linux-kernel - [PATCH rcu 07/32] rcu-tasks: Make trc_read_check_handler() fetch ->trc_reader

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Message-Id: <20220620225411.3842519-7-paulmck@kernel.org>
Date:   Mon, 20 Jun 2022 15:53:46 -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>,
        Eric Dumazet <edumazet@...gle.com>,
        Alexei Starovoitov <ast@...nel.org>,
        Andrii Nakryiko <andrii@...nel.org>,
        Martin KaFai Lau <kafai@...com>,
        KP Singh <kpsingh@...nel.org>
Subject: [PATCH rcu 07/32] rcu-tasks: Make trc_read_check_handler() fetch ->trc_reader_nesting only once

This commit replaces the pair of READ_ONCE(t->trc_reader_nesting) calls
with a single such call and a local variable.  This makes the code's
intent more clear.

Signed-off-by: Paul E. McKenney <paulmck@...nel.org>
Cc: Neeraj Upadhyay <quic_neeraju@...cinc.com>
Cc: Eric Dumazet <edumazet@...gle.com>
Cc: Alexei Starovoitov <ast@...nel.org>
Cc: Andrii Nakryiko <andrii@...nel.org>
Cc: Martin KaFai Lau <kafai@...com>
Cc: KP Singh <kpsingh@...nel.org>
---
 kernel/rcu/tasks.h | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/kernel/rcu/tasks.h b/kernel/rcu/tasks.h
index 561d24f7f73cc..8fe78a7fecafd 100644
--- a/kernel/rcu/tasks.h
+++ b/kernel/rcu/tasks.h
@@ -1278,6 +1278,7 @@ static void trc_del_holdout(struct task_struct *t)
 /* IPI handler to check task state. */
 static void trc_read_check_handler(void *t_in)
 {
+	int nesting;
 	struct task_struct *t = current;
 	struct task_struct *texp = t_in;
 
@@ -1287,12 +1288,13 @@ static void trc_read_check_handler(void *t_in)
 
 	// If the task is not in a read-side critical section, and
 	// if this is the last reader, awaken the grace-period kthread.
-	if (likely(!READ_ONCE(t->trc_reader_nesting))) {
+	nesting = READ_ONCE(t->trc_reader_nesting);
+	if (likely(!nesting)) {
 		rcu_trc_cmpxchg_need_qs(t, 0, TRC_NEED_QS_CHECKED);
 		goto reset_ipi;
 	}
 	// If we are racing with an rcu_read_unlock_trace(), try again later.
-	if (unlikely(READ_ONCE(t->trc_reader_nesting) < 0))
+	if (unlikely(nesting < 0))
 		goto reset_ipi;
 
 	// Get here if the task is in a read-side critical section.  Set
-- 
2.31.1.189.g2e36527f23