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locking/pvqspinlock: Use try_cmpxchg_acquire() in trylock_clear_pending()

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Replace this pattern in trylock_clear_pending():

    cmpxchg_acquire(*ptr, old, new) == old

... with the simpler and faster:

    try_cmpxchg_acquire(*ptr, &old, new)

The x86 CMPXCHG instruction returns success in the ZF flag, so this change
saves a compare after the CMPXCHG.

Also change the return type of the function to bool and streamline
the control flow in the _Q_PENDING_BITS == 8 variant a bit.

No functional change intended.

Signed-off-by: Uros Bizjak <ubizjak@gmail.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Reviewed-by: Waiman Long <longman@redhat.com>
Reviewed-by: Linus Torvalds <torvalds@linux-foundation.org>
Link: https://lore.kernel.org/r/20240325140943.815051-1-ubizjak@gmail.com
This commit is contained in:
Uros Bizjak 2024-03-25 15:09:32 +01:00 committed by Ingo Molnar
parent 79a34e3d84
commit 6a97734f22
1 changed files with 13 additions and 18 deletions
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31
kernel/locking/qspinlock_paravirt.h
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@ -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 */