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From:   "Neeraj Upadhyay (AMD)" <neeraj.iitr10@gmail.com>
To:     rcu@vger.kernel.org
Cc:     linux-kernel@vger.kernel.org, kernel-team@meta.com,
        rostedt@goodmis.org, paulmck@kernel.org, Neeraj.Upadhyay@amd.com,
        Frederic Weisbecker <frederic@kernel.org>,
        Yong He <zhuangel570@gmail.com>,
        Joel Fernandes <joel@joelfernandes.org>,
        Neeraj Upadhyay <neeraj.iitr10@gmail.com>
Subject: [PATCH rcu 1/3] srcu: Remove superfluous callbacks advancing from srcu_gp_start()
Date:   Tue, 12 Dec 2023 23:18:15 +0530
Message-Id: <20231212174817.11919-1-neeraj.iitr10@gmail.com>
In-Reply-To: <20231212174750.GA11886@neeraj.linux>
References: <20231212174750.GA11886@neeraj.linux>
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From: Frederic Weisbecker <frederic@kernel.org>

Callbacks advancing on SRCU must be performed on two specific places:

1) On enqueue time in order to make room for the acceleration of the
   new callback.

2) On invocation time in order to move the callbacks ready to invoke.

Any other callback advancing callsite is needless. Remove the remaining
one in srcu_gp_start().

Co-developed-by: Yong He <zhuangel570@gmail.com>
Signed-off-by: Yong He <zhuangel570@gmail.com>
Co-developed-by: Joel Fernandes <joel@joelfernandes.org>
Signed-off-by: Joel Fernandes <joel@joelfernandes.org>
Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Co-developed-by: Neeraj Upadhyay (AMD) <neeraj.iitr10@gmail.com>
Signed-off-by: Neeraj Upadhyay (AMD) <neeraj.iitr10@gmail.com>
---
 kernel/rcu/srcutree.c | 10 ----------
 1 file changed, 10 deletions(-)

diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c
index 560e99ec5333..e9356a103626 100644
--- a/kernel/rcu/srcutree.c
+++ b/kernel/rcu/srcutree.c
@@ -772,20 +772,10 @@ EXPORT_SYMBOL_GPL(__srcu_read_unlock_nmisafe);
  */
 static void srcu_gp_start(struct srcu_struct *ssp)
 {
-	struct srcu_data *sdp;
 	int state;
 
-	if (smp_load_acquire(&ssp->srcu_sup->srcu_size_state) < SRCU_SIZE_WAIT_BARRIER)
-		sdp = per_cpu_ptr(ssp->sda, get_boot_cpu_id());
-	else
-		sdp = this_cpu_ptr(ssp->sda);
 	lockdep_assert_held(&ACCESS_PRIVATE(ssp->srcu_sup, lock));
 	WARN_ON_ONCE(ULONG_CMP_GE(ssp->srcu_sup->srcu_gp_seq, ssp->srcu_sup->srcu_gp_seq_needed));
-	spin_lock_rcu_node(sdp);  /* Interrupts already disabled. */
-	rcu_segcblist_advance(&sdp->srcu_cblist,
-			      rcu_seq_current(&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);
 	smp_mb(); /* Order prior store to ->srcu_gp_seq_needed vs. GP start. */
-- 
2.40.1