On Wed, Mar 30, 2022 at 07:20:14PM +0800, Zqiang wrote:
> When the trc_wait waitqueue timeout, the atomic variable
> trc_n_readers_need_end need to be checked again, perhaps the
> conditions have been established at this time, avoid invalid stall
> information output.
>
>But are you actually seeing this invalid stall information? Either way, please seem my comments and question below.
>
>Please don't get me wrong, we do have similar checks for normal vanilla RCU stall warnings, for example, this statement in print_other_cpu_stall() in kernel/rcu/tree_stall.h:
>
> pr_err("INFO: Stall ended before state dump start\n");
>
>However, the approach used there did benefit from significant real-world experience with its absence. ;-)
>
> Thanx, Paul
>
> Signed-off-by: Zqiang <qiang1.zhang@xxxxxxxxx>
> ---
> kernel/rcu/tasks.h | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/kernel/rcu/tasks.h b/kernel/rcu/tasks.h index
> 65d6e21a607a..b73a2b362d6b 100644
> --- a/kernel/rcu/tasks.h
> +++ b/kernel/rcu/tasks.h
> @@ -1544,7 +1544,7 @@ static void rcu_tasks_trace_postgp(struct rcu_tasks *rtp)
> trc_wait,
> atomic_read(&trc_n_readers_need_end) == 0,
> READ_ONCE(rcu_task_stall_timeout));
>If I understand correctly, this patch is intended to handle a situation where the wait_event_idle_exclusive_timeout() timed out, but where the value of trc_n_readers_need_end decreased to zero just at this point.
Yes, this patch is intended to handle a situation.
>If so, please see my question below. If not, please show me the exact sequence of events leading up to the problem.
> - if (ret)
> + if (ret || !atomic_read(&trc_n_readers_need_end))
> break; // Count reached zero.
>Couldn't the value of trc_n_readers_need_end decrease to zero right here, still resulting in invalid stall information?
The value of trc_n_readers_need_end decrease to zero right here, indicates that the current grace period has been completed.
Even if we go to print some task with condition 't->trc_reader_special.b.need_qs' as true, and these task will not affect the current grace period. Is my understanding correct?
Thanks
Zqiang
> // Stall warning time, so make a list of the offenders.
> rcu_read_lock();
> --
> 2.25.1
>
