On Tue, Oct 24, 2023 at 11:46:23PM +0200, Frederic Weisbecker wrote:
> +/* Check for quiescent states since the pregp's synchronize_rcu() */
> +static bool rcu_tasks_is_holdout(struct task_struct *t)
> +{
> + int cpu;
> +
> + /* Has the task been seen voluntarily sleeping? */
> + if (!READ_ONCE(t->on_rq))
> + return false;
> +
> + cpu = task_cpu(t);
> +
> + /*
> + * Idle tasks within the idle loop or offline CPUs are RCU-tasks
> + * quiescent states. But CPU boot code performed by the idle task
> + * isn't a quiescent state.
> + */
> + if (t == idle_task(cpu)) {
> + if (is_idle_task(t))
> + return false;
> +
> + if (!rcu_cpu_online(cpu))
> + return false;
> + }
Hmm, why is this guarded by t == idle_task() ?
Notably, there is the idle-injection thing that uses FIFO tasks to run
'idle', see play_idle_precise(). This will (temporarily) get PF_IDLE on
tasks that are not idle_task().
> +
> + return true;
> +}
> +
> /* Per-task initial processing. */
> static void rcu_tasks_pertask(struct task_struct *t, struct list_head *hop)
> {
> - if (t != current && READ_ONCE(t->on_rq) && !is_idle_task(t)) {
> + if (t != current && rcu_tasks_is_holdout(t)) {
