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

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Message-Id: <20221019225846.2501109-10-paulmck@kernel.org>
Date:   Wed, 19 Oct 2022 15:58:45 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     rcu@...r.kernel.org
Cc:     linux-kernel@...r.kernel.org, kernel-team@...com,
        rostedt@...dmis.org, Frederic Weisbecker <frederic@...nel.org>,
        "Paul E . McKenney" <paulmck@...nel.org>
Subject: [PATCH v3 rcu 10/11] srcu: Explain the reason behind the read side critical section on GP start

From: Frederic Weisbecker <frederic@...nel.org>

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@...nel.org>
Signed-off-by: Frederic Weisbecker <frederic@...nel.org>
Signed-off-by: Paul E. McKenney <paulmck@...nel.org>
---
 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