rcutorture: Add trivial RCU implementation

I have been showing off a trivial RCU implementation for non-preemptive
environments for some time now:

	#define rcu_read_lock()
	#define rcu_read_unlock()
	#define rcu_dereference(p) READ_ONCE(p)
	#define rcu_assign_pointer(p, v) smp_store_release(&(p), (v))
	void synchronize_rcu(void)
	{
	int cpu;
		for_each_online_cpu(cpu)
			sched_setaffinity(current->pid, cpumask_of(cpu));
	}

Trivial or not, as the old saying goes, "if it ain't tested, it don't
work!".  This commit therefore adds a "trivial" flavor to rcutorture
and a corresponding TRIVIAL test scenario.  This variant does not handle
CPU hotplug, which is unconditionally enabled on x86 for post-v5.1-rc3
kernels, which is why the TRIVIAL.boot says "rcutorture.onoff_interval=0".
This commit actually does handle CONFIG_PREEMPT=y kernels, but only
because it turns back the Linux-kernel clock in order to provide these
alternative definitions (or the moral equivalent thereof):

	#define rcu_read_lock() preempt_disable()
	#define rcu_read_unlock() preempt_enable()

In CONFIG_PREEMPT=n kernels without debugging, these are equivalent to
empty macros give or take a compiler barrier.  However, the have been
successfully tested with actual empty macros as well.

Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com>
[ paulmck: Fix symbol issue reported by kbuild test robot <lkp@intel.com>. ]
[ paulmck: Work around sched_setaffinity() issue noted by Andrea Parri. ]
[ paulmck: Add rcutorture.shuffle_interval=0 to TRIVIAL.boot to fix
  interaction with shuffler task noted by Peter Zijlstra. ]
Tested-by: Andrea Parri <andrea.parri@amarulasolutions.com>

1 files changed, 5 insertions, 0 deletions

diff --git a/kernel/rcu/rcu.h b/kernel/rcu/rcu.h
index 390aab20115e..5290b01de534 100644
--- a/kernel/rcu/rcu.h
+++ b/kernel/rcu/rcu.h
@@ -446,6 +446,7 @@ void rcu_request_urgent_qs_task(struct task_struct *t);
 enum rcutorture_type {
 	RCU_FLAVOR,
 	RCU_TASKS_FLAVOR,
+	RCU_TRIVIAL_FLAVOR,
 	SRCU_FLAVOR,
 	INVALID_RCU_FLAVOR
 };
@@ -479,6 +480,10 @@ void do_trace_rcu_torture_read(const char *rcutorturename,
 #endif
 #endif
 
+#if IS_ENABLED(CONFIG_RCU_TORTURE_TEST) || IS_MODULE(CONFIG_RCU_TORTURE_TEST)
+long rcutorture_sched_setaffinity(pid_t pid, const struct cpumask *in_mask);
+#endif
+
 #ifdef CONFIG_TINY_SRCU
 
 static inline void srcutorture_get_gp_data(enum rcutorture_type test_type,