linux-kernel - Re: [PATCH] rcu/tree: Remove CONFIG_PREMPT_RCU check in force_qs

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Message-ID: <20200622231829.GV9247@paulmck-ThinkPad-P72>
Date:   Mon, 22 Jun 2020 16:18:29 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Neeraj Upadhyay <neeraju@...eaurora.org>
Cc:     josh@...htriplett.org, rostedt@...dmis.org,
        mathieu.desnoyers@...icios.com, jiangshanlai@...il.com,
        joel@...lfernandes.org, rcu@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] rcu/tree: Remove CONFIG_PREMPT_RCU check in force_qs_rnp

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@...eaurora.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

> ---
>  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@...eaurora.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@...eaurora.org>
    Signed-off-by: Paul E. McKenney <paulmck@...nel.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