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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>,
        Joel Fernandes <joel@joelfernandes.org>,
        Neeraj Upadhyay <neeraj.iitr10@gmail.com>
Subject: [PATCH rcu 3/3] srcu: Explain why callbacks invocations can't run concurrently
Date:   Tue, 12 Dec 2023 23:18:17 +0530
Message-Id: <20231212174817.11919-3-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>

If an SRCU barrier is queued while callbacks are running and a new
callbacks invocator for the same sdp were to run concurrently, the
RCU barrier might execute too early. As this requirement is non-obvious,
make sure to keep a record.

Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Reviewed-by: Joel Fernandes (Google) <joel@joelfernandes.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Neeraj Upadhyay (AMD) <neeraj.iitr10@gmail.com>
---
 kernel/rcu/srcutree.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c
index 2bfc8ed1eed2..0351a4e83529 100644
--- a/kernel/rcu/srcutree.c
+++ b/kernel/rcu/srcutree.c
@@ -1715,6 +1715,11 @@ static void srcu_invoke_callbacks(struct work_struct *work)
 	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));
+	/*
+	 * Although this function is theoretically re-entrant, concurrent
+	 * callbacks invocation is disallowed to avoid executing an SRCU barrier
+	 * too early.
+	 */
 	if (sdp->srcu_cblist_invoking ||
 	    !rcu_segcblist_ready_cbs(&sdp->srcu_cblist)) {
 		spin_unlock_irq_rcu_node(sdp);
@@ -1745,6 +1750,7 @@ static void srcu_invoke_callbacks(struct work_struct *work)
 	sdp->srcu_cblist_invoking = false;
 	more = rcu_segcblist_ready_cbs(&sdp->srcu_cblist);
 	spin_unlock_irq_rcu_node(sdp);
+	/* An SRCU barrier or callbacks from previous nesting work pending */
 	if (more)
 		srcu_schedule_cbs_sdp(sdp, 0);
 }
-- 
2.40.1