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@kernel.org>
Cc: Neeraj Upadhyay <quic_neeraju@quicinc.com>
Cc: Eric Dumazet <edumazet@google.com>
Cc: Alexei Starovoitov <ast@kernel.org>
Cc: Andrii Nakryiko <andrii@kernel.org>
Cc: Martin KaFai Lau <kafai@fb.com>
Cc: KP Singh <kpsingh@kernel.org>

diff --git a/kernel/rcu/tasks.h b/kernel/rcu/tasks.h
index 66d8473f1bda10bfd21319b9d69f77838349a1df..1aa6a24a9bc2bfddbd3e9b5e2dfafd06de12b813 100644 (file)
--- 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);