linux-kernel - [tip: locking/core] locking/pvqspinlock: Use try_cmpxchg() in qspinlock

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Message-ID: <171291517671.10875.5289616784830783285.tip-bot2@tip-bot2>
Date: Fri, 12 Apr 2024 09:46:16 -0000
From: "tip-bot2 for Uros Bizjak" <tip-bot2@...utronix.de>
To: linux-tip-commits@...r.kernel.org
Cc: Uros Bizjak <ubizjak@...il.com>, Ingo Molnar <mingo@...nel.org>,
 Waiman Long <longman@...hat.com>,
 Linus Torvalds <torvalds@...ux-foundation.org>, x86@...nel.org,
 linux-kernel@...r.kernel.org
Subject: [tip: locking/core] locking/pvqspinlock: Use try_cmpxchg() in
 qspinlock_paravirt.h

The following commit has been merged into the locking/core branch of tip:

Commit-ID:     fea0e1820b51fff95c85518eb9cf3386f367908d
Gitweb:        https://git.kernel.org/tip/fea0e1820b51fff95c85518eb9cf3386f367908d
Author:        Uros Bizjak <ubizjak@...il.com>
AuthorDate:    Thu, 11 Apr 2024 21:22:55 +02:00
Committer:     Ingo Molnar <mingo@...nel.org>
CommitterDate: Fri, 12 Apr 2024 11:42:39 +02:00

locking/pvqspinlock: Use try_cmpxchg() in qspinlock_paravirt.h

Use try_cmpxchg(*ptr, &old, new) instead of
cmpxchg(*ptr, old, new) == old in qspinlock_paravirt.h
x86 CMPXCHG instruction returns success in ZF flag, so
this change saves a compare after cmpxchg.

No functional change intended.

Signed-off-by: Uros Bizjak <ubizjak@...il.com>
Signed-off-by: Ingo Molnar <mingo@...nel.org>
Reviewed-by: Waiman Long <longman@...hat.com>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>
Link: https://lore.kernel.org/r/20240411192317.25432-2-ubizjak@gmail.com
---
 kernel/locking/qspinlock_paravirt.h | 16 ++++++++--------
 1 file changed, 8 insertions(+), 8 deletions(-)

diff --git a/kernel/locking/qspinlock_paravirt.h b/kernel/locking/qspinlock_paravirt.h
index 77ba80b..f5a36e6 100644
--- a/kernel/locking/qspinlock_paravirt.h
+++ b/kernel/locking/qspinlock_paravirt.h
@@ -86,9 +86,10 @@ static inline bool pv_hybrid_queued_unfair_trylock(struct qspinlock *lock)
 	 */
 	for (;;) {
 		int val = atomic_read(&lock->val);
+		u8 old = 0;
 
 		if (!(val & _Q_LOCKED_PENDING_MASK) &&
-		   (cmpxchg_acquire(&lock->locked, 0, _Q_LOCKED_VAL) == 0)) {
+		    try_cmpxchg_acquire(&lock->locked, &old, _Q_LOCKED_VAL)) {
 			lockevent_inc(pv_lock_stealing);
 			return true;
 		}
@@ -211,8 +212,9 @@ static struct qspinlock **pv_hash(struct qspinlock *lock, struct pv_node *node)
 	int hopcnt = 0;
 
 	for_each_hash_entry(he, offset, hash) {
+		struct qspinlock *old = NULL;
 		hopcnt++;
-		if (!cmpxchg(&he->lock, NULL, lock)) {
+		if (try_cmpxchg(&he->lock, &old, lock)) {
 			WRITE_ONCE(he->node, node);
 			lockevent_pv_hop(hopcnt);
 			return &he->lock;
@@ -355,7 +357,7 @@ static void pv_wait_node(struct mcs_spinlock *node, struct mcs_spinlock *prev)
 static void pv_kick_node(struct qspinlock *lock, struct mcs_spinlock *node)
 {
 	struct pv_node *pn = (struct pv_node *)node;
-
+	enum vcpu_state old = vcpu_halted;
 	/*
 	 * If the vCPU is indeed halted, advance its state to match that of
 	 * pv_wait_node(). If OTOH this fails, the vCPU was running and will
@@ -372,8 +374,7 @@ static void pv_kick_node(struct qspinlock *lock, struct mcs_spinlock *node)
 	 * subsequent writes.
 	 */
 	smp_mb__before_atomic();
-	if (cmpxchg_relaxed(&pn->state, vcpu_halted, vcpu_hashed)
-	    != vcpu_halted)
+	if (!try_cmpxchg_relaxed(&pn->state, &old, vcpu_hashed))
 		return;
 
 	/*
@@ -541,15 +542,14 @@ __pv_queued_spin_unlock_slowpath(struct qspinlock *lock, u8 locked)
 #ifndef __pv_queued_spin_unlock
 __visible __lockfunc void __pv_queued_spin_unlock(struct qspinlock *lock)
 {
-	u8 locked;
+	u8 locked = _Q_LOCKED_VAL;
 
 	/*
 	 * We must not unlock if SLOW, because in that case we must first
 	 * unhash. Otherwise it would be possible to have multiple @lock
 	 * entries, which would be BAD.
 	 */
-	locked = cmpxchg_release(&lock->locked, _Q_LOCKED_VAL, 0);
-	if (likely(locked == _Q_LOCKED_VAL))
+	if (try_cmpxchg_release(&lock->locked, &locked, 0))
 		return;
 
 	__pv_queued_spin_unlock_slowpath(lock, locked);