On 02/09, Raghavendra K T wrote:
+static inline void __ticket_check_and_clear_slowpath(arch_spinlock_t *lock)
+{
+ arch_spinlock_t old, new;
+ __ticket_t diff;
+
+ old.tickets = READ_ONCE(lock->tickets);
+ diff = (old.tickets.tail & ~TICKET_SLOWPATH_FLAG) - old.tickets.head;
+
+ /* try to clear slowpath flag when there are no contenders */
+ if ((old.tickets.tail & TICKET_SLOWPATH_FLAG) &&
+ (diff == TICKET_LOCK_INC)) {
+ new = old;
+ new.tickets.tail &= ~TICKET_SLOWPATH_FLAG;
+ cmpxchg(&lock->head_tail, old.head_tail, new.head_tail);
+ }
+}
Can't we simplify it? We own .head, and we already know it. We only need
to clear TICKET_SLOWPATH_FLAG in .tail atomically?
IOW,
static inline void __ticket_check_and_clear_slowpath(arch_spinlock_t *lock, __ticket_t head)
{
__ticket_t old_tail, new_tail;
new_tail = head + TICKET_LOCK_INC;
old_tail = new_tail | TICKET_SLOWPATH_FLAG;
if (READ_ONCE(lock->tickets.tail) == old_tail)
cmpxchg(&lock->tickets.tail, old_tail, new_tail);
}
Plus
- __ticket_check_and_clear_slowpath(lock);
+ __ticket_check_and_clear_slowpath(lock, inc.tail);
Or I missed something?
And I think it would be better to avoid ifdef(CONFIG_PARAVIRT_SPINLOCKS),
ww can just do
if (TICKET_SLOWPATH_FLAG)
__ticket_check_and_clear_slowpath();