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From: David Laight <David.Laight@ACULAB.COM>
To: "'linux-kernel@vger.kernel.org'" <linux-kernel@vger.kernel.org>,
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Subject: [PATCH next 3/5] locking/osq_lock: Clarify osq_wait_next()
Thread-Topic: [PATCH next 3/5] locking/osq_lock: Clarify osq_wait_next()
Thread-Index: Ado6mW78N1Y4ctAHQ36yzrF5D7u/HQ==
Date: Fri, 29 Dec 2023 20:56:03 +0000
Message-ID: <7c8828aec72e42eeb841ca0ee3397e9a@AcuMS.aculab.com>
References: <73a4b31c9c874081baabad9e5f2e5204@AcuMS.aculab.com>
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osq_wait_next() is passed 'prev' from osq_lock() and NULL from osq_unlock()
but only needs the 'cpu' value to write to lock->tail.
Just pass prev->cpu or OSQ_UNLOCKED_VAL instead.

Also directly return NULL or 'next' instead of breaking the loop.

Should have no effect on the generated code since gcc manages to
assume that 'prev !=3D NULL' due to an earlier dereference.

Signed-off-by: David Laight <david.laight@aculab.com>
---
 kernel/locking/osq_lock.c | 23 ++++++++++-------------
 1 file changed, 10 insertions(+), 13 deletions(-)

diff --git a/kernel/locking/osq_lock.c b/kernel/locking/osq_lock.c
index 55f5db896c02..9bb3a077ba92 100644
--- a/kernel/locking/osq_lock.c
+++ b/kernel/locking/osq_lock.c
@@ -48,18 +48,17 @@ static inline struct optimistic_spin_node *decode_cpu(i=
nt encoded_cpu_val)
 static inline struct optimistic_spin_node *
 osq_wait_next(struct optimistic_spin_queue *lock,
 =09      struct optimistic_spin_node *node,
-=09      struct optimistic_spin_node *prev)
+=09      int old)
 {
-=09struct optimistic_spin_node *next =3D NULL;
+=09struct optimistic_spin_node *next;
 =09int curr =3D node->cpu;
-=09int old;
=20
 =09/*
-=09 * If there is a prev node in queue, then the 'old' value will be
-=09 * the prev node's CPU #, else it's set to OSQ_UNLOCKED_VAL since if
-=09 * we're currently last in queue, then the queue will then become empty=
.
+=09 * If osq_lock() is being cancelled there must be a previous node
+=09 * and 'old' is its CPU #.
+=09 * For osq_unlock() there is never a previous node and old is set
+=09 * to OSQ_UNLOCKED_VAL.
 =09 */
-=09old =3D prev ? prev->cpu : OSQ_UNLOCKED_VAL;
=20
 =09for (;;) {
 =09=09if (atomic_read(&lock->tail) =3D=3D curr &&
@@ -69,7 +68,7 @@ osq_wait_next(struct optimistic_spin_queue *lock,
 =09=09=09 * will now observe @lock and will complete its
 =09=09=09 * unlock()/unqueue().
 =09=09=09 */
-=09=09=09break;
+=09=09=09return NULL;
 =09=09}
=20
 =09=09/*
@@ -85,13 +84,11 @@ osq_wait_next(struct optimistic_spin_queue *lock,
 =09=09if (node->next) {
 =09=09=09next =3D xchg(&node->next, NULL);
 =09=09=09if (next)
-=09=09=09=09break;
+=09=09=09=09return next;
 =09=09}
=20
 =09=09cpu_relax();
 =09}
-
-=09return next;
 }
=20
 bool osq_lock(struct optimistic_spin_queue *lock)
@@ -192,7 +189,7 @@ bool osq_lock(struct optimistic_spin_queue *lock)
 =09 * back to @prev.
 =09 */
=20
-=09next =3D osq_wait_next(lock, node, prev);
+=09next =3D osq_wait_next(lock, node, prev->cpu);
 =09if (!next)
 =09=09return false;
=20
@@ -232,7 +229,7 @@ void osq_unlock(struct optimistic_spin_queue *lock)
 =09=09return;
 =09}
=20
-=09next =3D osq_wait_next(lock, node, NULL);
+=09next =3D osq_wait_next(lock, node, OSQ_UNLOCKED_VAL);
 =09if (next)
 =09=09WRITE_ONCE(next->locked, 1);
 }
--=20
2.17.1

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