lkml.org 
[lkml]   [2024]   [Mar]   [27]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
SubjectRe: [PATCH 2/2] locking/pvqspinlock: Use try_cmpxchg() in qspinlock_paravirt.h
From

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@gmail.com>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: Ingo Molnar <mingo@redhat.com>
> Cc: Will Deacon <will@kernel.org>
> Cc: Waiman Long <longman@redhat.com>
> Cc: Boqun Feng <boqun.feng@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 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@redhat.com>


\
 
 \ /
  Last update: 2024-05-27 16:13    [W:0.287 / U:0.884 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site