linux-kernel - [PATCH rcu 15/21] srcu: Explain srcu_funnel_gp_start() call to list

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Message-Id: <20220419000322.3948903-15-paulmck@kernel.org>
Date:   Mon, 18 Apr 2022 17:03:16 -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, "Paul E. McKenney" <paulmck@...nel.org>,
        Neeraj Upadhyay <quic_neeraju@...cinc.com>
Subject: [PATCH rcu 15/21] srcu: Explain srcu_funnel_gp_start() call to list_add() is safe

This commit adds a comment explaining why an unprotected call to
list_add() from srcu_funnel_gp_start() can be safe.  TL;DR: It is only
called during very early boot when we don't have no steeking concurrency!

Co-developed-by: Neeraj Upadhyay <quic_neeraju@...cinc.com>
Signed-off-by: Neeraj Upadhyay <quic_neeraju@...cinc.com>
Signed-off-by: Paul E. McKenney <paulmck@...nel.org>
---
 kernel/rcu/srcutree.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c
index 7209fd95dde9..64993a172cff 100644
--- a/kernel/rcu/srcutree.c
+++ b/kernel/rcu/srcutree.c
@@ -776,6 +776,12 @@ static void srcu_funnel_gp_start(struct srcu_struct *ssp, struct srcu_data *sdp,
 	    rcu_seq_state(ssp->srcu_gp_seq) == SRCU_STATE_IDLE) {
 		WARN_ON_ONCE(ULONG_CMP_GE(ssp->srcu_gp_seq, ssp->srcu_gp_seq_needed));
 		srcu_gp_start(ssp);
+
+		// And how can that list_add() in the "else" clause
+		// possibly be safe for concurrent execution?  Well,
+		// it isn't.  And it does not have to be.  After all, it
+		// can only be executed during early boot when there is only
+		// the one boot CPU running with interrupts still disabled.
 		if (likely(srcu_init_done))
 			queue_delayed_work(rcu_gp_wq, &ssp->work,
 					   srcu_get_delay(ssp));
-- 
2.31.1.189.g2e36527f23