Re: [PATCH] rcu/tree: Remove CONFIG_PREMPT_RCU check in force_qs_rnp

[Neeraj Upadhyay <neeraju@codeaurora.org>]

Hi Paul,

On 6/23/2020 4:48 AM, Paul E. McKenney wrote:
> On Mon, Jun 22, 2020 at 11:37:03PM +0530, Neeraj Upadhyay wrote:
>> Remove CONFIG_PREMPT_RCU check in force_qs_rnp(). Originally,
>> this check was required to skip executing fqs failsafe
>> for rcu-sched, which was added in commit a77da14ce9af ("rcu:
>> Yet another fix for preemption and CPU hotplug"). However,
>> this failsafe has been removed, since then. So, cleanup the
>> code to avoid any confusion around the need for boosting,
>> for !CONFIG_PREMPT_RCU.
>>
>> Signed-off-by: Neeraj Upadhyay <neeraju@codeaurora.org>
> 
> Good point, there is a !PREEMPT definition of the function
> rcu_preempt_blocked_readers_cgp() that unconditionally returns zero.
> And if !PREEMPT kernels, the same things happens in the "if"
> body as after it, so behavior is not changed.
> 
> I have queued and pushed this with an upgraded commit log as
> shown below.
> 
> 						Thanx, Paul
> 

Thanks! patch looks good to me!

Thanks
Neeraj

>> ---
>>   kernel/rcu/tree.c | 3 +--
>>   1 file changed, 1 insertion(+), 2 deletions(-)
>>
>> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
>> index 6226bfb..57c904b 100644
>> --- a/kernel/rcu/tree.c
>> +++ b/kernel/rcu/tree.c
>> @@ -2514,8 +2514,7 @@ static void force_qs_rnp(int (*f)(struct rcu_data *rdp))
>>   		raw_spin_lock_irqsave_rcu_node(rnp, flags);
>>   		rcu_state.cbovldnext |= !!rnp->cbovldmask;
>>   		if (rnp->qsmask == 0) {
>> -			if (!IS_ENABLED(CONFIG_PREEMPT_RCU) ||
>> -			    rcu_preempt_blocked_readers_cgp(rnp)) {
>> +			if (rcu_preempt_blocked_readers_cgp(rnp)) {
>>   				/*
>>   				 * No point in scanning bits because they
>>   				 * are all zero.  But we might need to
>> -- 
>> The Qualcomm Innovation Center, Inc. is a member of the Code Aurora Forum,
>> a Linux Foundation Collaborative Project
> 
> ------------------------------------------------------------------------
> 
> commit a4600389c35010aef414b89e2817d4a527e751b5
> Author: Neeraj Upadhyay <neeraju@codeaurora.org>
> Date:   Mon Jun 22 23:37:03 2020 +0530
> 
>      rcu/tree: Remove CONFIG_PREMPT_RCU check in force_qs_rnp()
>      
>      Originally, the call to rcu_preempt_blocked_readers_cgp() from
>      force_qs_rnp() had to be conditioned on CONFIG_PREEMPT_RCU=y, as in
>      commit a77da14ce9af ("rcu: Yet another fix for preemption and CPU
>      hotplug").  However, there is now a CONFIG_PREEMPT_RCU=n definition of
>      rcu_preempt_blocked_readers_cgp() that unconditionally returns zero, so
>      invoking it is now safe.  In addition, the CONFIG_PREEMPT_RCU=n definition
>      of rcu_initiate_boost() simply releases the rcu_node structure's ->lock,
>      which is what happens when the "if" condition evaluates to false.
>      
>      This commit therefore drops the IS_ENABLED(CONFIG_PREEMPT_RCU) check,
>      so that rcu_initiate_boost() is called only in CONFIG_PREEMPT_RCU=y
>      kernels when there are readers blocking the current grace period.
>      This does not change the behavior, but reduces code-reader confusion by
>      eliminating non-CONFIG_PREEMPT_RCU=y calls to rcu_initiate_boost().
>      
>      Signed-off-by: Neeraj Upadhyay <neeraju@codeaurora.org>
>      Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
> 
> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> index 6226bfb..57c904b 100644
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -2514,8 +2514,7 @@ static void force_qs_rnp(int (*f)(struct rcu_data *rdp))
>   		raw_spin_lock_irqsave_rcu_node(rnp, flags);
>   		rcu_state.cbovldnext |= !!rnp->cbovldmask;
>   		if (rnp->qsmask == 0) {
> -			if (!IS_ENABLED(CONFIG_PREEMPT_RCU) ||
> -			    rcu_preempt_blocked_readers_cgp(rnp)) {
> +			if (rcu_preempt_blocked_readers_cgp(rnp)) {
>   				/*
>   				 * No point in scanning bits because they
>   				 * are all zero.  But we might need to
> 

-- 
QUALCOMM INDIA, on behalf of Qualcomm Innovation Center, Inc. is a 
member of the Code Aurora Forum, hosted by The Linux Foundation