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/* rwsem.h: R/W semaphores implemented using XADD/CMPXCHG for i486+
 *
 * Written by David Howells (dhowells@redhat.com).
 *
 * Derived from asm-i386/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 _I386_RWSEM_H
#define _I386_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>

struct rwsem_waiter;

extern struct rw_semaphore *FASTCALL(rwsem_down_read_failed(struct rw_semaphore *sem));
extern struct rw_semaphore *FASTCALL(rwsem_down_write_failed(struct rw_semaphore *sem));
extern struct rw_semaphore *FASTCALL(rwsem_wake(struct rw_semaphore *));
extern struct rw_semaphore *FASTCALL(rwsem_downgrade_wake(struct rw_semaphore *sem));

/*
 * the semaphore definition
 */

struct rw_semaphore {
        signed long             count;
#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)
        spinlock_t              wait_lock;
        struct list_head        wait_list;
#if RWSEM_DEBUG
        int                     debug;
#endif
};

/*
 * initialisation
 */

#if RWSEM_DEBUG
#define __RWSEM_DEBUG_INIT      , 0
#else
#define __RWSEM_DEBUG_INIT      /* */
#endif

#define __RWSEM_INITIALIZER(name) \
{ RWSEM_UNLOCKED_VALUE, SPIN_LOCK_UNLOCKED, LIST_HEAD_INIT((name).wait_list) \
        __RWSEM_DEBUG_INIT }


#define DECLARE_RWSEM(name) \
        struct rw_semaphore name = __RWSEM_INITIALIZER(name)


static inline void init_rwsem(struct rw_semaphore *sem)
{
        sem->count = RWSEM_UNLOCKED_VALUE;
        spin_lock_init(&sem->wait_lock);
        INIT_LIST_HEAD(&sem->wait_list);
#if RWSEM_DEBUG
        sem->debug = 0;
#endif
}

/*
 * 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 */
                "  js        2f\n\t" /* jump if we weren't granted the lock */
                "1:\n\t"
                LOCK_SECTION_START("")
                "2:\n\t"
                "  pushl     %%ecx\n\t"
                "  pushl     %%edx\n\t"
                "  call      rwsem_down_read_failed\n\t"
                "  popl      %%edx\n\t"
                "  popl      %%ecx\n\t"
                "  jmp       1b\n"
                LOCK_SECTION_END
                "# ending down_read\n\t"
                : "=m"(sem->count)
                : "a"(sem), "m"(sem->count)
                : "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(struct rw_semaphore *sem)
{
        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? */
                "  jnz       2f\n\t" /* jump if we weren't granted the lock */
                "1:\n\t"
                LOCK_SECTION_START("")
                "2:\n\t"
                "  pushl     %%ecx\n\t"
                "  call      rwsem_down_write_failed\n\t"
                "  popl      %%ecx\n\t"
                "  jmp       1b\n"
                LOCK_SECTION_END
                "# ending down_write"
                : "=m"(sem->count), "=d"(tmp)
                : "a"(sem), "1"(tmp), "m"(sem->count)
                : "memory", "cc");
}

/*
 * 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 */
                "  js        2f\n\t" /* jump if the lock is being waited upon */
                "1:\n\t"
                LOCK_SECTION_START("")
                "2:\n\t"
                "  decw      %%dx\n\t" /* do nothing if still outstanding active readers */
                "  jnz       1b\n\t"
                "  pushl     %%ecx\n\t"
                "  call      rwsem_wake\n\t"
                "  popl      %%ecx\n\t"
                "  jmp       1b\n"
                LOCK_SECTION_END
                "# ending __up_read\n"
                : "=m"(sem->count), "=d"(tmp)
                : "a"(sem), "1"(tmp), "m"(sem->count)
                : "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 */
                "  jnz       2f\n\t" /* jump if the lock is being waited upon */
                "1:\n\t"
                LOCK_SECTION_START("")
                "2:\n\t"
                "  decw      %%dx\n\t" /* did the active count reduce to 0? */
                "  jnz       1b\n\t" /* jump back if not */
                "  pushl     %%ecx\n\t"
                "  call      rwsem_wake\n\t"
                "  popl      %%ecx\n\t"
                "  jmp       1b\n"
                LOCK_SECTION_END
                "# ending __up_write\n"
                : "=m"(sem->count)
                : "a"(sem), "i"(-RWSEM_ACTIVE_WRITE_BIAS), "m"(sem->count)
                : "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 */
                "  js        2f\n\t" /* jump if the lock is being waited upon */
                "1:\n\t"
                LOCK_SECTION_START("")
                "2:\n\t"
                "  pushl     %%ecx\n\t"
                "  pushl     %%edx\n\t"
                "  call      rwsem_downgrade_wake\n\t"
                "  popl      %%edx\n\t"
                "  popl      %%ecx\n\t"
                "  jmp       1b\n"
                LOCK_SECTION_END
                "# ending __downgrade_write\n"
                : "=m"(sem->count)
                : "a"(sem), "i"(-RWSEM_WAITING_BIAS), "m"(sem->count)
                : "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), "m"(sem->count));
}

/*
 * 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,(%2)"
                : "+r"(tmp), "=m"(sem->count)
                : "r"(sem), "m"(sem->count)
                : "memory");

        return tmp+delta;
}

#endif /* __KERNEL__ */
#endif /* _I386_RWSEM_H */