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" <[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?
> >
> 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 <[email protected]>
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 <[email protected]>
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;
