From nobody Fri Dec 26 21:39:22 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 ABEC9DF42 for ; Sun, 31 Dec 2023 21:54:27 +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-254-BKaLleaNN2m97ul2XwiUmA-1; Sun, 31 Dec 2023 21:54:24 +0000 X-MC-Unique: BKaLleaNN2m97ul2XwiUmA-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 21:54:02 +0000 Received: from AcuMS.Aculab.com ([::1]) by AcuMS.aculab.com ([::1]) with mapi id 15.00.1497.048; Sun, 31 Dec 2023 21:54:02 +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 3/5] locking/osq_lock: Use node->prev_cpu instead of saving node->prev. Thread-Topic: [PATCH next v2 3/5] locking/osq_lock: Use node->prev_cpu instead of saving node->prev. Thread-Index: Ado8M93l5L/nOR+URXCdpqS8EUheiQ== Date: Sun, 31 Dec 2023 21:54:02 +0000 Message-ID: <7906aaa73f93493c873e6286c1f96645@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" node->prev is only used to update 'prev' in the unlikely case of concurrent unqueues. This can be replaced by a check for node->prev_cpu changing and then calling decode_cpu() to get the changed 'prev' pointer. node->cpu (or more particularly) prev->cpu is only used for the osq_wait_next() call in the unqueue path. Normally this is exactly the value that the initial xchg() read from lock->tail (used to obtain 'prev'), but can get updated by concurrent unqueues. Both the 'prev' and 'cpu' members of optimistic_spin_node are now unused and can be deleted. Signed-off-by: David Laight Reviewed-by: Waiman Long --- kernel/locking/osq_lock.c | 31 +++++++++++++++++-------------- 1 file changed, 17 insertions(+), 14 deletions(-) diff --git a/kernel/locking/osq_lock.c b/kernel/locking/osq_lock.c index eb8a6dfdb79d..27324b509f68 100644 --- a/kernel/locking/osq_lock.c +++ b/kernel/locking/osq_lock.c @@ -13,9 +13,8 @@ */ =20 struct optimistic_spin_node { - struct optimistic_spin_node *next, *prev; + struct optimistic_spin_node *next; int locked; /* 1 if lock acquired */ - int cpu; /* encoded CPU # + 1 value */ int prev_cpu; /* encoded CPU # + 1 value */ }; =20 @@ -91,10 +90,9 @@ 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; int curr =3D encode_cpu(smp_processor_id()); - int old; + int prev_cpu; =20 node->next =3D NULL; - node->cpu =3D curr; =20 /* * We need both ACQUIRE (pairs with corresponding RELEASE in @@ -102,13 +100,12 @@ bool osq_lock(struct optimistic_spin_queue *lock) * the node fields we just initialised) semantics when updating * the lock tail. */ - old =3D atomic_xchg(&lock->tail, curr); - if (old =3D=3D OSQ_UNLOCKED_VAL) + prev_cpu =3D atomic_xchg(&lock->tail, curr); + if (prev_cpu =3D=3D OSQ_UNLOCKED_VAL) return true; =20 - node->prev_cpu =3D old; - prev =3D decode_cpu(old); - node->prev =3D prev; + node->prev_cpu =3D prev_cpu; + prev =3D decode_cpu(prev_cpu); node->locked =3D 0; =20 /* @@ -174,9 +171,16 @@ bool osq_lock(struct optimistic_spin_queue *lock) =20 /* * Or we race against a concurrent unqueue()'s step-B, in which - * case its step-C will write us a new @node->prev pointer. + * case its step-C will write us a new @node->prev_cpu value. */ - prev =3D READ_ONCE(node->prev); + { + int new_prev_cpu =3D READ_ONCE(node->prev_cpu); + + if (new_prev_cpu =3D=3D prev_cpu) + continue; + prev_cpu =3D new_prev_cpu; + prev =3D decode_cpu(prev_cpu); + } } =20 /* @@ -186,7 +190,7 @@ bool osq_lock(struct optimistic_spin_queue *lock) * back to @prev. */ =20 - next =3D osq_wait_next(lock, node, prev->cpu); + next =3D osq_wait_next(lock, node, prev_cpu); if (!next) return false; =20 @@ -198,8 +202,7 @@ bool osq_lock(struct optimistic_spin_queue *lock) * it will wait in Step-A. */ =20 - WRITE_ONCE(next->prev_cpu, prev->cpu); - WRITE_ONCE(next->prev, prev); + WRITE_ONCE(next->prev_cpu, prev_cpu); WRITE_ONCE(prev->next, next); =20 return false; --=20 2.17.1 - Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1= PT, UK Registration No: 1397386 (Wales)