Hi Jiangshan,
I haven't checked the correctness of this patch carefully, but..
On Sat, Nov 02, 2019 at 12:45:54PM +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.
rcu_preempt_deferred_qs_irqrestore() will report RCU qs, and may
eventually call swake_up() or its friends to wake up, say, the gp
kthread, and the wake up functions could go into the scheduler code
path which might have RCU read-side critical section in it, IOW,
accessing ->rcu_read_lock_nesting.
Again, haven't checked closely, but this argument in the commit log
seems untrue.
Regards,
Boqun
>
> 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.
>
> Signed-off-by: Lai Jiangshan <laijs@xxxxxxxxxxxxxxxxx>
> ---
> 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 aba5896d67e3..2fab8be2061f 100644
> --- a/kernel/rcu/tree_plugin.h
> +++ b/kernel/rcu/tree_plugin.h
> @@ -552,16 +552,11 @@ 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
>
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