$NetBSD: patch-xen_include_asm-x86_spinlock.h,v 1.1 2014/12/30 08:15:01 spz Exp $

from XSA-114:
switch to write-biased r/w locks

This is to improve fairness: A permanent flow of read acquires can
otherwise lock out eventual writers indefinitely.

This is XSA-114 / CVE-2014-9065.

--- xen/include/asm-x86/spinlock.h.orig	2014-09-02 06:22:57.000000000 +0000
+++ xen/include/asm-x86/spinlock.h
@@ -31,58 +31,4 @@ static always_inline int _raw_spin_trylo
     return (oldval > 0);
 }
 
-typedef struct {
-    volatile int lock;
-} raw_rwlock_t;
-
-#define RW_WRITE_BIAS 0x7fffffff
-#define _RAW_RW_LOCK_UNLOCKED /*(raw_rwlock_t)*/ { 0 }
-
-static always_inline int _raw_read_trylock(raw_rwlock_t *rw)
-{
-    int acquired;
-
-    asm volatile (
-        "    lock; decl %0         \n"
-        "    jns 2f                \n"
-#ifdef __clang__ /* clang's builtin assember can't do .subsection */
-        "1:  .pushsection .fixup,\"ax\"\n"
-#else
-        "1:  .subsection 1         \n"
-#endif
-        "2:  lock; incl %0         \n"
-        "    decl %1               \n"
-        "    jmp 1b                \n"
-#ifdef __clang__
-        "    .popsection           \n"
-#else
-        "    .subsection 0         \n"
-#endif
-        : "=m" (rw->lock), "=r" (acquired) : "1" (1) : "memory" );
-
-    return acquired;
-}
-
-static always_inline int _raw_write_trylock(raw_rwlock_t *rw)
-{
-    return (cmpxchg(&rw->lock, 0, RW_WRITE_BIAS) == 0);
-}
-
-static always_inline void _raw_read_unlock(raw_rwlock_t *rw)
-{
-    asm volatile (
-        "lock ; incl %0"
-        : "=m" ((rw)->lock) : : "memory" );
-}
-
-static always_inline void _raw_write_unlock(raw_rwlock_t *rw)
-{
-    asm volatile (
-        "lock ; subl %1,%0"
-        : "=m" ((rw)->lock) : "i" (RW_WRITE_BIAS) : "memory" );
-}
-
-#define _raw_rw_is_locked(x) ((x)->lock != 0)
-#define _raw_rw_is_write_locked(x) ((x)->lock > 0)
-
 #endif /* __ASM_SPINLOCK_H */