linux-kernel - [PATCH 1/2] rcu: Delete a redundant check in check_cpu

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Date:   Fri, 21 Jul 2023 15:57:15 +0800
From:   thunder.leizhen@...weicloud.com
To:     "Paul E . McKenney" <paulmck@...nel.org>,
        Frederic Weisbecker <frederic@...nel.org>,
        Neeraj Upadhyay <quic_neeraju@...cinc.com>,
        Joel Fernandes <joel@...lfernandes.org>,
        Josh Triplett <josh@...htriplett.org>,
        Boqun Feng <boqun.feng@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        Zqiang <qiang.zhang1211@...il.com>, rcu@...r.kernel.org,
        linux-kernel@...r.kernel.org
Cc:     Zhen Lei <thunder.leizhen@...wei.com>
Subject: [PATCH 1/2] rcu: Delete a redundant check in check_cpu_stall()

From: Zhen Lei <thunder.leizhen@...wei.com>

j = jiffies;
js = READ_ONCE(rcu_state.jiffies_stall);			(1)
if (ULONG_CMP_LT(j, js))					(2)
	return;

if (cmpxchg(&rcu_state.jiffies_stall, js, jn) == js)		(3)
	didstall = true;

if (didstall && READ_ONCE(rcu_state.jiffies_stall) == jn) {	(4)
	jn = jiffies + 3 * rcu_jiffies_till_stall_check() + 3;
	WRITE_ONCE(rcu_state.jiffies_stall, jn);
}

For ease of description, the pseudo code is extracted as above. First,
assume that only one CPU is operating, the condition 'didstall' is true
means that (3) succeeds. That is, the value of rcu_state.jiffies_stall
must be 'jn'.

Then, assuming that another CPU is also operating at the same time, there
are two cases:
1. That CPU sees the updated result at (1), it returns at (2).
2. That CPU does not see the updated result at (1), it fails at (3) and
   cmpxchg returns jn. So that, didstall cannot be true.

Signed-off-by: Zhen Lei <thunder.leizhen@...wei.com>
---
 kernel/rcu/tree_stall.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/rcu/tree_stall.h b/kernel/rcu/tree_stall.h
index cc884cd49e026a3..371713f3f7d15d9 100644
--- a/kernel/rcu/tree_stall.h
+++ b/kernel/rcu/tree_stall.h
@@ -794,7 +794,7 @@ static void check_cpu_stall(struct rcu_data *rdp)
 			rcu_ftrace_dump(DUMP_ALL);
 		didstall = true;
 	}
-	if (didstall && READ_ONCE(rcu_state.jiffies_stall) == jn) {
+	if (didstall) {
 		jn = jiffies + 3 * rcu_jiffies_till_stall_check() + 3;
 		WRITE_ONCE(rcu_state.jiffies_stall, jn);
 	}
-- 
2.25.1