[PATCH 2/3] rcu: Add warning to ensure rcu_seq_done_exact() is working

[Joel Fernandes]

The previous patch improved the rcu_seq_done_exact() function by adding
a meaningful constant for the guardband.

Ensure that this is working for the future by a quick check during
rcu_gp_init().

Signed-off-by: Joel Fernandes <joelagn...@nvidia.com>
---
 kernel/rcu/tree.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 659f83e71048..29ddbcbea25e 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -1798,6 +1798,7 @@ static noinline_for_stack bool rcu_gp_init(void)
        struct rcu_data *rdp;
        struct rcu_node *rnp = rcu_get_root();
        bool start_new_poll;
+       unsigned long old_gp_seq;
 
        WRITE_ONCE(rcu_state.gp_activity, jiffies);
        raw_spin_lock_irq_rcu_node(rnp);
@@ -1825,7 +1826,11 @@ static noinline_for_stack bool rcu_gp_init(void)
         */
        start_new_poll = rcu_sr_normal_gp_init();
        /* Record GP times before starting GP, hence rcu_seq_start(). */
+       old_gp_seq = rcu_state.gp_seq;
        rcu_seq_start(&rcu_state.gp_seq);
+       /* Ensure that rcu_seq_done_exact() guardband doesn't give false 
positives. */
+       WARN_ON_ONCE(rcu_seq_done_exact(&old_gp_seq, 
rcu_seq_snap(&rcu_state.gp_seq)));
+
        ASSERT_EXCLUSIVE_WRITER(rcu_state.gp_seq);
        trace_rcu_grace_period(rcu_state.name, rcu_state.gp_seq, TPS("start"));
        rcu_poll_gp_seq_start(&rcu_state.gp_seq_polled_snap);
-- 
2.43.0