On Tue, Jul 12, 2022 at 04:26:06PM +0800, Zqiang wrote:
> When the rcu_scheduler_active variable is RCU_SCHEDULER_INACTIVE and not
> yet converted to RCU_SCHEDULER_INIT, there is only idle task, any legal
> call synchronize_rcu_tasks_generic() is a quiescent state. this commit
> make synchronize_rcu_tasks_generic() no-ops when the rcu_scheduler_active
> variable is RCU_SCHEDULER_INACTIVE.
>
> Signed-off-by: Zqiang <qiang1.zhang@xxxxxxxxx>
> ---
>It looks like this would be a good way to provide early boot access
>to synchronize_rcu_tasks(), synchronize_rcu_tasks_rude(), and
>synchronize_rcu_tasks_trace().
>
>But do we really need early boot access to these functions? As in has
>the below WARN_ON() actually triggered?
when the rcu_scheduler_active variable is RCU_SCHEDULER_INACTIVE,
invoke synchronize_rcu_tasks_generic(), in addition to triggering a warning,
also need to make it return directly, if not, the rcu_tasks_one_gp() will be
called directly, but due to the rtp structure's -> pregp_func is not initialized,
A null pointer bug will appear.
But like said, I don't see the need to call synchronize_rcu_tasks_generic() on
early booting. maybe this change is not necessary.
Thanks
Zqiang
>
>And if it has triggered, and in a context that means that these functions
>really are needed during early boot, how should the testing strategy
>change to test these at the relevant portions of the boot sequence?
>
>From what I know, hitting these during early boot would indicate that
>something was removing a trace during early boot, and I know of no way
>to make that happen. Hence my skepticism. ;-)
>
>But *if* this was really needed, this looks to be a reasonable way to
>implement it.
>
> Thanx, Paul
> kernel/rcu/tasks.h | 5 +++--
> 1 file changed, 3 insertions(+), 2 deletions(-)
>
> diff --git a/kernel/rcu/tasks.h b/kernel/rcu/tasks.h
> index 469bf2a3b505..0237e765c28e 100644
> --- a/kernel/rcu/tasks.h
> +++ b/kernel/rcu/tasks.h
> @@ -560,8 +560,9 @@ static int __noreturn rcu_tasks_kthread(void *arg)
> static void synchronize_rcu_tasks_generic(struct rcu_tasks *rtp)
> {
> /* Complain if the scheduler has not started. */
> - WARN_ONCE(rcu_scheduler_active == RCU_SCHEDULER_INACTIVE,
> - "synchronize_rcu_tasks called too soon");
> + if (WARN_ONCE(rcu_scheduler_active == RCU_SCHEDULER_INACTIVE,
> + "synchronize_rcu_tasks called too soon"))
> + return;
>
> // If the grace-period kthread is running, use it.
> if (READ_ONCE(rtp->kthread_ptr)) {
> --
> 2.25.1
>
