[PATCH v2 2/5] kvfree_rcu: Use [READ/WRITE]_ONCE() macros to access to nr_bkv_objs

From: "Uladzislau Rezki (Sony)" <urezki@gmail.com>
To: LKML <linux-kernel@vger.kernel.org>, RCU <rcu@vger.kernel.org>,
	"Paul E . McKenney" <paulmck@kernel.org>
Cc: Michal Hocko <mhocko@suse.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	Daniel Axtens <dja@axtens.net>,
	Frederic Weisbecker <frederic@kernel.org>,
	Neeraj Upadhyay <neeraju@codeaurora.org>,
	Joel Fernandes <joel@joelfernandes.org>,
	Peter Zijlstra <peterz@infradead.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	"Theodore Y . Ts'o" <tytso@mit.edu>,
	Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
	Uladzislau Rezki <urezki@gmail.com>,
	Oleksiy Avramchenko <oleksiy.avramchenko@sonymobile.com>
Subject: [PATCH v2 2/5] kvfree_rcu: Use [READ/WRITE]_ONCE() macros to access to nr_bkv_objs
Date: Thu, 15 Apr 2021 19:19:57 +0200	[thread overview]
Message-ID: <20210415172000.15086-3-urezki@gmail.com> (raw)
In-Reply-To: <20210415172000.15086-1-urezki@gmail.com>

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@gmail.com>
---
 kernel/rcu/tree.c | 14 ++++----------
 1 file changed, 4 insertions(+), 10 deletions(-)

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 742152d6b952..07e718fdea12 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -3223,7 +3223,7 @@ get_cached_bnode(struct kfree_rcu_cpu *krcp)
 	if (!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);
 }
@@ -3237,9 +3237,8 @@ put_cached_bnode(struct kfree_rcu_cpu *krcp,
 		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

