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" <[email protected]>
> > >
> > > 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 <[email protected]>
> > > Signed-off-by: Paul E. McKenney <[email protected]>
> > > Signed-off-by: Uladzislau Rezki (Sony) <[email protected]>
> > > ---
> > > 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?
Thanx, Paul
