[PATCH RFC 12/13] rcutorture: Bracket Tasks RCU readers with trampoline nesting

[Josef Bacik <josef@xxxxxxxxxxxxxx>]

rcutorture's tasks flavor models a Tasks RCU reader as "any stretch of
kernel code", and rcu_read_delay() deliberately preempts inside it to
check that a preemption does not end the read-side critical section.
Once preemption outside a trampoline becomes a quiescent state that
model no longer matches what Tasks RCU protects, and the readers would
report false too-short grace periods.

Have tasks_torture_read_lock()/unlock() raise and drop
current->rcu_tramp_nesting so the reader models a trampoline, which is
the thing Tasks RCU actually guards; the deliberate preemption inside it
then continues to be, correctly, not a quiescent state.

Assisted-by: LLM
Signed-off-by: Josef Bacik <josef@xxxxxxxxxxxxxx>
---
 kernel/rcu/rcutorture.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/kernel/rcu/rcutorture.c b/kernel/rcu/rcutorture.c
index 794937e13e7c..df6dd708cea7 100644
--- a/kernel/rcu/rcutorture.c
+++ b/kernel/rcu/rcutorture.c
@@ -1144,11 +1144,17 @@ static struct rcu_torture_ops trivial_preempt_ops = {
 
 static int tasks_torture_read_lock(void)
 {
+       /*
+        * Model a trampoline: with CONFIG_RCU_TASKS_PREEMPT_QS a preemption is
+        * otherwise a quiescent state and rcu_read_delay() preempts on purpose.
+        */
+       rcu_tasks_trampoline_enter();
        return 0;
 }
 
 static void tasks_torture_read_unlock(int idx)
 {
+       rcu_tasks_trampoline_exit();
 }
 
 static void rcu_tasks_torture_deferred_free(struct rcu_torture *p)

-- 
2.55.0