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

[Zhang, Qiang1]

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_.
>

Yes, for CONFIG_PREEMPT_COUNT=y and CONFIG_PREEMPT=n kernel
The rcu_read_lock/unlock are replaced by preempt_disbale/enable, and the 
preempt-count is exists,  so can report exp QS when not being an  RCU 
read-side critical(preempt_count & (PREEMPT_MASK | SOFTIRQ_MASK )return zero).
in IPI handler. 

For CONFIG_PREEMPT_COUNT=n and CONFIG_PREEMPT=n kernel, 
The rcu_read_lock/unlock is just barrier(). 


So I add IS_ENABLED(CONFIG_PREEMPT_COUNT) check in code.

Of course, for CONFIG_PREEMPT_COUNT=y  kernel, in RCU softirq, the 
preempt-count is also checked

/* Report any deferred quiescent states if preemption enabled. */
 if (IS_ENABLED(CONFIG_PREEMPT_COUNT) && (!(preempt_count() & PREEMPT_MASK))) {
                 rcu_preempt_deferred_qs(current);

but the RCU softirq may not be triggered in time and reported exp QS, for
example a kernel loop exist on NO_HZ_FULL CPU

this change, It is to capture the exp QS state earlier and report it.


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

Usually access to pointers of type rcu needs to be protected.

Any thoughts?

>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
>