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

[Joel Fernandes <joelagnelf@nvidia.com>]

On Mon, Nov 02, 2026 at 01:50:17PM +0000, Zqiang wrote:
> > 
> > > 
> > > 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.
> 
> 
> Hello, Joel
> 
> I have make simple testing for all rcu flavor:

Thanks Zqiang, I will apply it.

thanks,

Joel Fernandes

> 
> rcu:
> 
> [    3.871868][  T158] rcu-torture: rcu_torture_fwd_progress task started
> [    3.874347][    T1] tasks-torture: rcu_torture_fwd_prog_init: Disabled, unsupported by RCU flavor under test
> [    3.862128][    T1] tasks-tracing-torture: Creating rcu_torture_fwd_prog task
> [    3.857800][    T1] tasks-rude-torture: rcu_torture_fwd_prog_init: Disabled, unsupported by RCU flavor under test
> [    3.435733][    T1] busted-torture: rcu_torture_fwd_prog_init: Disabled, unsupported by RCU flavor under test
> 
> srcu:
> 
> [    3.676067][    T1] srcu-torture: srcu_torture_init normal SRCU
> [    3.691425][    T1] srcu-torture: Creating rcu_torture_fwd_prog task
> 
> [    3.467289][    T1] srcu-torture: srcu_torture_init NMI-safe SRCU
> [    3.482983][    T1] srcu-torture: Creating rcu_torture_fwd_prog task
> 
> [    3.457597][    T1] srcu-torture: srcu_torture_init fast SRCU
> [    3.475676][    T1] srcu-torture: Creating rcu_torture_fwd_prog task
> 
> [    3.381378][    T1] srcu-torture: srcu_torture_init fast-up/down SRCU
> [    3.397417][    T1] srcu-torture: Creating rcu_torture_fwd_prog task
> 
> [    3.479642][    T1] srcu-torture: srcu_torture_init atomic SRCU
> [    3.488532][    T1] srcu-torture: rcu_torture_fwd_prog_init: Disabled, unsupported by RCU flavor under test
> 
> srcud:
> 
> [    3.898271][    T1] srcud-torture: srcud_torture_init normal SRCU
> [    3.915141][    T1] srcud-torture: Creating rcu_torture_fwd_prog task
> 
> [    3.586829][    T1] srcud-torture: srcud_torture_init NMI-safe SRCU
> [    3.604943][  T146] srcud-torture: rcu_torture_fwd_progress task started
> 
> [    3.617095][    T1] srcud-torture: srcud_torture_init fast SRCU
> [    3.635608][    T1] srcud-torture: Creating rcu_torture_fwd_prog task
> 
> [    3.915194][    T1] srcud-torture: srcud_torture_init fast-up/down SRCU
> [    3.933086][    T1] srcud-torture: Creating rcu_torture_fwd_prog task
> 
> [    3.866575][    T1] srcud-torture: srcud_torture_init atomic SRCU
> [    3.883949][    T1] srcud-torture: rcu_torture_fwd_prog_init: Disabled, unsupported by RCU flavor under test
> 
> srcud-busted:
> 
> [    3.382474][    T1] busted_srcud-torture: srcu_torture_init normal SRCU
> [    3.389525][    T1] busted_srcud-torture: rcu_torture_fwd_prog_init: Disabled, unsupported by RCU flavor under test
> 
> [    3.423111][    T1] busted_srcud-torture: srcu_torture_init NMI-safe SRCU
> [    3.432941][    T1] busted_srcud-torture: rcu_torture_fwd_prog_init: Disabled, unsupported by RCU flavor under test
> 
> [    3.364002][    T1] busted_srcud-torture: srcu_torture_init fast SRCU
> [    3.372654][    T1] busted_srcud-torture: rcu_torture_fwd_prog_init: Disabled, unsupported by RCU flavor under test
> 
> [    3.860358][    T1] busted_srcud-torture: srcu_torture_init fast-up/down SRCU
> [    3.870796][    T1] busted_srcud-torture: rcu_torture_fwd_prog_init: Disabled, unsupported by RCU flavor under test
> 
> [    3.364909][    T1] busted_srcud-torture: srcu_torture_init atomic SRCU
> [    3.372280][    T1] busted_srcud-torture: rcu_torture_fwd_prog_init: Disabled, unsupported by RCU flavor under test
> 
> trivial:
> 
> [    3.858080][    T1] trivial-torture: rcu_torture_fwd_prog_init: Disabled, unsupported by RCU flavor under test
> [    3.689060][    T1] trivial-preempt-torture: rcu_torture_fwd_prog_init: Disabled, unsupported by RCU flavor under test
> 
> 
> 
> Thanks
> Zqiang
> 
> > 
> > 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
> > >  
> > > 
> > > > 
> > > > 
> > > >
> > >
> >