The following commit has been merged into the locking/core branch of tip:
Commit-ID: 6a97734f2222e0352f1900e3eb3167e9069b91bb
Gitweb: https://git.kernel.org/tip/6a97734f2222e0352f1900e3eb3167e9069b91bb
Author: Uros Bizjak <ubizjak@xxxxxxxxx>
AuthorDate: Mon, 25 Mar 2024 15:09:32 +01:00
Committer: Ingo Molnar <mingo@xxxxxxxxxx>
CommitterDate: Fri, 12 Apr 2024 11:40:51 +02:00
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: Uros Bizjak <ubizjak@xxxxxxxxx>
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
Reviewed-by: Waiman Long <longman@xxxxxxxxxx>
Reviewed-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Link: https://lore.kernel.org/r/20240325140943.815051-1-ubizjak@xxxxxxxxx
---
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 169950f..77ba80b 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 */
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