linux-kernel - [tip: core/rcu] kvfree_rcu: Use [READ/WRITE]_ONCE() macros to access to nr_bkv

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Message-ID: <162506087956.395.5513533942041405498.tip-bot2@tip-bot2>
Date:   Wed, 30 Jun 2021 13:47:59 -0000
From:   "tip-bot2 for Uladzislau Rezki (Sony)" <tip-bot2@...utronix.de>
To:     linux-tip-commits@...r.kernel.org
Cc:     "Uladzislau Rezki (Sony)" <urezki@...il.com>,
        "Paul E. McKenney" <paulmck@...nel.org>, x86@...nel.org,
        linux-kernel@...r.kernel.org
Subject: [tip: core/rcu] kvfree_rcu: Use [READ/WRITE]_ONCE() macros to access
 to nr_bkv_objs

The following commit has been merged into the core/rcu branch of tip:

Commit-ID:     ac7625ebd5f7bad93f821b7397fe50635f58aa4b
Gitweb:        https://git.kernel.org/tip/ac7625ebd5f7bad93f821b7397fe50635f58aa4b
Author:        Uladzislau Rezki (Sony) <urezki@...il.com>
AuthorDate:    Thu, 15 Apr 2021 19:19:57 +02:00
Committer:     Paul E. McKenney <paulmck@...nel.org>
CommitterDate: Mon, 10 May 2021 16:00:48 -07:00

kvfree_rcu: Use [READ/WRITE]_ONCE() macros to access to nr_bkv_objs

nr_bkv_objs is a count of the objects in the kvfree_rcu page cache.
Accessing it requires holding the ->lock.  Switch to READ_ONCE() and
WRITE_ONCE() macros to provide lockless access to this counter.
This lockless access is used for the shrinker.

Signed-off-by: Uladzislau Rezki (Sony) <urezki@...il.com>
Signed-off-by: Paul E. McKenney <paulmck@...nel.org>
---
 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 74d840a..676a49a 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -3250,7 +3250,7 @@ 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);
 }
@@ -3264,9 +3264,8 @@ 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
@@ -3278,7 +3277,7 @@ 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) {
@@ -3682,18 +3681,13 @@ 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);
 	}