[PATCH v3 12/14] rcutorture: tighten boost-WARN to exclude any implicit-reader context

[Joel Fernandes <joelagnelf@nvidia.com>]

The Slow-deboost WARN_ON_ONCE in rcu_torture_one_read() currently uses
only the handler-context predicates to decide whether the calling
context is "safe" to assert immediate deboosting:

    !in_serving_softirq() && !in_hardirq() && !in_nmi()

They do NOT exclude several other contexts in which the task is still
holding an implicit RCU read-side critical section.

  1. local_bh_disable()-but-not-in-softirq.

  2. preempt_disable()-held.

  3. irqs_disabled().

  4. rcu_preempt_depth() > 0.

The current predicate set lets the WARN fire while these implicit
readers are still active, which is a false positive: the user-visible
"tardy deboost" finding becomes ambiguous between an actual
deferred-QS bug and a valid caller still inside an implicit-reader
section.

Replace the three handler-only predicates with the following which
covers everything.

    preempt_count() == 0 && !irqs_disabled() && rcu_preempt_depth() == 0

Note that we only check in contexts where rcu_read_unlock_special() deboosts
synchronously, i.e. where it is not inside an implicit RCU read-side
critical section. This matches rcu_read_unlock_special()'s own gate
and rcu_flavor_sched_clock_irq()'s:
 "rcu_preempt_depth() > 0 || (preempt_count() & (PREEMPT_MASK | SOFTIRQ_MASK))".

Also note, in PREEMPT_RT, softirqs always run under local_bh_disable(),
which itself takes rcu_read_lock() to satisfy RCU's bottom-half
requirement (see softirq.c) — so rcu_preempt_depth() is non-zero
throughout any RT softirq thus making in_serving_softirq() redundant.

Signed-off-by: Joel Fernandes <joelagnelf@nvidia.com>
---
 kernel/rcu/rcutorture.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/kernel/rcu/rcutorture.c b/kernel/rcu/rcutorture.c
index 13e0eec1e1ce..e0bea4039c42 100644
--- a/kernel/rcu/rcutorture.c
+++ b/kernel/rcu/rcutorture.c
@@ -2603,7 +2603,8 @@ static bool rcu_torture_one_read(struct torture_random_state *trsp, long myid)
 	// rcu_read_unlock() does not end the full segmented RCU read-side
 	// critical section.
 	if (WARN_ON_ONCE(cur_ops->is_task_rcu_boosted && cur_ops->is_task_rcu_boosted() &&
-			 !in_serving_softirq() && !in_hardirq() && !in_nmi()) &&
+			 preempt_count() == 0 && !irqs_disabled() &&
+			 rcu_preempt_depth() == 0) &&
 	    READ_ONCE(firsttime) && xchg(&firsttime, 0)) {
 		nsegs = rtors.rtrsp - rtors.rtseg;
 		nsegs = clamp_val(nsegs, 0, RCUTORTURE_RDR_MAX_SEGS);
-- 
2.34.1