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
> ---
> 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
