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From: David Laight <David.Laight@ACULAB.COM>
To: "'linux-kernel@vger.kernel.org'" <linux-kernel@vger.kernel.org>,
	"'peterz@infradead.org'" <peterz@infradead.org>, "'longman@redhat.com'"
	<longman@redhat.com>
CC: "'mingo@redhat.com'" <mingo@redhat.com>, "'will@kernel.org'"
	<will@kernel.org>, "'boqun.feng@gmail.com'" <boqun.feng@gmail.com>, "'Linus
 Torvalds'" <torvalds@linux-foundation.org>,
	"'virtualization@lists.linux-foundation.org'"
	<virtualization@lists.linux-foundation.org>, 'Zeng Heng'
	<zengheng4@huawei.com>
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>
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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 <david.laight@aculab.com>
---
 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)
 {
-=09struct optimistic_spin_node *node =3D this_cpu_ptr(&osq_node);
-=09struct optimistic_spin_node *prev, *next;
+=09struct optimistic_spin_node *node, *prev, *next;
 =09int curr =3D encode_cpu(smp_processor_id());
 =09int prev_cpu;
=20
-=09node->next =3D NULL;
+=09/*
+=09 * node->next should be NULL on entry.
+=09 * Check just in case there is a race somewhere.
+=09 * Note that this is probably an unnecessary cache miss in the fast pat=
h.
+=09 */
+=09if (WARN_ON_ONCE(raw_cpu_read(osq_node.next) !=3D NULL))
+=09=09raw_cpu_write(osq_node.next, NULL);
=20
 =09/*
 =09 * We need both ACQUIRE (pairs with corresponding RELEASE in
@@ -104,8 +109,9 @@ bool osq_lock(struct optimistic_spin_queue *lock)
 =09if (prev_cpu =3D=3D OSQ_UNLOCKED_VAL)
 =09=09return true;
=20
-=09node->prev_cpu =3D prev_cpu;
+=09node =3D this_cpu_ptr(&osq_node);
 =09prev =3D decode_cpu(prev_cpu);
+=09node->prev_cpu =3D prev_cpu;
 =09node->locked =3D 0;
=20
 =09/*
--=20
2.17.1

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