linux-kernel - Re: [PATCH 2/2] locking/pvqspinlock: Use try_cmpxchg() in qspinlock

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Date: Wed, 27 Mar 2024 22:22:27 -0400
From: Waiman Long <longman@...hat.com>
To: Uros Bizjak <ubizjak@...il.com>, linux-kernel@...r.kernel.org
Cc: Peter Zijlstra <peterz@...radead.org>, Ingo Molnar <mingo@...hat.com>,
 Will Deacon <will@...nel.org>, Boqun Feng <boqun.feng@...il.com>
Subject: Re: [PATCH 2/2] locking/pvqspinlock: Use try_cmpxchg() in
 qspinlock_paravirt.h


On 3/25/24 10:09, Uros Bizjak wrote:
> 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>
> Cc: Peter Zijlstra <peterz@...radead.org>
> Cc: Ingo Molnar <mingo@...hat.com>
> Cc: Will Deacon <will@...nel.org>
> Cc: Waiman Long <longman@...hat.com>
> Cc: Boqun Feng <boqun.feng@...il.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 77ba80bd95f9..3db5f811260f 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);
Reviewed-by: Waiman Long <longman@...hat.com>