linux-kernel - [PATCH rcu 22/32] rcu-tasks: Avoid rcu_tasks_trace

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Message-Id: <20220620225411.3842519-22-paulmck@kernel.org>
Date:   Mon, 20 Jun 2022 15:54:01 -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 22/32] rcu-tasks: Avoid rcu_tasks_trace_pertask() duplicate list additions

This commit adds checks within rcu_tasks_trace_pertask() to avoid
duplicate (and destructive) additions to the holdouts list.  These checks
will be required later due to the possibility of a given task having
blocked while in an RCU Tasks Trace read-side critical section, but now
running on a CPU.

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 | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/kernel/rcu/tasks.h b/kernel/rcu/tasks.h
index 66d8473f1bda1..1aa6a24a9bc2b 100644
--- a/kernel/rcu/tasks.h
+++ b/kernel/rcu/tasks.h
@@ -1454,9 +1454,10 @@ static void rcu_tasks_trace_pertask(struct task_struct *t,
 				    struct list_head *hop)
 {
 	// During early boot when there is only the one boot CPU, there
-	// is no idle task for the other CPUs.  Also, the grace-period
-	// kthread is always in a quiescent state.  Either way, just return.
-	if (unlikely(t == NULL) || t == current)
+	// is no idle task for the other CPUs.	Also, the grace-period
+	// kthread is always in a quiescent state.  In addition, just return
+	// if this task is already on the list.
+	if (unlikely(t == NULL) || t == current || !list_empty(&t->trc_holdout_list))
 		return;
 
 	rcu_st_need_qs(t, 0);
-- 
2.31.1.189.g2e36527f23