1 files changed, 265 insertions, 0 deletions
diff --git a/arch/x86/include/asm/rwsem.h b/arch/x86/include/asm/rwsem.h
new file mode 100644
index 0000000..3ff3015
--- /dev/null
+++ b/arch/x86/include/asm/rwsem.h
@@ -0,0 +1,265 @@
+/* rwsem.h: R/W semaphores implemented using XADD/CMPXCHG for i486+
+ *
+ * Written by David Howells (dhowells@redhat.com).
+ *
+ * Derived from asm-x86/semaphore.h
+ *
+ *
+ * The MSW of the count is the negated number of active writers and waiting
+ * lockers, and the LSW is the total number of active locks
+ *
+ * The lock count is initialized to 0 (no active and no waiting lockers).
+ *
+ * When a writer subtracts WRITE_BIAS, it'll get 0xffff0001 for the case of an
+ * uncontended lock. This can be determined because XADD returns the old value.
+ * Readers increment by 1 and see a positive value when uncontended, negative
+ * if there are writers (and maybe) readers waiting (in which case it goes to
+ * sleep).
+ *
+ * The value of WAITING_BIAS supports up to 32766 waiting processes. This can
+ * be extended to 65534 by manually checking the whole MSW rather than relying
+ * on the S flag.
+ *
+ * The value of ACTIVE_BIAS supports up to 65535 active processes.
+ *
+ * This should be totally fair - if anything is waiting, a process that wants a
+ * lock will go to the back of the queue. When the currently active lock is
+ * released, if there's a writer at the front of the queue, then that and only
+ * that will be woken up; if there's a bunch of consequtive readers at the
+ * front, then they'll all be woken up, but no other readers will be.
+ */
+
+#ifndef ASM_X86__RWSEM_H
+#define ASM_X86__RWSEM_H
+
+#ifndef _LINUX_RWSEM_H
+#error "please don't include asm/rwsem.h directly, use linux/rwsem.h instead"
+#endif
+
+#ifdef __KERNEL__
+
+#include <linux/list.h>
+#include <linux/spinlock.h>
+#include <linux/lockdep.h>
+
+struct rwsem_waiter;
+
+extern asmregparm struct rw_semaphore *
+ rwsem_down_read_failed(struct rw_semaphore *sem);
+extern asmregparm struct rw_semaphore *
+ rwsem_down_write_failed(struct rw_semaphore *sem);
+extern asmregparm struct rw_semaphore *
+ rwsem_wake(struct rw_semaphore *);
+extern asmregparm struct rw_semaphore *
+ rwsem_downgrade_wake(struct rw_semaphore *sem);
+
+/*
+ * the semaphore definition
+ */
+
+#define RWSEM_UNLOCKED_VALUE		0x00000000
+#define RWSEM_ACTIVE_BIAS		0x00000001
+#define RWSEM_ACTIVE_MASK		0x0000ffff
+#define RWSEM_WAITING_BIAS		(-0x00010000)
+#define RWSEM_ACTIVE_READ_BIAS		RWSEM_ACTIVE_BIAS
+#define RWSEM_ACTIVE_WRITE_BIAS		(RWSEM_WAITING_BIAS + RWSEM_ACTIVE_BIAS)
+
+struct rw_semaphore {
+	signed long		count;
+	spinlock_t		wait_lock;
+	struct list_head	wait_list;
+#ifdef CONFIG_DEBUG_LOCK_ALLOC
+	struct lockdep_map dep_map;
+#endif
+};
+
+#ifdef CONFIG_DEBUG_LOCK_ALLOC
+# define __RWSEM_DEP_MAP_INIT(lockname) , .dep_map = { .name = #lockname }
+#else
+# define __RWSEM_DEP_MAP_INIT(lockname)
+#endif
+
+
+#define __RWSEM_INITIALIZER(name)				\
+{								\
+	RWSEM_UNLOCKED_VALUE, __SPIN_LOCK_UNLOCKED((name).wait_lock), \
+	LIST_HEAD_INIT((name).wait_list) __RWSEM_DEP_MAP_INIT(name) \
+}
+
+#define DECLARE_RWSEM(name)					\
+	struct rw_semaphore name = __RWSEM_INITIALIZER(name)
+
+extern void __init_rwsem(struct rw_semaphore *sem, const char *name,
+			 struct lock_class_key *key);
+
+#define init_rwsem(sem)						\
+do {								\
+	static struct lock_class_key __key;			\
+								\
+	__init_rwsem((sem), #sem, &__key);			\
+} while (0)
+
+/*
+ * lock for reading
+ */
+static inline void __down_read(struct rw_semaphore *sem)
+{
+	asm volatile("# beginning down_read\n\t"
+		     LOCK_PREFIX "  incl      (%%eax)\n\t"
+		     /* adds 0x00000001, returns the old value */
+		     "  jns        1f\n"
+		     "  call call_rwsem_down_read_failed\n"
+		     "1:\n\t"
+		     "# ending down_read\n\t"
+		     : "+m" (sem->count)
+		     : "a" (sem)
+		     : "memory", "cc");
+}
+
+/*
+ * trylock for reading -- returns 1 if successful, 0 if contention
+ */
+static inline int __down_read_trylock(struct rw_semaphore *sem)
+{
+	__s32 result, tmp;
+	asm volatile("# beginning __down_read_trylock\n\t"
+		     "  movl      %0,%1\n\t"
+		     "1:\n\t"
+		     "  movl	     %1,%2\n\t"
+		     "  addl      %3,%2\n\t"
+		     "  jle	     2f\n\t"
+		     LOCK_PREFIX "  cmpxchgl  %2,%0\n\t"
+		     "  jnz	     1b\n\t"
+		     "2:\n\t"
+		     "# ending __down_read_trylock\n\t"
+		     : "+m" (sem->count), "=&a" (result), "=&r" (tmp)
+		     : "i" (RWSEM_ACTIVE_READ_BIAS)
+		     : "memory", "cc");
+	return result >= 0 ? 1 : 0;
+}
+
+/*
+ * lock for writing
+ */
+static inline void __down_write_nested(struct rw_semaphore *sem, int subclass)
+{
+	int tmp;
+
+	tmp = RWSEM_ACTIVE_WRITE_BIAS;
+	asm volatile("# beginning down_write\n\t"
+		     LOCK_PREFIX "  xadd      %%edx,(%%eax)\n\t"
+		     /* subtract 0x0000ffff, returns the old value */
+		     "  testl     %%edx,%%edx\n\t"
+		     /* was the count 0 before? */
+		     "  jz        1f\n"
+		     "  call call_rwsem_down_write_failed\n"
+		     "1:\n"
+		     "# ending down_write"
+		     : "+m" (sem->count), "=d" (tmp)
+		     : "a" (sem), "1" (tmp)
+		     : "memory", "cc");
+}
+
+static inline void __down_write(struct rw_semaphore *sem)
+{
+	__down_write_nested(sem, 0);
+}
+
+/*
+ * trylock for writing -- returns 1 if successful, 0 if contention
+ */
+static inline int __down_write_trylock(struct rw_semaphore *sem)
+{
+	signed long ret = cmpxchg(&sem->count,
+				  RWSEM_UNLOCKED_VALUE,
+				  RWSEM_ACTIVE_WRITE_BIAS);
+	if (ret == RWSEM_UNLOCKED_VALUE)
+		return 1;
+	return 0;
+}
+
+/*
+ * unlock after reading
+ */
+static inline void __up_read(struct rw_semaphore *sem)
+{
+	__s32 tmp = -RWSEM_ACTIVE_READ_BIAS;
+	asm volatile("# beginning __up_read\n\t"
+		     LOCK_PREFIX "  xadd      %%edx,(%%eax)\n\t"
+		     /* subtracts 1, returns the old value */
+		     "  jns        1f\n\t"
+		     "  call call_rwsem_wake\n"
+		     "1:\n"
+		     "# ending __up_read\n"
+		     : "+m" (sem->count), "=d" (tmp)
+		     : "a" (sem), "1" (tmp)
+		     : "memory", "cc");
+}
+
+/*
+ * unlock after writing
+ */
+static inline void __up_write(struct rw_semaphore *sem)
+{
+	asm volatile("# beginning __up_write\n\t"
+		     "  movl      %2,%%edx\n\t"
+		     LOCK_PREFIX "  xaddl     %%edx,(%%eax)\n\t"
+		     /* tries to transition
+			0xffff0001 -> 0x00000000 */
+		     "  jz       1f\n"
+		     "  call call_rwsem_wake\n"
+		     "1:\n\t"
+		     "# ending __up_write\n"
+		     : "+m" (sem->count)
+		     : "a" (sem), "i" (-RWSEM_ACTIVE_WRITE_BIAS)
+		     : "memory", "cc", "edx");
+}
+
+/*
+ * downgrade write lock to read lock
+ */
+static inline void __downgrade_write(struct rw_semaphore *sem)
+{
+	asm volatile("# beginning __downgrade_write\n\t"
+		     LOCK_PREFIX "  addl      %2,(%%eax)\n\t"
+		     /* transitions 0xZZZZ0001 -> 0xYYYY0001 */
+		     "  jns       1f\n\t"
+		     "  call call_rwsem_downgrade_wake\n"
+		     "1:\n\t"
+		     "# ending __downgrade_write\n"
+		     : "+m" (sem->count)
+		     : "a" (sem), "i" (-RWSEM_WAITING_BIAS)
+		     : "memory", "cc");
+}
+
+/*
+ * implement atomic add functionality
+ */
+static inline void rwsem_atomic_add(int delta, struct rw_semaphore *sem)
+{
+	asm volatile(LOCK_PREFIX "addl %1,%0"
+		     : "+m" (sem->count)
+		     : "ir" (delta));
+}
+
+/*
+ * implement exchange and add functionality
+ */
+static inline int rwsem_atomic_update(int delta, struct rw_semaphore *sem)
+{
+	int tmp = delta;
+
+	asm volatile(LOCK_PREFIX "xadd %0,%1"
+		     : "+r" (tmp), "+m" (sem->count)
+		     : : "memory");
+
+	return tmp + delta;
+}
+
+static inline int rwsem_is_locked(struct rw_semaphore *sem)
+{
+	return (sem->count != 0);
+}
+
+#endif /* __KERNEL__ */
+#endif /* ASM_X86__RWSEM_H */