Re: [PATCH v2] rcutorture: Avoid create fwd_prog_tasks for tasks-rude and srcu-atomic

[Joel Fernandes <joelagnelf@nvidia.com>]

> On Oct 30, 2026, at 8:14 AM, Zqiang <qiang.zhang@linux.dev> wrote:
> 
> 
>> 
>> 
>>> 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. ;-)
> 
> Ok, I see. thanks Paul ;-) .

So let me know Zqiang once you are confident on the testing from and I will apply it for 7.5.

thanks,

Joel Fernandes


> 
>> 
>>> 
>>> ---
>>> 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
>>> 
>>