rcu-tasks: Remove second argument of rcu_read_unlock_trace_special()

The second argument of rcu_read_unlock_trace_special() is always zero.
When called from exit_tasks_rcu_finish_trace(), it is the constant
zero, and rcu_read_unlock_trace_special() doesn't get called from
rcu_read_unlock_trace() unless the value of local variable "nesting"
is zero because in that case the early return is taken instead.

This commit therefore removes the "nesting" argument from the
rcu_read_unlock_trace_special() function, substituting the constant
zero within that function.  This commit also adds a WARN_ON_ONCE()
to rcu_read_lock_trace_held() in case non-zeroness some day appears.
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

include/linux/rcupdate_trace.h

@@ -31,7 +31,7 @@ static inline int rcu_read_lock_trace_held(void)
 
 #ifdef CONFIG_TASKS_TRACE_RCU
 
-void rcu_read_unlock_trace_special(struct task_struct *t, int nesting);
+void rcu_read_unlock_trace_special(struct task_struct *t);
 
 /**
  * rcu_read_lock_trace - mark beginning of RCU-trace read-side critical section
@@ -80,7 +80,8 @@ static inline void rcu_read_unlock_trace(void)
 		WRITE_ONCE(t->trc_reader_nesting, nesting);
 		return;  // We assume shallow reader nesting.
 	}
-	rcu_read_unlock_trace_special(t, nesting);
+	WARN_ON_ONCE(nesting != 0);
+	rcu_read_unlock_trace_special(t);
 }
 
 void call_rcu_tasks_trace(struct rcu_head *rhp, rcu_callback_t func);

kernel/rcu/tasks.h

@@ -848,7 +848,7 @@ static void rcu_read_unlock_iw(struct irq_work *iwp)
 static DEFINE_IRQ_WORK(rcu_tasks_trace_iw, rcu_read_unlock_iw);
 
 /* If we are the last reader, wake up the grace-period kthread. */
-void rcu_read_unlock_trace_special(struct task_struct *t, int nesting)
+void rcu_read_unlock_trace_special(struct task_struct *t)
 {
 	int nq = READ_ONCE(t->trc_reader_special.b.need_qs);
 
@@ -858,7 +858,7 @@ void rcu_read_unlock_trace_special(struct task_struct *t, int nesting)
 	// Update .need_qs before ->trc_reader_nesting for irq/NMI handlers.
 	if (nq)
 		WRITE_ONCE(t->trc_reader_special.b.need_qs, false);
-	WRITE_ONCE(t->trc_reader_nesting, nesting);
+	WRITE_ONCE(t->trc_reader_nesting, 0);
 	if (nq && atomic_dec_and_test(&trc_n_readers_need_end))
 		irq_work_queue(&rcu_tasks_trace_iw);
 }
@@ -1200,7 +1200,7 @@ static void exit_tasks_rcu_finish_trace(struct task_struct *t)
 	WARN_ON_ONCE(READ_ONCE(t->trc_reader_nesting));
 	WRITE_ONCE(t->trc_reader_nesting, 0);
 	if (WARN_ON_ONCE(READ_ONCE(t->trc_reader_special.b.need_qs)))
-		rcu_read_unlock_trace_special(t, 0);
+		rcu_read_unlock_trace_special(t);
 }
 
 /**