srcu: Use ->srcu_gp_seq for rcutorture reader batch

This commit stops using ->srcu_idx for rcutorture's reader-batch
consistency checking, using ->srcu_gp_seq instead.  This is a first
step towards a faster srcu_read_{,un}lock_lite() that avoids the array
accesses that use ->srcu_idx.

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Cc: Alexei Starovoitov <ast@kernel.org>
Cc: Andrii Nakryiko <andrii@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Kent Overstreet <kent.overstreet@linux.dev>
Cc: <bpf@vger.kernel.org>
Signed-off-by: Boqun Feng <boqun.feng@gmail.com>

Author: paulmckrcu

kernel/rcu/rcutorture.c

@@ -791,6 +791,7 @@ static struct rcu_torture_ops srcu_ops = {
 	.readunlock	= srcu_torture_read_unlock,
 	.readlock_held	= torture_srcu_read_lock_held,
 	.get_gp_seq	= srcu_torture_completed,
+	.gp_diff	= rcu_seq_diff,
 	.deferred_free	= srcu_torture_deferred_free,
 	.sync		= srcu_torture_synchronize,
 	.exp_sync	= srcu_torture_synchronize_expedited,
@@ -834,6 +835,7 @@ static struct rcu_torture_ops srcud_ops = {
 	.readunlock	= srcu_torture_read_unlock,
 	.readlock_held	= torture_srcu_read_lock_held,
 	.get_gp_seq	= srcu_torture_completed,
+	.gp_diff	= rcu_seq_diff,
 	.deferred_free	= srcu_torture_deferred_free,
 	.sync		= srcu_torture_synchronize,
 	.exp_sync	= srcu_torture_synchronize_expedited,

kernel/rcu/srcutree.c

@@ -1675,7 +1675,7 @@ EXPORT_SYMBOL_GPL(srcu_barrier);
  */
 unsigned long srcu_batches_completed(struct srcu_struct *ssp)
 {
-	return READ_ONCE(ssp->srcu_idx);
+	return READ_ONCE(ssp->srcu_sup->srcu_gp_seq);
 }
 EXPORT_SYMBOL_GPL(srcu_batches_completed);