linux-kernel - [PATCH 2/6] kvfree_rcu: use [READ/WRITE]_ONCE() macros to access to nr_bkv

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Message-Id: <20210414121226.2650-2-urezki@gmail.com>
Date:   Wed, 14 Apr 2021 14:12:22 +0200
From:   "Uladzislau Rezki (Sony)" <urezki@...il.com>
To:     LKML <linux-kernel@...r.kernel.org>, RCU <rcu@...r.kernel.org>,
        "Paul E . McKenney" <paulmck@...nel.org>
Cc:     Michal Hocko <mhocko@...e.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Daniel Axtens <dja@...ens.net>,
        Frederic Weisbecker <frederic@...nel.org>,
        Neeraj Upadhyay <neeraju@...eaurora.org>,
        Joel Fernandes <joel@...lfernandes.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        "Theodore Y . Ts'o" <tytso@....edu>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Uladzislau Rezki <urezki@...il.com>,
        Oleksiy Avramchenko <oleksiy.avramchenko@...ymobile.com>
Subject: [PATCH 2/6] kvfree_rcu: use [READ/WRITE]_ONCE() macros to access to nr_bkv_objs

nr_bkv_objs represents the counter of objects in the page-cache.
Accessing to it requires taking the lock. Switch to READ_ONCE()
WRITE_ONCE() macros to provide an atomic access to that counter.
A shrinker is one of the user of it.

Signed-off-by: Uladzislau Rezki (Sony) <urezki@...il.com>
---
 kernel/rcu/tree.c | 18 ++++++------------
 1 file changed, 6 insertions(+), 12 deletions(-)

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 8b74edcd11d4..1b0289fa1cdd 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -3220,10 +3220,10 @@ krc_this_cpu_unlock(struct kfree_rcu_cpu *krcp, unsigned long flags)
 static inline struct kvfree_rcu_bulk_data *
 get_cached_bnode(struct kfree_rcu_cpu *krcp)
 {
-	if (!krcp->nr_bkv_objs)
+	if (!READ_ONCE(krcp->nr_bkv_objs))
 		return NULL;
 
-	krcp->nr_bkv_objs--;
+	WRITE_ONCE(krcp->nr_bkv_objs, krcp->nr_bkv_objs - 1);
 	return (struct kvfree_rcu_bulk_data *)
 		llist_del_first(&krcp->bkvcache);
 }
@@ -3233,13 +3233,12 @@ put_cached_bnode(struct kfree_rcu_cpu *krcp,
 	struct kvfree_rcu_bulk_data *bnode)
 {
 	// Check the limit.
-	if (krcp->nr_bkv_objs >= rcu_min_cached_objs)
+	if (READ_ONCE(krcp->nr_bkv_objs) >= rcu_min_cached_objs)
 		return false;
 
 	llist_add((struct llist_node *) bnode, &krcp->bkvcache);
-	krcp->nr_bkv_objs++;
+	WRITE_ONCE(krcp->nr_bkv_objs, krcp->nr_bkv_objs + 1);
 	return true;
-
 }
 
 static int
@@ -3251,7 +3250,7 @@ drain_page_cache(struct kfree_rcu_cpu *krcp)
 
 	raw_spin_lock_irqsave(&krcp->lock, flags);
 	page_list = llist_del_all(&krcp->bkvcache);
-	krcp->nr_bkv_objs = 0;
+	WRITE_ONCE(krcp->nr_bkv_objs, 0);
 	raw_spin_unlock_irqrestore(&krcp->lock, flags);
 
 	llist_for_each_safe(pos, n, page_list) {
@@ -3655,18 +3654,13 @@ kfree_rcu_shrink_count(struct shrinker *shrink, struct shrink_control *sc)
 {
 	int cpu;
 	unsigned long count = 0;
-	unsigned long flags;
 
 	/* Snapshot count of all CPUs */
 	for_each_possible_cpu(cpu) {
 		struct kfree_rcu_cpu *krcp = per_cpu_ptr(&krc, cpu);
 
 		count += READ_ONCE(krcp->count);
-
-		raw_spin_lock_irqsave(&krcp->lock, flags);
-		count += krcp->nr_bkv_objs;
-		raw_spin_unlock_irqrestore(&krcp->lock, flags);
-
+		count += READ_ONCE(krcp->nr_bkv_objs);
 		atomic_set(&krcp->backoff_page_cache_fill, 1);
 	}
 
-- 
2.20.1