rcutorture: Add a textbook-style trivial preemptible RCU

This commit adds a trivial textbook implementation of preemptible RCU
to rcutorture ("torture_type=trivial-preempt"), similar in spirit to the
existing "torture_type=trivial" textbook implementation of non-preemptible
RCU.  Neither trivial RCU implementation has any value for production use,
and are intended only to keep Paul honest in his introductory writings
and presentations.

[ paulmck: Apply kernel test robot feedback. ]

Signed-off-by: Joel Fernandes <joelagnelf@nvidia.com>

include/linux/sched.h

@@ -949,6 +949,10 @@ struct task_struct {
 	struct srcu_ctr __percpu	*trc_reader_scp;
 #endif /* #ifdef CONFIG_TASKS_TRACE_RCU */
 
+#ifdef CONFIG_TRIVIAL_PREEMPT_RCU
+	int				rcu_trivial_preempt_nesting;
+#endif /* #ifdef CONFIG_TRIVIAL_PREEMPT_RCU */
+
 	struct sched_info		sched_info;
 
 	struct list_head		tasks;

kernel/rcu/Kconfig.debug

@@ -228,4 +228,15 @@ config RCU_DYNTICKS_TORTURE
 
 	  This has no value for production and is only for testing.
 
+config TRIVIAL_PREEMPT_RCU
+	bool "Textbook trivial preemptible RCU in rcutorture"
+	depends on RCU_EXPERT && RCU_TORTURE_TEST
+	default n
+	help
+	  This option enables a textbook preemptible RCU that is
+	  implemented in rcutorture.  Its sole purpose is to validate
+	  code used in books, papers, and presentations.
+
+	  This has no value for production and is only for testing.
+
 endmenu # "RCU Debugging"

kernel/rcu/rcu.h

@@ -691,4 +691,8 @@ int rcu_stall_notifier_call_chain(unsigned long val, void *v);
 static inline int rcu_stall_notifier_call_chain(unsigned long val, void *v) { return NOTIFY_DONE; }
 #endif // #else // #if defined(CONFIG_RCU_STALL_COMMON) && defined(CONFIG_RCU_CPU_STALL_NOTIFIER)
 
+#ifdef CONFIG_TRIVIAL_PREEMPT_RCU
+void synchronize_rcu_trivial_preempt(void);
+#endif // #ifdef CONFIG_TRIVIAL_PREEMPT_RCU
+
 #endif /* __LINUX_RCU_H */

kernel/rcu/rcutorture.c

@@ -1061,6 +1061,61 @@ static struct rcu_torture_ops trivial_ops = {
 	.name		= "trivial"
 };
 
+#ifdef CONFIG_TRIVIAL_PREEMPT_RCU
+
+/*
+ * Definitions for trivial CONFIG_PREEMPT=y torture testing.  This
+ * implementation does not work well with large numbers of tasks or with
+ * long-term preemption.  Either or both get you RCU CPU stall warnings.
+ */
+
+static void rcu_sync_torture_init_trivial_preempt(void)
+{
+	rcu_sync_torture_init();
+	if (WARN_ONCE(onoff_interval || shuffle_interval, "%s: Non-zero onoff_interval (%d) or shuffle_interval (%d) breaks trivial RCU, resetting to zero", __func__, onoff_interval, shuffle_interval)) {
+		onoff_interval = 0;
+		shuffle_interval = 0;
+	}
+}
+
+static int rcu_torture_read_lock_trivial_preempt(void)
+{
+	struct task_struct *t = current;
+
+	WRITE_ONCE(t->rcu_trivial_preempt_nesting, t->rcu_trivial_preempt_nesting + 1);
+	smp_mb();
+	return 0;
+}
+
+static void rcu_torture_read_unlock_trivial_preempt(int idx)
+{
+	struct task_struct *t = current;
+
+	smp_store_release(&t->rcu_trivial_preempt_nesting, t->rcu_trivial_preempt_nesting - 1);
+}
+
+static struct rcu_torture_ops trivial_preempt_ops = {
+	.ttype		= RCU_TRIVIAL_FLAVOR,
+	.init		= rcu_sync_torture_init_trivial_preempt,
+	.readlock	= rcu_torture_read_lock_trivial_preempt,
+	.read_delay	= rcu_read_delay,  // just reuse rcu's version.
+	.readunlock	= rcu_torture_read_unlock_trivial_preempt,
+	.readlock_held	= torture_readlock_not_held,
+	.get_gp_seq	= rcu_no_completed,
+	.sync		= synchronize_rcu_trivial_preempt,
+	.exp_sync	= synchronize_rcu_trivial_preempt,
+	.irq_capable	= 0, // In theory it should be, but let's keep it trivial.
+	.name		= "trivial-preempt"
+};
+
+#define TRIVIAL_PREEMPT_OPS &trivial_preempt_ops,
+
+#else // #ifdef CONFIG_TRIVIAL_PREEMPT_RCU
+
+#define TRIVIAL_PREEMPT_OPS
+
+#endif // #else // #ifdef CONFIG_TRIVIAL_PREEMPT_RCU
+
 #ifdef CONFIG_TASKS_RCU
 
 /*
@@ -4449,7 +4504,7 @@ rcu_torture_init(void)
 	static struct rcu_torture_ops *torture_ops[] = {
 		&rcu_ops, &rcu_busted_ops, &srcu_ops, &srcud_ops, &busted_srcud_ops,
 		TASKS_OPS TASKS_RUDE_OPS TASKS_TRACING_OPS
-		&trivial_ops,
+		&trivial_ops, TRIVIAL_PREEMPT_OPS
 	};
 
 	if (!torture_init_begin(torture_type, verbose))

kernel/rcu/update.c

@@ -538,6 +538,28 @@ long torture_sched_setaffinity(pid_t pid, const struct cpumask *in_mask, bool do
 EXPORT_SYMBOL_GPL(torture_sched_setaffinity);
 #endif
 
+#if IS_ENABLED(CONFIG_TRIVIAL_PREEMPT_RCU)
+// Trivial and stupid grace-period wait.  Defined here so that lockdep
+// kernels can find tasklist_lock.
+void synchronize_rcu_trivial_preempt(void)
+{
+	struct task_struct *g;
+	struct task_struct *t;
+
+	smp_mb(); // Order prior accesses before grace-period start.
+	rcu_read_lock(); // Protect task list.
+	for_each_process_thread(g, t) {
+		if (t == current)
+			continue;  // Don't deadlock on ourselves!
+		// Order later rcu_read_lock() on other tasks after QS.
+		while (smp_load_acquire(&t->rcu_trivial_preempt_nesting))
+			continue;
+	}
+	rcu_read_unlock();
+}
+EXPORT_SYMBOL_GPL(synchronize_rcu_trivial_preempt);
+#endif // #if IS_ENABLED(CONFIG_TRIVIAL_PREEMPT_RCU)
+
 int rcu_cpu_stall_notifiers __read_mostly; // !0 = provide stall notifiers (rarely useful)
 EXPORT_SYMBOL_GPL(rcu_cpu_stall_notifiers);
 

tools/testing/selftests/rcutorture/configs/rcu/TRIVIAL-PREEMPT

@@ -0,0 +1,12 @@
+CONFIG_SMP=y
+CONFIG_NR_CPUS=8
+CONFIG_PREEMPT_NONE=n
+CONFIG_PREEMPT_VOLUNTARY=n
+CONFIG_PREEMPT=y
+CONFIG_HZ_PERIODIC=n
+CONFIG_NO_HZ_IDLE=y
+CONFIG_NO_HZ_FULL=n
+CONFIG_DEBUG_LOCK_ALLOC=n
+CONFIG_DEBUG_OBJECTS_RCU_HEAD=n
+CONFIG_RCU_EXPERT=y
+CONFIG_TRIVIAL_PREEMPT_RCU=y

tools/testing/selftests/rcutorture/configs/rcu/TRIVIAL-PREEMPT.boot

@@ -0,0 +1,3 @@
+rcutorture.torture_type=trivial-preempt
+rcutorture.onoff_interval=0
+rcutorture.shuffle_interval=0