linux-kernel - [PATCH 2/3] srcu: Explain the reason behind the read side critical section on GP start

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Date:   Thu, 13 Oct 2022 19:22:43 +0200
From:   Frederic Weisbecker <frederic@...nel.org>
To:     "Paul E . McKenney" <paulmck@...nel.org>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Frederic Weisbecker <frederic@...nel.org>,
        quic_neeraju@...cinc.com, joel@...lfernandes.org,
        rcu@...r.kernel.org, Lai Jiangshan <jiangshanlai@...il.com>
Subject: [PATCH 2/3] srcu: Explain the reason behind the read side critical section on GP start

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>
---
 kernel/rcu/srcutree.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c
index 8b7ef1031d89..5bf67d997796 100644
--- a/kernel/rcu/srcutree.c
+++ b/kernel/rcu/srcutree.c
@@ -1154,6 +1154,11 @@ static unsigned long srcu_gp_start_if_needed(struct srcu_struct *ssp,
 	int ss_state;
 
 	check_init_srcu_struct(ssp);
+	/*
+	 * While starting the new gp if needed, make sure we are in an SRCU read
+	 * side critical section so that the gp sequence can't wrap around in
+	 * the middle.
+	 */
 	idx = __srcu_read_lock_nmisafe(ssp, false);
 	ss_state = smp_load_acquire(&ssp->srcu_size_state);
 	if (ss_state < SRCU_SIZE_WAIT_CALL)
-- 
2.25.1