[PATCH rcu 22/32] rcu-tasks: Avoid rcu_tasks_trace_pertask() duplicate list additions

Paul E. McKenney
2022-06-20T22:54:38Z

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>
---
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