linux-kernel - [PATCH] rcu: use try_cmpxchg in check_cpu

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Message-Id: <20230228155121.3416-1-ubizjak@gmail.com>
Date:   Tue, 28 Feb 2023 16:51:21 +0100
From:   Uros Bizjak <ubizjak@...il.com>
To:     rcu@...r.kernel.org, linux-kernel@...r.kernel.org
Cc:     Uros Bizjak <ubizjak@...il.com>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Frederic Weisbecker <frederic@...nel.org>,
        Neeraj Upadhyay <quic_neeraju@...cinc.com>,
        Josh Triplett <josh@...htriplett.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        Joel Fernandes <joel@...lfernandes.org>
Subject: [PATCH] rcu: use try_cmpxchg in check_cpu_stall

Use try_cmpxchg instead of cmpxchg (*ptr, old, new) == old in
check_cpu_stall.  x86 CMPXCHG instruction returns success in ZF flag, so
this change saves a compare after cmpxchg (and related move instruction in
front of cmpxchg).

No functional change intended.

Cc: "Paul E. McKenney" <paulmck@...nel.org>
Cc: Frederic Weisbecker <frederic@...nel.org>
Cc: Neeraj Upadhyay <quic_neeraju@...cinc.com>
Cc: Josh Triplett <josh@...htriplett.org>
Cc: Steven Rostedt <rostedt@...dmis.org>
Cc: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc: Lai Jiangshan <jiangshanlai@...il.com>
Cc: Joel Fernandes <joel@...lfernandes.org>
Signed-off-by: Uros Bizjak <ubizjak@...il.com>
---
 kernel/rcu/tree_stall.h | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/kernel/rcu/tree_stall.h b/kernel/rcu/tree_stall.h
index b10b8349bb2a..d81c88e66b42 100644
--- a/kernel/rcu/tree_stall.h
+++ b/kernel/rcu/tree_stall.h
@@ -760,7 +760,7 @@ static void check_cpu_stall(struct rcu_data *rdp)
 	jn = jiffies + ULONG_MAX / 2;
 	if (rcu_gp_in_progress() &&
 	    (READ_ONCE(rnp->qsmask) & rdp->grpmask) &&
-	    cmpxchg(&rcu_state.jiffies_stall, js, jn) == js) {
+	    try_cmpxchg(&rcu_state.jiffies_stall, &js, jn)) {
 
 		/*
 		 * If a virtual machine is stopped by the host it can look to
@@ -778,7 +778,7 @@ static void check_cpu_stall(struct rcu_data *rdp)
 
 	} else if (rcu_gp_in_progress() &&
 		   ULONG_CMP_GE(j, js + RCU_STALL_RAT_DELAY) &&
-		   cmpxchg(&rcu_state.jiffies_stall, js, jn) == js) {
+		   try_cmpxchg(&rcu_state.jiffies_stall, &js, jn)) {
 
 		/*
 		 * If a virtual machine is stopped by the host it can look to
-- 
2.39.2