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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

 *

 * Trylock by Brian Watson (Brian.J.Watson@compaq.com).

 *

 *

 * 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 dont 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 *));

/*

 * 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)

                : "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      %0,(%%eax)\n\t" /* subtract 0x0000ffff, returns the old value */

                "  testl     %0,%0\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"

                : "+d"(tmp), "+m"(sem->count)

                : "a"(sem)

                : "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)

                : "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)

                : "memory", "cc", "edx");

}

/*

 * 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 */