* [PATCH v2] rcutorture: Avoid create fwd_prog_tasks for tasks-rude and srcu-atomic
@ 2026-10-23 9:37 Zqiang
2026-10-23 19:23 ` Paul E. McKenney
0 siblings, 1 reply; 2+ messages in thread
From: Zqiang @ 2026-10-23 9:37 UTC (permalink / raw)
To: paulmck, frederic, neeraj.upadhyay, joelagnelf, urezki, boqun
Cc: qiang.zhang, rcu, linux-kernel
When running the tasks-rude and srcu-atomic tests, the fwd_prog_tasks
kthread is created because the default value of fwd_progress is 1.
however, the two RCU flavors not support ->call():
1. tasks-rude: commit b428a9de9bba ("rcutorture: Stop testing RCU Tasks
Rude asynchronous APIs") removed .call and .cb_barrier from tasks-rude,
but left the .cbflood_max = 50000.
2. srcu-atomic: srcu_torture_init_forbidden_apis() sets ->call to NULL
for SRCU_READ_FLAVOR_ATOMIC runs, but leaves ->cbflood_max untouched.
In both cases, the non-zero ->cbflood_max fools the check above into
creating the fwd_prog_tasks kthreads, even though those kthreads cannot
possibly carry out any forward-progress test:
1. The rcu_torture_fwd_prog_cr() returns immediately due to cur_ops->call
is NULL:
[ 68.022674] rcu_torture_fwd_prog_cr: No ->call(), so no forward-progress test 0
[ 129.462046] rcu_torture_fwd_prog_cr: No ->call(), so no forward-progress test 0
[ 190.902123] rcu_torture_fwd_prog_cr: No ->call(), so no forward-progress test 0
[ 252.343809] rcu_torture_fwd_prog_cr: No ->call(), so no forward-progress test 0
2. The rcu_torture_fwd_prog_nr() is never even invoked, because its caller
requires cur_ops->stall_dur, and neither tasks-rude nor srcu-atomic
provide ->stall_dur (only rcu_ops does).
Therefore, creating fwd_prog_tasks kthreads is pointless, this commit adds
->call and ->cb_barrier to the forward-progress check, rejecting flavors
that lack the cbflood_max set and a positive ->stall_dur.
simple verification output:
[ 3.062959][ T1] srcu-torture: rcu_torture_fwd_prog_init: Disabled, unsupported by RCU flavor under test
[ 2.922323][ T1] tasks-rude-torture: rcu_torture_fwd_prog_init: Disabled, unsupported by RCU flavor under test
Suggested-by: Joel Fernandes <joelagnelf@nvidia.com>
Signed-off-by: Zqiang <qiang.zhang@linux.dev>
---
kernel/rcu/rcutorture.c | 5 ++---
1 file changed, 2 insertions(+), 3 deletions(-)
diff --git a/kernel/rcu/rcutorture.c b/kernel/rcu/rcutorture.c
index 968343be4e0d..92ff197f4789 100644
--- a/kernel/rcu/rcutorture.c
+++ b/kernel/rcu/rcutorture.c
@@ -4007,9 +4007,8 @@ static int __init rcu_torture_fwd_prog_init(void)
} else if (fwd_progress < 0) {
fwd_progress = nr_cpu_ids;
}
- if ((!cur_ops->sync && !cur_ops->call) ||
- (!cur_ops->cbflood_max && (!cur_ops->stall_dur || cur_ops->stall_dur() <= 0)) ||
- cur_ops == &rcu_busted_ops) {
+ if (!cur_ops->sync || ((!cur_ops->call || !cur_ops->cb_barrier || !cur_ops->cbflood_max) &&
+ (!cur_ops->stall_dur || cur_ops->stall_dur() <= 0))) {
VERBOSE_TOROUT_STRING("rcu_torture_fwd_prog_init: Disabled, unsupported by RCU flavor under test");
fwd_progress = 0;
return 0;
--
2.17.1
^ permalink raw reply related [flat|nested] 2+ messages in thread
* Re: [PATCH v2] rcutorture: Avoid create fwd_prog_tasks for tasks-rude and srcu-atomic
2026-10-23 9:37 [PATCH v2] rcutorture: Avoid create fwd_prog_tasks for tasks-rude and srcu-atomic Zqiang
@ 2026-10-23 19:23 ` Paul E. McKenney
0 siblings, 0 replies; 2+ messages in thread
From: Paul E. McKenney @ 2026-10-23 19:23 UTC (permalink / raw)
To: Zqiang
Cc: frederic, neeraj.upadhyay, joelagnelf, urezki, boqun, rcu,
linux-kernel
On Fri, Oct 23, 2026 at 05:37:16PM +0800, Zqiang wrote:
> When running the tasks-rude and srcu-atomic tests, the fwd_prog_tasks
> kthread is created because the default value of fwd_progress is 1.
> however, the two RCU flavors not support ->call():
>
> 1. tasks-rude: commit b428a9de9bba ("rcutorture: Stop testing RCU Tasks
> Rude asynchronous APIs") removed .call and .cb_barrier from tasks-rude,
> but left the .cbflood_max = 50000.
>
> 2. srcu-atomic: srcu_torture_init_forbidden_apis() sets ->call to NULL
> for SRCU_READ_FLAVOR_ATOMIC runs, but leaves ->cbflood_max untouched.
>
> In both cases, the non-zero ->cbflood_max fools the check above into
> creating the fwd_prog_tasks kthreads, even though those kthreads cannot
> possibly carry out any forward-progress test:
>
> 1. The rcu_torture_fwd_prog_cr() returns immediately due to cur_ops->call
> is NULL:
>
> [ 68.022674] rcu_torture_fwd_prog_cr: No ->call(), so no forward-progress test 0
> [ 129.462046] rcu_torture_fwd_prog_cr: No ->call(), so no forward-progress test 0
> [ 190.902123] rcu_torture_fwd_prog_cr: No ->call(), so no forward-progress test 0
> [ 252.343809] rcu_torture_fwd_prog_cr: No ->call(), so no forward-progress test 0
>
> 2. The rcu_torture_fwd_prog_nr() is never even invoked, because its caller
> requires cur_ops->stall_dur, and neither tasks-rude nor srcu-atomic
> provide ->stall_dur (only rcu_ops does).
>
> Therefore, creating fwd_prog_tasks kthreads is pointless, this commit adds
> ->call and ->cb_barrier to the forward-progress check, rejecting flavors
> that lack the cbflood_max set and a positive ->stall_dur.
>
> simple verification output:
>
> [ 3.062959][ T1] srcu-torture: rcu_torture_fwd_prog_init: Disabled, unsupported by RCU flavor under test
> [ 2.922323][ T1] tasks-rude-torture: rcu_torture_fwd_prog_init: Disabled, unsupported by RCU flavor under test
>
> Suggested-by: Joel Fernandes <joelagnelf@nvidia.com>
> Signed-off-by: Zqiang <qiang.zhang@linux.dev>
Reviewed-by: Paul E. McKenney <paulmck@kernel.org>
It would nevertheless be good to run tests demonstrating that
forward-progress testing happens and does not happen on the same
rcu_torture_ops structures as before. Just in case I lost my
way through that boolean expression. ;-)
> ---
> kernel/rcu/rcutorture.c | 5 ++---
> 1 file changed, 2 insertions(+), 3 deletions(-)
>
> diff --git a/kernel/rcu/rcutorture.c b/kernel/rcu/rcutorture.c
> index 968343be4e0d..92ff197f4789 100644
> --- a/kernel/rcu/rcutorture.c
> +++ b/kernel/rcu/rcutorture.c
> @@ -4007,9 +4007,8 @@ static int __init rcu_torture_fwd_prog_init(void)
> } else if (fwd_progress < 0) {
> fwd_progress = nr_cpu_ids;
> }
> - if ((!cur_ops->sync && !cur_ops->call) ||
> - (!cur_ops->cbflood_max && (!cur_ops->stall_dur || cur_ops->stall_dur() <= 0)) ||
> - cur_ops == &rcu_busted_ops) {
> + if (!cur_ops->sync || ((!cur_ops->call || !cur_ops->cb_barrier || !cur_ops->cbflood_max) &&
> + (!cur_ops->stall_dur || cur_ops->stall_dur() <= 0))) {
> VERBOSE_TOROUT_STRING("rcu_torture_fwd_prog_init: Disabled, unsupported by RCU flavor under test");
> fwd_progress = 0;
> return 0;
> --
> 2.17.1
>
^ permalink raw reply [flat|nested] 2+ messages in thread
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2026-10-23 9:37 [PATCH v2] rcutorture: Avoid create fwd_prog_tasks for tasks-rude and srcu-atomic Zqiang
2026-10-23 19:23 ` Paul E. McKenney
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