LCOM1920-T02G10

/*        $NetBSD: lock.h,v 1.27 2013/01/22 22:09:44 christos Exp $        */

/*-

 * Copyright (c) 2000, 2006 The NetBSD Foundation, Inc.

 * All rights reserved.

 *

 * This code is derived from software contributed to The NetBSD Foundation

 * by Jason R. Thorpe and Andrew Doran.

 *

 * Redistribution and use in source and binary forms, with or without

 * modification, are permitted provided that the following conditions

 * are met:

 * 1. Redistributions of source code must retain the above copyright

 *    notice, this list of conditions and the following disclaimer.

 * 2. Redistributions in binary form must reproduce the above copyright

 *    notice, this list of conditions and the following disclaimer in the

 *    documentation and/or other materials provided with the distribution.

 *

 * THIS SOFTWARE IS PROVIDED BY THE NETBSD FOUNDATION, INC. AND CONTRIBUTORS

 * ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED

 * TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR

 * PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL THE FOUNDATION OR CONTRIBUTORS

 * BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR

 * CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF

 * SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS

 * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN

 * CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)

 * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE

 * POSSIBILITY OF SUCH DAMAGE.

 */

/*

 * Machine-dependent spin lock operations.

 */

#ifndef _X86_LOCK_H_

#define        _X86_LOCK_H_

#include <sys/param.h>

static __inline int

__SIMPLELOCK_LOCKED_P(__cpu_simple_lock_t *__ptr)

{

        return *__ptr == __SIMPLELOCK_LOCKED;

}

static __inline int

__SIMPLELOCK_UNLOCKED_P(__cpu_simple_lock_t *__ptr)

{

        return *__ptr == __SIMPLELOCK_UNLOCKED;

}

static __inline void

__cpu_simple_lock_set(__cpu_simple_lock_t *__ptr)

{

        *__ptr = __SIMPLELOCK_LOCKED;

}

static __inline void

__cpu_simple_lock_clear(__cpu_simple_lock_t *__ptr)

{

        *__ptr = __SIMPLELOCK_UNLOCKED;

}

#ifdef _HARDKERNEL

# include <machine/cpufunc.h>

# define SPINLOCK_SPIN_HOOK        /* nothing */

# ifdef SPINLOCK_BACKOFF_HOOK

#  undef SPINLOCK_BACKOFF_HOOK

# endif

# define SPINLOCK_BACKOFF_HOOK        x86_pause()

# define SPINLOCK_INLINE

#else /* !_HARDKERNEL */

# define SPINLOCK_BODY

# define SPINLOCK_INLINE static __inline __unused

#endif /* _HARDKERNEL */

SPINLOCK_INLINE void        __cpu_simple_lock_init(__cpu_simple_lock_t *);

SPINLOCK_INLINE void        __cpu_simple_lock(__cpu_simple_lock_t *);

SPINLOCK_INLINE int        __cpu_simple_lock_try(__cpu_simple_lock_t *);

SPINLOCK_INLINE void        __cpu_simple_unlock(__cpu_simple_lock_t *);

#ifdef SPINLOCK_BODY

SPINLOCK_INLINE void

__cpu_simple_lock_init(__cpu_simple_lock_t *lockp)

{

        *lockp = __SIMPLELOCK_UNLOCKED;

        __insn_barrier();

}

SPINLOCK_INLINE int

__cpu_simple_lock_try(__cpu_simple_lock_t *lockp)

{

        uint8_t val;

        val = __SIMPLELOCK_LOCKED;

        __asm volatile ("xchgb %0,(%2)" : 

            "=qQ" (val)

            :"0" (val), "r" (lockp));

        __insn_barrier();

        return val == __SIMPLELOCK_UNLOCKED;

}

SPINLOCK_INLINE void

__cpu_simple_lock(__cpu_simple_lock_t *lockp)

{

        while (!__cpu_simple_lock_try(lockp))

                /* nothing */;

        __insn_barrier();

}

/*

 * Note on x86 memory ordering

 *

 * When releasing a lock we must ensure that no stores or loads from within

 * the critical section are re-ordered by the CPU to occur outside of it:

 * they must have completed and be visible to other processors once the lock

 * has been released.

 *

 * NetBSD usually runs with the kernel mapped (via MTRR) in a WB (write

 * back) memory region.  In that case, memory ordering on x86 platforms

 * looks like this:

 *

 * i386                All loads/stores occur in instruction sequence.

 *

 * i486                All loads/stores occur in instruction sequence.  In

 * Pentium        exceptional circumstances, loads can be re-ordered around

 *                stores, but for the purposes of releasing a lock it does

 *                not matter.  Stores may not be immediately visible to other

 *                processors as they can be buffered.  However, since the

 *                stores are buffered in order the lock release will always be

 *                the last operation in the critical section that becomes

 *                visible to other CPUs.

 *

 * Pentium Pro        The "Intel 64 and IA-32 Architectures Software Developer's

 * onwards        Manual" volume 3A (order number 248966) says that (1) "Reads

 *                can be carried out speculatively and in any order" and (2)

 *                "Reads can pass buffered stores, but the processor is

 *                self-consistent.".  This would be a problem for the below,

 *                and would mandate a locked instruction cycle or load fence

 *                before releasing the simple lock.

 *

 *                The "Intel Pentium 4 Processor Optimization" guide (order

 *                number 253668-022US) says: "Loads can be moved before stores

 *                that occurred earlier in the program if they are not

 *                predicted to load from the same linear address.".  This is

 *                not a problem since the only loads that can be re-ordered

 *                take place once the lock has been released via a store.

 *

 *                The above two documents seem to contradict each other,

 *                however with the exception of early steppings of the Pentium

 *                Pro, the second document is closer to the truth: a store

 *                will always act as a load fence for all loads that precede

 *                the store in instruction order.

 *

 *                Again, note that stores can be buffered and will not always

 *                become immediately visible to other CPUs: they are however

 *                buffered in order.

 *

 * AMD64        Stores occur in order and are buffered.  Loads can be

 *                reordered, however stores act as load fences, meaning that

 *                loads can not be reordered around stores.

 */

SPINLOCK_INLINE void

__cpu_simple_unlock(__cpu_simple_lock_t *lockp)

{

        __insn_barrier();

        *lockp = __SIMPLELOCK_UNLOCKED;

}

#endif        /* SPINLOCK_BODY */

#endif /* _X86_LOCK_H_ */