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>

2 files changed, 17 insertions, 0 deletions

diff --git a/tools/testing/selftests/rcutorture/configs/rcu/TRIVIAL b/tools/testing/selftests/rcutorture/configs/rcu/TRIVIAL
new file mode 100644
index 000000000000..4d8eb5bfb6f6
--- /dev/null
+++ b/tools/testing/selftests/rcutorture/configs/rcu/TRIVIAL
@@ -0,0 +1,14 @@
+CONFIG_SMP=y
+CONFIG_NR_CPUS=8
+CONFIG_PREEMPT_NONE=y
+CONFIG_PREEMPT_VOLUNTARY=n
+CONFIG_PREEMPT=n
+CONFIG_HZ_PERIODIC=n
+CONFIG_NO_HZ_IDLE=y
+CONFIG_NO_HZ_FULL=n
+CONFIG_HOTPLUG_CPU=n
+CONFIG_SUSPEND=n
+CONFIG_HIBERNATION=n
+CONFIG_DEBUG_LOCK_ALLOC=n
+CONFIG_DEBUG_OBJECTS_RCU_HEAD=n
+CONFIG_RCU_EXPERT=y
diff --git a/tools/testing/selftests/rcutorture/configs/rcu/TRIVIAL.boot b/tools/testing/selftests/rcutorture/configs/rcu/TRIVIAL.boot
new file mode 100644
index 000000000000..7017f5f5a55f
--- /dev/null
+++ b/tools/testing/selftests/rcutorture/configs/rcu/TRIVIAL.boot
@@ -0,0 +1,3 @@
+rcutorture.torture_type=trivial
+rcutorture.onoff_interval=0
+rcutorture.shuffle_interval=0