo);
> > > > > kfree(rgo);
> > > > > rcu_torture_writer_state = RTWS_STOPPING;
> > > > > 
> > > > I agree, we seem miss destroying the work via destroy_work_on_stack()
> > > > and "sync" the lazy_work work via cancel_work_sync().
> > > > 
> > > > Paul, any thoughts?
> > > 
> > > Good eyes!!!  I will take a closer look tomorrow.
> > > 
> > > In the meantime, one question...  Do we want to test for your new
> > > ->support_lazy instead of IS_ENABLED(CONFIG_RCU_LAZY) for that
> > > cancel_work_sync() and destroy_work_on_stack()?  If we really need
> > > IS_ENABLED(CONFIG_RCU_LAZY), could you please suggest a comment giving
> > > the reasoning?
> > > 
> > To me IS_ENABLED(CONFIG_RCU_LAZY) is enough :) I am not sure we really
> > need ->support_lazy flag.
> 
> Good point, given that the queue_work() is also guarded by that same
> IS_ENABLED(CONFIG_RCU_LAZY).
> 
> How does the patch below, based on the patch I sent initially, look?
> 
> I can fold it in and resend, or you can do so, your choice.
> 
> 							Thanx, Paul
> 
> ------------------------------------------------------------------------
> 
> commit 68284a54b40f4fb1a6595fb9263aae47a56f20bb
> Author: Paul E. McKenney <paulmck@xxxxxxxxxx>
> Date:   Mon May 18 17:36:10 2026 -0700
> 
>     squash! rcutorture: Fully test lazy RCU
>     
>     [ paulmck: Apply Z qiang feedback. ]
>     
>     Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
> 
> diff --git a/kernel/rcu/rcutorture.c b/kernel/rcu/rcutorture.c
> index e1b4f4dbbb5819..878706beea01cd 100644
> --- a/kernel/rcu/rcutorture.c
> +++ b/kernel/rcu/rcutorture.c
> @@ -1682,7 +1682,6 @@ rcu_torture_writer(void *arg)
>  		stallsdone += (stall_cpu_holdoff + stall_gp_kthread + stall_cpu + 60) *
>  			      HZ * (stall_cpu_repeat + 1);
>  	VERBOSE_TOROUT_STRING("rcu_torture_writer task started");
> -	INIT_WORK_ONSTACK(&lazy_work, rcu_torture_writer_work);
>  	if (!can_expedite)
>  		pr_alert("%s" TORTURE_FLAG
>  			 " GP expediting controlled from boot/sysfs for %s.\n",
> @@ -1719,6 +1718,8 @@ rcu_torture_writer(void *arg)
>  		pr_alert("%s" TORTURE_FLAG " Waited %lu jiffies for boot to complete.\n",
>  			 torture_type, jiffies - j);
>  
> +	if (IS_ENABLED(CONFIG_RCU_LAZY))
> +		INIT_WORK_ONSTACK(&lazy_work, rcu_torture_writer_work);
>  	do {
>  		rcu_torture_writer_state = RTWS_FIXED_DELAY;
>  		torture_hrtimeout_us(500, 1000, &rand);
> @@ -1943,6 +1944,10 @@ rcu_torture_writer(void *arg)
>  		pr_alert("%s" TORTURE_FLAG
>  			 " Dynamic grace-period expediting was disabled.\n",
>  			 torture_type);
> +	if (IS_ENABLED(CONFIG_RCU_LAZY)) {
> +		cancel_work_sync(&lazy_work);
> +		destroy_work_on_stack(&lazy_work);
> +	}
>  	kfree(ulo);
>  	kfree(rgo);
>  	rcu_torture_writer_state = RTWS_STOPPING;
>
I will fold it on my own, Paul!

--
Uladzislau Rezki