linux-kernel - [PATCH 1/1] rcu/kvfree: Use READ_ONCE() when access to krcp->head

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Date:   Fri,  2 Dec 2022 14:18:37 +0100
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:     Frederic Weisbecker <frederic@...nel.org>,
        Neeraj Upadhyay <neeraj.iitr10@...il.com>,
        Joel Fernandes <joel@...lfernandes.org>,
        Uladzislau Rezki <urezki@...il.com>,
        Oleksiy Avramchenko <oleksiy.avramchenko@...y.com>
Subject: [PATCH 1/1] rcu/kvfree: Use READ_ONCE() when access to krcp->head

A need_offload_krc() function is now lock-free. A compiler
can optimize readers in way that they see an old value even
though writers already updated the krcp->head from another
path.

Signed-off-by: Uladzislau Rezki (Sony) <urezki@...il.com>
---
 kernel/rcu/tree.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index efc79f942b30..d155f2594317 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -3029,7 +3029,7 @@ need_offload_krc(struct kfree_rcu_cpu *krcp)
 		if (!list_empty(&krcp->bulk_head[i]))
 			return true;
 
-	return !!krcp->head;
+	return !!READ_ONCE(krcp->head);
 }
 
 static void
@@ -3070,7 +3070,7 @@ static void kfree_rcu_monitor(struct work_struct *work)
 		// in that case the monitor work is rearmed.
 		if ((!list_empty(&krcp->bulk_head[0]) && list_empty(&krwp->bulk_head_free[0])) ||
 			(!list_empty(&krcp->bulk_head[1]) && list_empty(&krwp->bulk_head_free[1])) ||
-				(krcp->head && !krwp->head_free)) {
+				(READ_ONCE(krcp->head) && !krwp->head_free)) {
 
 			// Channel 1 corresponds to the SLAB-pointer bulk path.
 			// Channel 2 corresponds to vmalloc-pointer bulk path.
-- 
2.30.2