[Xen-devel] [PATCH 2/6] x86: move decision about clearing slowpath flag into arch_spin_lock()

[Juergen Gross <jgross@xxxxxxxx>]

The decision whether the slowpath flag is to be cleared for
paravirtualized spinlocks is located in
__ticket_check_and_clear_slowpath() today.

Move that decision into arch_spin_lock() and add an unlikely attribute
to it to avoid calling a function in case the compiler chooses not to
inline __ticket_check_and_clear_slowpath() and the slowpath flag isn't
set.

Signed-off-by: Juergen Gross <jgross@xxxxxxxx>
---
 arch/x86/include/asm/spinlock.h | 23 +++++++++++------------
 1 file changed, 11 insertions(+), 12 deletions(-)

diff --git a/arch/x86/include/asm/spinlock.h b/arch/x86/include/asm/spinlock.h
index 8ceec8d..268b9da 100644
--- a/arch/x86/include/asm/spinlock.h
+++ b/arch/x86/include/asm/spinlock.h
@@ -66,20 +66,18 @@ static inline int  __tickets_equal(__ticket_t one, 
__ticket_t two)
        return !((one ^ two) & ~TICKET_SLOWPATH_FLAG);
 }
 
-static inline void __ticket_check_and_clear_slowpath(arch_spinlock_t *lock,
-                                                       __ticket_t head)
+static inline void __ticket_clear_slowpath(arch_spinlock_t *lock,
+                                          __ticket_t head)
 {
-       if (head & TICKET_SLOWPATH_FLAG) {
-               arch_spinlock_t old, new;
+       arch_spinlock_t old, new;
 
-               old.tickets.head = head;
-               new.tickets.head = head & ~TICKET_SLOWPATH_FLAG;
-               old.tickets.tail = new.tickets.head + TICKET_LOCK_INC;
-               new.tickets.tail = old.tickets.tail;
+       old.tickets.head = head;
+       new.tickets.head = head & ~TICKET_SLOWPATH_FLAG;
+       old.tickets.tail = new.tickets.head + TICKET_LOCK_INC;
+       new.tickets.tail = old.tickets.tail;
 
-               /* try to clear slowpath flag when there are no contenders */
-               cmpxchg(&lock->head_tail, old.head_tail, new.head_tail);
-       }
+       /* try to clear slowpath flag when there are no contenders */
+       cmpxchg(&lock->head_tail, old.head_tail, new.head_tail);
 }
 
 static __always_inline int arch_spin_value_unlocked(arch_spinlock_t lock)
@@ -120,7 +118,8 @@ static __always_inline void arch_spin_lock(arch_spinlock_t 
*lock)
                __ticket_lock_spinning(lock, inc.tail);
        }
 clear_slowpath:
-       __ticket_check_and_clear_slowpath(lock, inc.head);
+       if (unlikely(inc.head & TICKET_SLOWPATH_FLAG))
+               __ticket_clear_slowpath(lock, inc.head);
 out:
        barrier();      /* make sure nothing creeps before the lock is taken */
 }
-- 
2.1.4


_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxx
http://lists.xen.org/xen-devel