Re: [PATCH] rcu: Add exp QS check in rcu_exp_handler() for no-preemptible expedited RCU

[Boqun Feng]

Hi,

On Wed, Jun 22, 2022 at 06:35:49PM +0800, Zqiang wrote:
> In CONFIG_PREEMPT=n and CONFIG_PREEMPT_COUNT=y kernel, after a exp
> grace period begins, if detected current CPU enters idle in
> rcu_exp_handler() IPI handler, will immediately report the exp QS of the
> current cpu, at this time, maybe not being in an RCU read-side critical
> section, but need wait until rcu-softirq or sched-clock irq or sched-switch
> occurs on current CPU to check and report exp QS.
> 

I think the idea is OK, however, this "optimization" is based on the
implementation detail that rcu_read_lock() counts preempt_count when
CONFIG_PREEMPT_COUNT=y, right? It's a little bit dangerous because the
preempt_count when CONFIG_PREEMPT_COUNT=y and CONFIG_PREEMPT=n is mostly
for debugging purposes IIUC, and in other words, _it could be gone_.

Also I'm not aware of any but there could be someone assuming that RCU
read-side critical sections can be formed without
rcu_read_{lock,unlock}() in CONFIG_PREEMPT=n kernel. For example, there
might be "creative" code like the following:

	void do_something_only_in_nonpreempt(void)
	{
		int *p;

		// This function only gets called in PREEMPT=n kernel,
		// which means everywhere is a RCU read-side critical
		// section, let's save some lines of code.

		p = rcu_dereference_check(gp, !IS_ENABLED(PREEMPT));
		... // of course no schedule() here.
		<access p>
	}

Again, I'm not aware of any existing code that does this but we need to
be sure.

Regards,
Boqun

> This commit add a exp QS check in rcu_exp_handler(), when not being
> in an RCU read-side critical section, report exp QS earlier.
> 
> Signed-off-by: Zqiang <qiang1.zhang@xxxxxxxxx>
> ---
>  kernel/rcu/tree_exp.h | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/rcu/tree_exp.h b/kernel/rcu/tree_exp.h
> index be667583a554..34f08267410f 100644
> --- a/kernel/rcu/tree_exp.h
> +++ b/kernel/rcu/tree_exp.h
> @@ -828,11 +828,14 @@ static void rcu_exp_handler(void *unused)
>  {
>  	struct rcu_data *rdp = this_cpu_ptr(&rcu_data);
>  	struct rcu_node *rnp = rdp->mynode;
> +	bool preempt_bh_disabled =
> +				!!(preempt_count() & (PREEMPT_MASK | SOFTIRQ_MASK));
>  
>  	if (!(READ_ONCE(rnp->expmask) & rdp->grpmask) ||
>  	    __this_cpu_read(rcu_data.cpu_no_qs.b.exp))
>  		return;
> -	if (rcu_is_cpu_rrupt_from_idle()) {
> +	if (rcu_is_cpu_rrupt_from_idle() ||
> +			(IS_ENABLED(CONFIG_PREEMPT_COUNT) && !preempt_bh_disabled)) {
>  		rcu_report_exp_rdp(this_cpu_ptr(&rcu_data));
>  		return;
>  	}
> -- 
> 2.25.1
>