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

["Zqiang" <qiang.zhang@linux.dev>]

> 
> 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 ;-) .

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