[PATCH v3 rcu 10/11] srcu: Explain the reason behind the read side critical section on GP start

[Paul E. McKenney]

From: Frederic Weisbecker <frederic@xxxxxxxxxx>

Tell about the need to protect against concurrent updaters who may
overflow the GP counter behind the current update.

Reported-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
Signed-off-by: Frederic Weisbecker <frederic@xxxxxxxxxx>
Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
---
 kernel/rcu/srcutree.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c
index 18bb696cff8ca..272830a87e566 100644
--- a/kernel/rcu/srcutree.c
+++ b/kernel/rcu/srcutree.c
@@ -1158,6 +1158,11 @@ static unsigned long srcu_gp_start_if_needed(struct srcu_struct *ssp,
 	int ss_state;
 
 	check_init_srcu_struct(ssp);
+	/*
+	 * While starting a new grace period, make sure we are in an
+	 * SRCU read-side critical section so that the grace-period
+	 * sequence number cannot wrap around in the meantime.
+	 */
 	idx = __srcu_read_lock_nmisafe(ssp, false);
 	ss_state = smp_load_acquire(&ssp->srcu_size_state);
 	if (ss_state < SRCU_SIZE_WAIT_CALL)
-- 
2.31.1.189.g2e36527f23