[PATCH for-v7.5 07/14] rcutorture: Avoid create fwd_prog_tasks for tasks-rude and srcu-atomic

[Joel Fernandes <jfern@kernel.org>]

From: Zqiang <qiang.zhang@linux.dev>

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 <jfern@kernel.org>
Signed-off-by: Zqiang <qiang.zhang@linux.dev>
Reviewed-by: Paul E. McKenney <paulmck@kernel.org>
Link: https://lore.kernel.org/r/20261023093716.25717-1-qiang.zhang@linux.dev
Signed-off-by: Joel Fernandes <jfern@kernel.org>
---
 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 cbe38aa7ee68..924705b7759d 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;
-- 
2.54.0