From nobody Fri Dec 26 21:39:27 2025 Received: from eu-smtp-delivery-151.mimecast.com (eu-smtp-delivery-151.mimecast.com [185.58.85.151]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 48BC1F9CC for ; Sun, 31 Dec 2023 21:55:23 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=ACULAB.COM Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=aculab.com Received: from AcuMS.aculab.com (156.67.243.121 [156.67.243.121]) by relay.mimecast.com with ESMTP with both STARTTLS and AUTH (version=TLSv1.2, cipher=TLS_ECDHE_RSA_WITH_AES_256_CBC_SHA384) id uk-mta-44-6gjeuhImOKioWY8extJLBA-1; Sun, 31 Dec 2023 21:55:20 +0000 X-MC-Unique: 6gjeuhImOKioWY8extJLBA-1 Received: from AcuMS.Aculab.com (10.202.163.6) by AcuMS.aculab.com (10.202.163.6) with Microsoft SMTP Server (TLS) id 15.0.1497.48; Sun, 31 Dec 2023 21:54:59 +0000 Received: from AcuMS.Aculab.com ([::1]) by AcuMS.aculab.com ([::1]) with mapi id 15.00.1497.048; Sun, 31 Dec 2023 21:54:59 +0000 From: David Laight To: "'linux-kernel@vger.kernel.org'" , "'peterz@infradead.org'" , "'longman@redhat.com'" CC: "'mingo@redhat.com'" , "'will@kernel.org'" , "'boqun.feng@gmail.com'" , "'Linus Torvalds'" , "'virtualization@lists.linux-foundation.org'" , 'Zeng Heng' Subject: [PATCH next v2 4/5] locking/osq_lock: Avoid writing to node->next in the osq_lock() fast path. Thread-Topic: [PATCH next v2 4/5] locking/osq_lock: Avoid writing to node->next in the osq_lock() fast path. Thread-Index: Ado8NAjQtRL812H3R1Kc4G+FOscjCQ== Date: Sun, 31 Dec 2023 21:54:59 +0000 Message-ID: <06a11b2c7d784f2d80dc8e81c7175c57@AcuMS.aculab.com> References: <2b4e8a5816a742d2bd23fdbaa8498e80@AcuMS.aculab.com> In-Reply-To: <2b4e8a5816a742d2bd23fdbaa8498e80@AcuMS.aculab.com> Accept-Language: en-GB, en-US X-MS-Has-Attach: X-MS-TNEF-Correlator: x-ms-exchange-transport-fromentityheader: Hosted Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: aculab.com Content-Language: en-US Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset="utf-8" When osq_lock() returns false or osq_unlock() returns static analysis shows that node->next should always be NULL. This means that it isn't necessary to explicitly set it to NULL prior to atomic_xchg(&lock->tail, curr) on extry to osq_lock(). Just in case there a non-obvious race condition that can leave it non-NULL check with WARN_ON_ONCE() and NULL if set. Note that without this check the fast path (adding at the list head) doesn't need to to access the per-cpu osq_node at all. Signed-off-by: David Laight Reviewed-by: Waiman Long --- kernel/locking/osq_lock.c | 14 ++++++++++---- 1 file changed, 10 insertions(+), 4 deletions(-) diff --git a/kernel/locking/osq_lock.c b/kernel/locking/osq_lock.c index 27324b509f68..35bb99e96697 100644 --- a/kernel/locking/osq_lock.c +++ b/kernel/locking/osq_lock.c @@ -87,12 +87,17 @@ osq_wait_next(struct optimistic_spin_queue *lock, =20 bool osq_lock(struct optimistic_spin_queue *lock) { - struct optimistic_spin_node *node =3D this_cpu_ptr(&osq_node); - struct optimistic_spin_node *prev, *next; + struct optimistic_spin_node *node, *prev, *next; int curr =3D encode_cpu(smp_processor_id()); int prev_cpu; =20 - node->next =3D NULL; + /* + * node->next should be NULL on entry. + * Check just in case there is a race somewhere. + * Note that this is probably an unnecessary cache miss in the fast path. + */ + if (WARN_ON_ONCE(raw_cpu_read(osq_node.next) !=3D NULL)) + raw_cpu_write(osq_node.next, NULL); =20 /* * We need both ACQUIRE (pairs with corresponding RELEASE in @@ -104,8 +109,9 @@ bool osq_lock(struct optimistic_spin_queue *lock) if (prev_cpu =3D=3D OSQ_UNLOCKED_VAL) return true; =20 - node->prev_cpu =3D prev_cpu; + node =3D this_cpu_ptr(&osq_node); prev =3D decode_cpu(prev_cpu); + node->prev_cpu =3D prev_cpu; node->locked =3D 0; =20 /* --=20 2.17.1 - Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1= PT, UK Registration No: 1397386 (Wales)