[PATCH] rcutorture: Clear cbflood_max for tasks-rude and srcu-atomic testing

[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
removes the .cbflood_max from tasks-rude and clears ->cbflood_max in the
srcu_torture_init_forbidden_apis().

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

Signed-off-by: Zqiang <qiang.zhang@linux.dev>
---
 kernel/rcu/rcutorture.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/rcu/rcutorture.c b/kernel/rcu/rcutorture.c
index 968343be4e0d..00db87256e78 100644
--- a/kernel/rcu/rcutorture.c
+++ b/kernel/rcu/rcutorture.c
@@ -723,6 +723,7 @@ static void srcu_torture_init_forbidden_apis(void)
 	cur_ops->deferred_free = NULL;
 	cur_ops->exp_current = NULL;
 	cur_ops->start_gp_poll = NULL;
+	cur_ops->cbflood_max = 0;
 }
 
 static void srcu_torture_init(void)
@@ -1250,7 +1251,6 @@ static struct rcu_torture_ops tasks_rude_ops = {
 	.exp_sync	= synchronize_rcu_tasks_rude,
 	.gp_kthread_dbg	= show_rcu_tasks_rude_gp_kthread,
 	.get_gp_data	= rcu_tasks_rude_get_gp_data,
-	.cbflood_max	= 50000,
 	.irq_capable	= 1,
 	.name		= "tasks-rude"
 };
-- 
2.17.1