Re: [PATCH 04/11] rcu: cleanup rcu_preempt_deferred_qs()

Paul E. McKenney
2019-10-31T14:10:59Z

On Thu, Oct 31, 2019 at 10:07:59AM +0000, Lai Jiangshan wrote:
> Don't need to set ->rcu_read_lock_nesting negative, irq-protected
> rcu_preempt_deferred_qs_irqrestore() doesn't expect
> ->rcu_read_lock_nesting to be negative to work, it even
> doesn't access to ->rcu_read_lock_nesting any more.
>
> It is true that NMI over rcu_preempt_deferred_qs_irqrestore()
> may access to ->rcu_read_lock_nesting, but it is still safe
> since rcu_read_unlock_special() can protect itself from NMI.

Hmmm... Testing identified the need for this one. But I will wait for
your responses on the earlier patches before going any further through
this series.

Thanx, Paul

> Signed-off-by: Lai Jiangshan <laijs@linux.alibaba.com>
> ---
> kernel/rcu/tree_plugin.h | 5 -----
> 1 file changed, 5 deletions(-)
>
> diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
> index 82595db04eec..9fe8138ed3c3 100644
> --- a/kernel/rcu/tree_plugin.h
> +++ b/kernel/rcu/tree_plugin.h
> @@ -555,16 +555,11 @@ static bool rcu_preempt_need_deferred_qs(struct task_struct *t)
> static void rcu_preempt_deferred_qs(struct task_struct *t)
> {
> unsigned long flags;
> - bool couldrecurse = t->rcu_read_lock_nesting >= 0;
>
> if (!rcu_preempt_need_deferred_qs(t))
> return;
> - if (couldrecurse)
> - t->rcu_read_lock_nesting -= RCU_NEST_BIAS;
> local_irq_save(flags);
> rcu_preempt_deferred_qs_irqrestore(t, flags);
> - if (couldrecurse)
> - t->rcu_read_lock_nesting += RCU_NEST_BIAS;
> }
>
> /*
> --
> 2.20.1
>