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

[Neeraj Upadhyay]

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@xxxxxxxxxxxxxx>
>
> 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@xxxxxxxxxxxxxx>
> 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@xxxxxxxxxxxxxx>
>      Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
>
> 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

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