Commit fea0e182 authored by Uros Bizjak's avatar Uros Bizjak Committed by Ingo Molnar
Browse files

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: default avatarUros Bizjak <ubizjak@gmail.com>
Signed-off-by: default avatarIngo Molnar <mingo@kernel.org>
Reviewed-by: default avatarWaiman Long <longman@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Link: https://lore.kernel.org/r/20240411192317.25432-2-ubizjak@gmail.com
parent 6a97734f

kernel/locking/qspinlock_paravirt.h

+8 −8
Original line number Diff line number Diff line
@@ -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);