/* 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 consecutive 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 <asm/asm.h>

#define FASTCALL __attribute__ ((fastcall)) __attribute__ ((dllimport))

void  FASTCALL InitRwsem(struct rw_semaphore *sem)__asm__("InitRwsem");
void  FASTCALL DownRead(struct rw_semaphore *sem)__asm__("DownRead");
void  FASTCALL DownWrite(struct rw_semaphore *sem)__asm__("DownWrite");
void  FASTCALL UpRead(struct rw_semaphore *sem)__asm__("UpRead");
void  FASTCALL UpWrite(struct rw_semaphore *sem)__asm__("UpWrite");

static inline void __init_rwsem(struct rw_semaphore *sem, const char *name,
                  struct lock_class_key *key)
{
    InitRwsem(sem);
}

/*
 * lock for reading
 */
static inline void down_read(struct rw_semaphore *sem)
{
    DownRead(sem);
}

static inline void down_write(struct rw_semaphore *sem)
{
    DownWrite(sem);
}

/*
 * unlock after reading
 */
static inline void up_read(struct rw_semaphore *sem)
{
    UpRead(sem);
}

/*
 * unlock after writing
 */
static inline void up_write(struct rw_semaphore *sem)
{
    UpWrite(sem);
}

#define RWSEM_UNLOCKED_VALUE            0x00000000L
#define RWSEM_ACTIVE_BIAS               0x00000001L
#define RWSEM_WAITING_BIAS              (-RWSEM_ACTIVE_MASK-1)
#define RWSEM_ACTIVE_READ_BIAS          RWSEM_ACTIVE_BIAS
#define RWSEM_ACTIVE_WRITE_BIAS         (RWSEM_WAITING_BIAS + RWSEM_ACTIVE_BIAS)

static inline int down_read_trylock(struct rw_semaphore *sem)
{
    long result, tmp;
    asm volatile("# beginning __down_read_trylock\n\t"
                 "  mov          %0,%1\n\t"
                 "1:\n\t"
                 "  mov          %1,%2\n\t"
                 "  add          %3,%2\n\t"
                 "  jle          2f\n\t"
                 LOCK_PREFIX "  cmpxchg  %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;
}


#endif /* __KERNEL__ */
#endif /* _ASM_X86_RWSEM_H */