[Xen-devel] [PATCH 14/20] x86/ticketlock: loosen ordering restraints on unlock

[Jeremy Fitzhardinge <jeremy@xxxxxxxx>]

From: Jeremy Fitzhardinge <jeremy.fitzhardinge@xxxxxxxxxx>

It's only necessary to prevent the compiler from reordering code out of
the locked region past the unlock.  Putting too many barriers in prevents
the compiler from generating the best code when PV spinlocks are enabled.

Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@xxxxxxxxxx>
---
 arch/x86/include/asm/spinlock.h |    8 +++++---
 1 files changed, 5 insertions(+), 3 deletions(-)

diff --git a/arch/x86/include/asm/spinlock.h b/arch/x86/include/asm/spinlock.h
index b9a1aae..d6de5e7 100644
--- a/arch/x86/include/asm/spinlock.h
+++ b/arch/x86/include/asm/spinlock.h
@@ -46,9 +46,7 @@ static __always_inline void __ticket_unlock_release(struct 
arch_spinlock *lock)
 #else
 static __always_inline void __ticket_unlock_release(struct arch_spinlock *lock)
 {
-       barrier();
        lock->tickets.head++;
-       barrier();
 }
 #endif
 
@@ -184,7 +182,11 @@ static __always_inline int 
arch_spin_trylock(arch_spinlock_t *lock)
 
 static __always_inline void arch_spin_unlock(arch_spinlock_t *lock)
 {
-       __ticket_t next = lock->tickets.head + 1;
+       __ticket_t next;
+
+       barrier();              /* prevent reordering out of locked region */
+
+       next = lock->tickets.head + 1;
        __ticket_unlock_release(lock);
        __ticket_unlock_kick(lock, next);
 }
-- 
1.7.2.3


_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxxxxxxxx
http://lists.xensource.com/xen-devel