rcu-tasks: Split ->trc_reader_need_end

This commit splits ->trc_reader_need_end by using the rcu_special union.
This change permits readers to check to see if a memory barrier is
required without any added overhead in the common case where no such
barrier is required.  This commit also adds the read-side checking.
Later commits will add the machinery to properly set the new
->trc_reader_special.b.need_mb field.

This commit also makes rcu_read_unlock_trace_special() tolerate nested
read-side critical sections within interrupt and NMI handlers.
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);
+void rcu_read_unlock_trace_special(struct task_struct *t, int nesting);
 
 /**
  * rcu_read_lock_trace - mark beginning of RCU-trace read-side critical section
@@ -50,6 +50,8 @@ static inline void rcu_read_lock_trace(void)
 	struct task_struct *t = current;
 
 	WRITE_ONCE(t->trc_reader_nesting, READ_ONCE(t->trc_reader_nesting) + 1);
+	if (t->trc_reader_special.b.need_mb)
+		smp_mb(); // Pairs with update-side barriers
 	rcu_lock_acquire(&rcu_trace_lock_map);
 }
 
@@ -69,10 +71,11 @@ static inline void rcu_read_unlock_trace(void)
 
 	rcu_lock_release(&rcu_trace_lock_map);
 	nesting = READ_ONCE(t->trc_reader_nesting) - 1;
-	WRITE_ONCE(t->trc_reader_nesting, nesting);
-	if (likely(!READ_ONCE(t->trc_reader_need_end)) || nesting)
+	if (likely(!READ_ONCE(t->trc_reader_special.s)) || nesting) {
+		WRITE_ONCE(t->trc_reader_nesting, nesting);
 		return;  // We assume shallow reader nesting.
-	rcu_read_unlock_trace_special(t);
+	}
+	rcu_read_unlock_trace_special(t, nesting);
 }
 
 void call_rcu_tasks_trace(struct rcu_head *rhp, rcu_callback_t func);

include/linux/sched.h

@@ -613,7 +613,7 @@ union rcu_special {
 		u8			blocked;
 		u8			need_qs;
 		u8			exp_hint; /* Hint for performance. */
-		u8			pad; /* No garbage from compiler! */
+		u8			need_mb; /* Readers need smp_mb(). */
 	} b; /* Bits. */
 	u32 s; /* Set of bits. */
 };
@@ -727,7 +727,7 @@ struct task_struct {
 #ifdef CONFIG_TASKS_TRACE_RCU
 	int				trc_reader_nesting;
 	int				trc_ipi_to_cpu;
-	bool				trc_reader_need_end;
+	union rcu_special		trc_reader_special;
 	bool				trc_reader_checked;
 	struct list_head		trc_holdout_list;
 #endif /* #ifdef CONFIG_TASKS_TRACE_RCU */

init/init_task.c

@@ -143,6 +143,7 @@ struct task_struct init_task
 #endif
 #ifdef CONFIG_TASKS_TRACE_RCU
 	.trc_reader_nesting = 0,
+	.trc_reader_special.s = 0,
 	.trc_holdout_list = LIST_HEAD_INIT(init_task.trc_holdout_list),
 #endif
 #ifdef CONFIG_CPUSETS

kernel/fork.c

@@ -1685,6 +1685,7 @@ static inline void rcu_copy_process(struct task_struct *p)
 #endif /* #ifdef CONFIG_TASKS_RCU */
 #ifdef CONFIG_TASKS_TRACE_RCU
 	p->trc_reader_nesting = 0;
+	p->trc_reader_special.s = 0;
 	INIT_LIST_HEAD(&p->trc_holdout_list);
 #endif /* #ifdef CONFIG_TASKS_TRACE_RCU */
 }

kernel/rcu/tasks.h

@@ -723,10 +723,17 @@ DEFINE_RCU_TASKS(rcu_tasks_trace, rcu_tasks_wait_gp, call_rcu_tasks_trace,
 		 "RCU Tasks Trace");
 
 /* If we are the last reader, wake up the grace-period kthread. */
-void rcu_read_unlock_trace_special(struct task_struct *t)
+void rcu_read_unlock_trace_special(struct task_struct *t, int nesting)
 {
-	WRITE_ONCE(t->trc_reader_need_end, false);
-	if (atomic_dec_and_test(&trc_n_readers_need_end))
+	int nq = t->trc_reader_special.b.need_qs;
+
+	if (t->trc_reader_special.b.need_mb)
+		smp_mb(); // Pairs with update-side barriers.
+	// 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);
+	if (nq && atomic_dec_and_test(&trc_n_readers_need_end))
 		wake_up(&trc_wait);
 }
 EXPORT_SYMBOL_GPL(rcu_read_unlock_trace_special);
@@ -777,8 +784,8 @@ static void trc_read_check_handler(void *t_in)
 	// Get here if the task is in a read-side critical section.  Set
 	// its state so that it will awaken the grace-period kthread upon
 	// exit from that critical section.
-	WARN_ON_ONCE(t->trc_reader_need_end);
-	WRITE_ONCE(t->trc_reader_need_end, true);
+	WARN_ON_ONCE(t->trc_reader_special.b.need_qs);
+	WRITE_ONCE(t->trc_reader_special.b.need_qs, true);
 
 reset_ipi:
 	// Allow future IPIs to be sent on CPU and for task.
@@ -804,8 +811,8 @@ static bool trc_inspect_reader(struct task_struct *t, void *arg)
 	// exit from that critical section.
 	if (unlikely(t->trc_reader_nesting)) {
 		atomic_inc(&trc_n_readers_need_end); // One more to wait on.
-		WARN_ON_ONCE(t->trc_reader_need_end);
-		WRITE_ONCE(t->trc_reader_need_end, true);
+		WARN_ON_ONCE(t->trc_reader_special.b.need_qs);
+		WRITE_ONCE(t->trc_reader_special.b.need_qs, true);
 	}
 	return true;
 }
@@ -884,7 +891,7 @@ static void rcu_tasks_trace_pregp_step(void)
 static void rcu_tasks_trace_pertask(struct task_struct *t,
 				    struct list_head *hop)
 {
-	WRITE_ONCE(t->trc_reader_need_end, false);
+	WRITE_ONCE(t->trc_reader_special.b.need_qs, false);
 	WRITE_ONCE(t->trc_reader_checked, false);
 	t->trc_ipi_to_cpu = -1;
 	trc_wait_for_one_reader(t, hop);
@@ -916,7 +923,7 @@ static void show_stalled_task_trace(struct task_struct *t, bool *firstreport)
 		 ".i"[is_idle_task(t)],
 		 ".N"[cpu > 0 && tick_nohz_full_cpu(cpu)],
 		 t->trc_reader_nesting,
-		 " N"[!!t->trc_reader_need_end],
+		 " N"[!!t->trc_reader_special.b.need_qs],
 		 cpu);
 	sched_show_task(t);
 }
@@ -980,11 +987,11 @@ static void rcu_tasks_trace_postgp(struct rcu_tasks *rtp)
 			break;  // Count reached zero.
 		// Stall warning time, so make a list of the offenders.
 		for_each_process_thread(g, t)
-			if (READ_ONCE(t->trc_reader_need_end))
+			if (READ_ONCE(t->trc_reader_special.b.need_qs))
 				trc_add_holdout(t, &holdouts);
 		firstreport = true;
 		list_for_each_entry_safe(t, g, &holdouts, trc_holdout_list)
-			if (READ_ONCE(t->trc_reader_need_end)) {
+			if (READ_ONCE(t->trc_reader_special.b.need_qs)) {
 				show_stalled_task_trace(t, &firstreport);
 				trc_del_holdout(t);
 			}
@@ -1003,8 +1010,8 @@ void exit_tasks_rcu_finish_trace(struct task_struct *t)
 	WRITE_ONCE(t->trc_reader_checked, true);
 	WARN_ON_ONCE(t->trc_reader_nesting);
 	WRITE_ONCE(t->trc_reader_nesting, 0);
-	if (WARN_ON_ONCE(READ_ONCE(t->trc_reader_need_end)))
-		rcu_read_unlock_trace_special(t);
+	if (WARN_ON_ONCE(READ_ONCE(t->trc_reader_special.b.need_qs)))
+		rcu_read_unlock_trace_special(t, 0);
 }
 
 /**