linux-kernel - [PATCH v10 2/5] locking/qspinlock: Refactor the qspinlock slow path

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Date:   Fri,  3 Apr 2020 16:59:27 -0400
From:   Alex Kogan <alex.kogan@...cle.com>
To:     linux@...linux.org.uk, peterz@...radead.org, mingo@...hat.com,
        will.deacon@....com, arnd@...db.de, longman@...hat.com,
        linux-arch@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        linux-kernel@...r.kernel.org, tglx@...utronix.de, bp@...en8.de,
        hpa@...or.com, x86@...nel.org, guohanjun@...wei.com,
        jglauber@...vell.com
Cc:     steven.sistare@...cle.com, daniel.m.jordan@...cle.com,
        alex.kogan@...cle.com, dave.dice@...cle.com
Subject: [PATCH v10 2/5] locking/qspinlock: Refactor the qspinlock slow path

Move some of the code manipulating the spin lock into separate functions.
This would allow easier integration of alternative ways to manipulate
that lock.

Signed-off-by: Alex Kogan <alex.kogan@...cle.com>
Reviewed-by: Steve Sistare <steven.sistare@...cle.com>
Reviewed-by: Waiman Long <longman@...hat.com>
---
 kernel/locking/qspinlock.c | 38 ++++++++++++++++++++++++++++++++++++--
 1 file changed, 36 insertions(+), 2 deletions(-)

diff --git a/kernel/locking/qspinlock.c b/kernel/locking/qspinlock.c
index ac1dedbe0237..6e63c72e3fbd 100644
--- a/kernel/locking/qspinlock.c
+++ b/kernel/locking/qspinlock.c
@@ -289,6 +289,34 @@ static __always_inline u32  __pv_wait_head_or_lock(struct qspinlock *lock,
 #define queued_spin_lock_slowpath	native_queued_spin_lock_slowpath
 #endif
 
+/*
+ * __try_clear_tail - try to clear tail by setting the lock value to
+ * _Q_LOCKED_VAL.
+ * @lock: Pointer to the queued spinlock structure
+ * @val: Current value of the lock
+ * @node: Pointer to the MCS node of the lock holder
+ */
+static __always_inline bool __try_clear_tail(struct qspinlock *lock,
+						   u32 val,
+						   struct mcs_spinlock *node)
+{
+	return atomic_try_cmpxchg_relaxed(&lock->val, &val, _Q_LOCKED_VAL);
+}
+
+/*
+ * __mcs_lock_handoff - pass the MCS lock to the next waiter
+ * @node: Pointer to the MCS node of the lock holder
+ * @next: Pointer to the MCS node of the first waiter in the MCS queue
+ */
+static __always_inline void __mcs_lock_handoff(struct mcs_spinlock *node,
+					    struct mcs_spinlock *next)
+{
+	arch_mcs_lock_handoff(&next->locked, 1);
+}
+
+#define try_clear_tail		__try_clear_tail
+#define mcs_lock_handoff	__mcs_lock_handoff
+
 #endif /* _GEN_PV_LOCK_SLOWPATH */
 
 /**
@@ -533,7 +561,7 @@ void queued_spin_lock_slowpath(struct qspinlock *lock, u32 val)
 	 *       PENDING will make the uncontended transition fail.
 	 */
 	if ((val & _Q_TAIL_MASK) == tail) {
-		if (atomic_try_cmpxchg_relaxed(&lock->val, &val, _Q_LOCKED_VAL))
+		if (try_clear_tail(lock, val, node))
 			goto release; /* No contention */
 	}
 
@@ -550,7 +578,7 @@ void queued_spin_lock_slowpath(struct qspinlock *lock, u32 val)
 	if (!next)
 		next = smp_cond_load_relaxed(&node->next, (VAL));
 
-	arch_mcs_lock_handoff(&next->locked, 1);
+	mcs_lock_handoff(node, next);
 	pv_kick_node(lock, next);
 
 release:
@@ -575,6 +603,12 @@ EXPORT_SYMBOL(queued_spin_lock_slowpath);
 #undef pv_kick_node
 #undef pv_wait_head_or_lock
 
+#undef try_clear_tail
+#define try_clear_tail		__try_clear_tail
+
+#undef mcs_lock_handoff
+#define mcs_lock_handoff			__mcs_lock_handoff
+
 #undef  queued_spin_lock_slowpath
 #define queued_spin_lock_slowpath	__pv_queued_spin_lock_slowpath
 
-- 
2.21.1 (Apple Git-122.3)