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From:   Frederic Weisbecker <frederic@kernel.org>
To:     "Paul E . McKenney" <paulmck@kernel.org>
Cc:     LKML <linux-kernel@vger.kernel.org>,
        Frederic Weisbecker <frederic@kernel.org>,
        Yong He <zhuangel570@gmail.com>,
        Neeraj upadhyay <neeraj.iitr10@gmail.com>,
        Joel Fernandes <joel@joelfernandes.org>,
        Boqun Feng <boqun.feng@gmail.com>,
        Uladzislau Rezki <urezki@gmail.com>, RCU <rcu@vger.kernel.org>
Subject: [PATCH 2/5] srcu: Only accelerate on enqueue time
Date:   Wed,  4 Oct 2023 01:29:00 +0200
Message-ID: <20231003232903.7109-3-frederic@kernel.org>
In-Reply-To: <20231003232903.7109-1-frederic@kernel.org>
References: <20231003232903.7109-1-frederic@kernel.org>
MIME-Version: 1.0
Content-Transfer-Encoding: 8bit
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Acceleration in SRCU happens on enqueue time for each new callback. This
operation is expected not to fail and therefore any similar attempt
from other places shouldn't find any remaining callbacks to accelerate.

Moreover accelerations performed beyond enqueue time are error prone
because rcu_seq_snap() then may return the snapshot for a new grace
period that is not going to be started.

Remove these dangerous and needless accelerations and introduce instead
assertions reporting leaking unaccelerated callbacks beyond enqueue
time.

Co-developed-by: Yong He <zhuangel570@gmail.com>
Co-developed-by: Joel Fernandes <joel@joelfernandes.org>
Co-developed-by: Neeraj upadhyay <neeraj.iitr10@gmail.com>
Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
---
 kernel/rcu/srcutree.c | 6 ++----
 1 file changed, 2 insertions(+), 4 deletions(-)

diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c
index 9fab9ac36996..560e99ec5333 100644
--- a/kernel/rcu/srcutree.c
+++ b/kernel/rcu/srcutree.c
@@ -784,8 +784,7 @@ static void srcu_gp_start(struct srcu_struct *ssp)
 	spin_lock_rcu_node(sdp);  /* Interrupts already disabled. */
 	rcu_segcblist_advance(&sdp->srcu_cblist,
 			      rcu_seq_current(&ssp->srcu_sup->srcu_gp_seq));
-	(void)rcu_segcblist_accelerate(&sdp->srcu_cblist,
-				       rcu_seq_snap(&ssp->srcu_sup->srcu_gp_seq));
+	WARN_ON_ONCE(!rcu_segcblist_segempty(&sdp->srcu_cblist, RCU_NEXT_TAIL));
 	spin_unlock_rcu_node(sdp);  /* Interrupts remain disabled. */
 	WRITE_ONCE(ssp->srcu_sup->srcu_gp_start, jiffies);
 	WRITE_ONCE(ssp->srcu_sup->srcu_n_exp_nodelay, 0);
@@ -1721,6 +1720,7 @@ static void srcu_invoke_callbacks(struct work_struct *work)
 	ssp = sdp->ssp;
 	rcu_cblist_init(&ready_cbs);
 	spin_lock_irq_rcu_node(sdp);
+	WARN_ON_ONCE(!rcu_segcblist_segempty(&sdp->srcu_cblist, RCU_NEXT_TAIL));
 	rcu_segcblist_advance(&sdp->srcu_cblist,
 			      rcu_seq_current(&ssp->srcu_sup->srcu_gp_seq));
 	if (sdp->srcu_cblist_invoking ||
@@ -1750,8 +1750,6 @@ static void srcu_invoke_callbacks(struct work_struct *work)
 	 */
 	spin_lock_irq_rcu_node(sdp);
 	rcu_segcblist_add_len(&sdp->srcu_cblist, -len);
-	(void)rcu_segcblist_accelerate(&sdp->srcu_cblist,
-				       rcu_seq_snap(&ssp->srcu_sup->srcu_gp_seq));
 	sdp->srcu_cblist_invoking = false;
 	more = rcu_segcblist_ready_cbs(&sdp->srcu_cblist);
 	spin_unlock_irq_rcu_node(sdp);
-- 
2.41.0