On x86, LOCK DEC is cheaper than LOCK CMPXCHG and doesn't require a
retry loop around it.
Signed-off-by: Jan Beulich <jbeul...@suse.com>
--- a/xen/common/spinlock.c
+++ b/xen/common/spinlock.c
@@ -333,14 +333,18 @@ int _read_trylock(rwlock_t *lock)
return 1;
}
-void _read_unlock(rwlock_t *lock)
-{
- uint32_t x, y;
+#ifndef _raw_read_unlock
+# define _raw_read_unlock(l) do { \
+ uint32_t x = (l)->lock, y; \
+ while ( (y = cmpxchg(&(l)->lock, x, x - 1)) != x ) \
+ x = y; \
+} while (0)
+#endif
+inline void _read_unlock(rwlock_t *lock)
+{
preempt_enable();
- x = lock->lock;
- while ( (y = cmpxchg(&lock->lock, x, x-1)) != x )
- x = y;
+ _raw_read_unlock(lock);
}
void _read_unlock_irq(rwlock_t *lock)
--- 2014-08-25.orig/xen/include/asm-x86/spinlock.h 2014-10-07
14:42:41.000000000 +0200
+++ 2014-08-25/xen/include/asm-x86/spinlock.h 2014-10-07 15:33:18.000000000
+0200
@@ -31,4 +31,7 @@ static always_inline int _raw_spin_trylo
return (oldval > 0);
}
+#define _raw_read_unlock(l) \
+ asm volatile ( "lock; dec%z0 %0" : "+m" ((l)->lock) :: "memory" )
+
#endif /* __ASM_SPINLOCK_H */
rwlock: allow arch to override read_unlock() atomic
On x86, LOCK DEC is cheaper than LOCK CMPXCHG and doesn't require a
retry loop around it.
Signed-off-by: Jan Beulich <jbeul...@suse.com>
--- a/xen/common/spinlock.c
+++ b/xen/common/spinlock.c
@@ -333,14 +333,18 @@ int _read_trylock(rwlock_t *lock)
return 1;
}
-void _read_unlock(rwlock_t *lock)
-{
- uint32_t x, y;
+#ifndef _raw_read_unlock
+# define _raw_read_unlock(l) do { \
+ uint32_t x = (l)->lock, y; \
+ while ( (y = cmpxchg(&(l)->lock, x, x - 1)) != x ) \
+ x = y; \
+} while (0)
+#endif
+inline void _read_unlock(rwlock_t *lock)
+{
preempt_enable();
- x = lock->lock;
- while ( (y = cmpxchg(&lock->lock, x, x-1)) != x )
- x = y;
+ _raw_read_unlock(lock);
}
void _read_unlock_irq(rwlock_t *lock)
--- 2014-08-25.orig/xen/include/asm-x86/spinlock.h 2014-10-07
14:42:41.000000000 +0200
+++ 2014-08-25/xen/include/asm-x86/spinlock.h 2014-10-07 15:33:18.000000000
+0200
@@ -31,4 +31,7 @@ static always_inline int _raw_spin_trylo
return (oldval > 0);
}
+#define _raw_read_unlock(l) \
+ asm volatile ( "lock; dec%z0 %0" : "+m" ((l)->lock) :: "memory" )
+
#endif /* __ASM_SPINLOCK_H */
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