locktorture: With nested locks, occasionally skip main lock

If we're using nested locking to stress things, occasionally
skip taking the main lock, so that we can get some different
contention patterns between the writers (to hopefully get two
disjoint blocked trees)

Cc: Davidlohr Bueso <dave@stgolabs.net>
Cc: "Paul E. McKenney" <paulmck@kernel.org>
Cc: Josh Triplett <josh@joshtriplett.org>
Cc: Joel Fernandes <joel@joelfernandes.org>
Cc: Juri Lelli <juri.lelli@redhat.com>
Cc: Valentin Schneider <vschneid@redhat.com>
Cc: Dietmar Eggemann <dietmar.eggemann@arm.com>
Cc: kernel-team@android.com
Co-developed-by: Connor O'Brien <connoro@google.com>
Signed-off-by: Connor O'Brien <connoro@google.com>
Signed-off-by: John Stultz <jstultz@google.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

kernel/locking/locktorture.c

@@ -760,6 +760,7 @@ static int lock_torture_writer(void *arg)
 	int tid = lwsp - cxt.lwsa;
 	DEFINE_TORTURE_RANDOM(rand);
 	u32 lockset_mask;
+	bool skip_main_lock;
 
 	VERBOSE_TOROUT_STRING("lock_torture_writer task started");
 	set_user_nice(current, MAX_NICE);
@@ -769,9 +770,22 @@ static int lock_torture_writer(void *arg)
 			schedule_timeout_uninterruptible(1);
 
 		lockset_mask = torture_random(&rand);
+		/*
+		 * When using nested_locks, we want to occasionally
+		 * skip the main lock so we can avoid always serializing
+		 * the lock chains on that central lock. By skipping the
+		 * main lock occasionally, we can create different
+		 * contention patterns (allowing for multiple disjoint
+		 * blocked trees)
+		 */
+		skip_main_lock = (nested_locks &&
+				 !(torture_random(&rand) % 100));
+
 		cxt.cur_ops->task_boost(&rand);
 		if (cxt.cur_ops->nested_lock)
 			cxt.cur_ops->nested_lock(tid, lockset_mask);
+
+		if (!skip_main_lock) {
 			cxt.cur_ops->writelock(tid);
 			if (WARN_ON_ONCE(lock_is_write_held))
 				lwsp->n_lock_fail++;
@@ -780,10 +794,13 @@ static int lock_torture_writer(void *arg)
 				lwsp->n_lock_fail++; /* rare, but... */
 
 			lwsp->n_lock_acquired++;
+		}
 		cxt.cur_ops->write_delay(&rand);
+		if (!skip_main_lock) {
 			lock_is_write_held = false;
 			WRITE_ONCE(last_lock_release, jiffies);
 			cxt.cur_ops->writeunlock(tid);
+		}
 		if (cxt.cur_ops->nested_unlock)
 			cxt.cur_ops->nested_unlock(tid, lockset_mask);