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Subject[PATCH tip/core/rcu 3/7] rcu: Make kfree_rcu() use a non-atomic ->monitor_todo
Date
From: Joel Fernandes <joel@joelfernandes.org>

Because the ->monitor_todo field is always protected by krcp->lock,
this commit downgrades from xchg() to non-atomic unmarked assignment
statements.

Signed-off-by: Joel Fernandes <joel@joelfernandes.org>
[ paulmck: Update to include early-boot kick code. ]
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
---
kernel/rcu/tree.c | 16 ++++++++++------
1 file changed, 10 insertions(+), 6 deletions(-)

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 0af016f..6106b9e 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -2708,7 +2708,7 @@ struct kfree_rcu_cpu {
struct rcu_head *head_free;
spinlock_t lock;
struct delayed_work monitor_work;
- int monitor_todo;
+ bool monitor_todo;
bool initialized;
};

@@ -2765,6 +2765,7 @@ static inline void kfree_rcu_drain_unlock(struct kfree_rcu_cpu *krcp,
unsigned long flags)
{
// Attempt to start a new batch.
+ krcp->monitor_todo = false;
if (queue_kfree_rcu_work(krcp)) {
// Success! Our job is done here.
spin_unlock_irqrestore(&krcp->lock, flags);
@@ -2772,8 +2773,8 @@ static inline void kfree_rcu_drain_unlock(struct kfree_rcu_cpu *krcp,
}

// Previous RCU batch still in progress, try again later.
- if (!xchg(&krcp->monitor_todo, true))
- schedule_delayed_work(&krcp->monitor_work, KFREE_DRAIN_JIFFIES);
+ krcp->monitor_todo = true;
+ schedule_delayed_work(&krcp->monitor_work, KFREE_DRAIN_JIFFIES);
spin_unlock_irqrestore(&krcp->lock, flags);
}

@@ -2788,7 +2789,7 @@ static void kfree_rcu_monitor(struct work_struct *work)
monitor_work.work);

spin_lock_irqsave(&krcp->lock, flags);
- if (xchg(&krcp->monitor_todo, false))
+ if (krcp->monitor_todo)
kfree_rcu_drain_unlock(krcp, flags);
else
spin_unlock_irqrestore(&krcp->lock, flags);
@@ -2837,8 +2838,10 @@ void kfree_call_rcu(struct rcu_head *head, rcu_callback_t func)

// Set timer to drain after KFREE_DRAIN_JIFFIES.
if (rcu_scheduler_active == RCU_SCHEDULER_RUNNING &&
- !xchg(&krcp->monitor_todo, true))
+ !krcp->monitor_todo) {
+ krcp->monitor_todo = true;
schedule_delayed_work(&krcp->monitor_work, KFREE_DRAIN_JIFFIES);
+ }

if (krcp->initialized)
spin_unlock(&krcp->lock);
@@ -2855,10 +2858,11 @@ void __init kfree_rcu_scheduler_running(void)
struct kfree_rcu_cpu *krcp = per_cpu_ptr(&krc, cpu);

spin_lock_irqsave(&krcp->lock, flags);
- if (!krcp->head || xchg(&krcp->monitor_todo, true)) {
+ if (!krcp->head || krcp->monitor_todo) {
spin_unlock_irqrestore(&krcp->lock, flags);
continue;
}
+ krcp->monitor_todo = true;
schedule_delayed_work(&krcp->monitor_work, KFREE_DRAIN_JIFFIES);
spin_unlock_irqrestore(&krcp->lock, flags);
}
--
2.9.5
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