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SubjectRe: [PATCH 1/2] locking/pvqspinlock: Use try_cmpxchg_acquire() in trylock_clear_pending()
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On 3/25/24 10:09, Uros Bizjak wrote:
> Use try_cmpxchg_acquire(*ptr, &old, new) instead of
> cmpxchg_relaxed(*ptr, old, new) == old in trylock_clear_pending().
> x86 CMPXCHG instruction returns success in ZF flag, so this change
> saves a compare after cmpxchg.
>
> Also change the return type of the function to bool.
>
> 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 | 31 ++++++++++++-----------------
> 1 file changed, 13 insertions(+), 18 deletions(-)
>
> diff --git a/kernel/locking/qspinlock_paravirt.h b/kernel/locking/qspinlock_paravirt.h
> index 169950fe1aad..77ba80bd95f9 100644
> --- a/kernel/locking/qspinlock_paravirt.h
> +++ b/kernel/locking/qspinlock_paravirt.h
> @@ -116,11 +116,12 @@ static __always_inline void set_pending(struct qspinlock *lock)
> * barrier. Therefore, an atomic cmpxchg_acquire() is used to acquire the
> * lock just to be sure that it will get it.
> */
> -static __always_inline int trylock_clear_pending(struct qspinlock *lock)
> +static __always_inline bool trylock_clear_pending(struct qspinlock *lock)
> {
> + u16 old = _Q_PENDING_VAL;
> +
> return !READ_ONCE(lock->locked) &&
> - (cmpxchg_acquire(&lock->locked_pending, _Q_PENDING_VAL,
> - _Q_LOCKED_VAL) == _Q_PENDING_VAL);
> + try_cmpxchg_acquire(&lock->locked_pending, &old, _Q_LOCKED_VAL);
> }
> #else /* _Q_PENDING_BITS == 8 */
> static __always_inline void set_pending(struct qspinlock *lock)
> @@ -128,27 +129,21 @@ static __always_inline void set_pending(struct qspinlock *lock)
> atomic_or(_Q_PENDING_VAL, &lock->val);
> }
>
> -static __always_inline int trylock_clear_pending(struct qspinlock *lock)
> +static __always_inline bool trylock_clear_pending(struct qspinlock *lock)
> {
> - int val = atomic_read(&lock->val);
> -
> - for (;;) {
> - int old, new;
> -
> - if (val & _Q_LOCKED_MASK)
> - break;
> + int old, new;
>
> + old = atomic_read(&lock->val);
> + do {
> + if (old & _Q_LOCKED_MASK)
> + return false;
> /*
> * Try to clear pending bit & set locked bit
> */
> - old = val;
> - new = (val & ~_Q_PENDING_MASK) | _Q_LOCKED_VAL;
> - val = atomic_cmpxchg_acquire(&lock->val, old, new);
> + new = (old & ~_Q_PENDING_MASK) | _Q_LOCKED_VAL;
> + } while (!atomic_try_cmpxchg_acquire (&lock->val, &old, new));
>
> - if (val == old)
> - return 1;
> - }
> - return 0;
> + return true;
> }
> #endif /* _Q_PENDING_BITS == 8 */
>
Reviewed-by: Waiman Long <longman@redhat.com>


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