Commit 6a97734f authored by Uros Bizjak's avatar Uros Bizjak Committed by Ingo Molnar
Browse files

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



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: default avatarUros Bizjak <ubizjak@gmail.com>
Signed-off-by: default avatarIngo Molnar <mingo@kernel.org>
Reviewed-by: default avatarWaiman Long <longman@redhat.com>
Reviewed-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
Link: https://lore.kernel.org/r/20240325140943.815051-1-ubizjak@gmail.com
parent 79a34e3d

kernel/locking/qspinlock_paravirt.h

+13 −18
Original line number Diff line number Diff line
@@ -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;



	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 */