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lock.h revision 1.20 
      1 /*	$NetBSD: lock.h,v 1.20 2007/12/20 23:46:11 ad Exp $	*/
      2 
      3 /*-
      4  * Copyright (c) 2000, 2006 The NetBSD Foundation, Inc.
      5  * All rights reserved.
      6  *
      7  * This code is derived from software contributed to The NetBSD Foundation
      8  * by Jason R. Thorpe and Andrew Doran.
      9  *
     10  * Redistribution and use in source and binary forms, with or without
     11  * modification, are permitted provided that the following conditions
     12  * are met:
     13  * 1. Redistributions of source code must retain the above copyright
     14  *    notice, this list of conditions and the following disclaimer.
     15  * 2. Redistributions in binary form must reproduce the above copyright
     16  *    notice, this list of conditions and the following disclaimer in the
     17  *    documentation and/or other materials provided with the distribution.
     18  * 3. All advertising materials mentioning features or use of this software
     19  *    must display the following acknowledgement:
     20  *	This product includes software developed by the NetBSD
     21  *	Foundation, Inc. and its contributors.
     22  * 4. Neither the name of The NetBSD Foundation nor the names of its
     23  *    contributors may be used to endorse or promote products derived
     24  *    from this software without specific prior written permission.
     25  *
     26  * THIS SOFTWARE IS PROVIDED BY THE NETBSD FOUNDATION, INC. AND CONTRIBUTORS
     27  * ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED
     28  * TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
     29  * PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL THE FOUNDATION OR CONTRIBUTORS
     30  * BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
     31  * CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
     32  * SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
     33  * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
     34  * CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
     35  * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
     36  * POSSIBILITY OF SUCH DAMAGE.
     37  */
     38 
     39 /*
     40  * Machine-dependent spin lock operations.
     41  */
     42 
     43 #ifndef _X86_LOCK_H_
     44 #define	_X86_LOCK_H_
     45 
     46 static __inline int
     47 __SIMPLELOCK_LOCKED_P(__cpu_simple_lock_t *__ptr)
     48 {
     49 	return *__ptr == __SIMPLELOCK_LOCKED;
     50 }
     51 
     52 static __inline int
     53 __SIMPLELOCK_UNLOCKED_P(__cpu_simple_lock_t *__ptr)
     54 {
     55 	return *__ptr == __SIMPLELOCK_UNLOCKED;
     56 }
     57 
     58 static __inline void
     59 __cpu_simple_lock_set(__cpu_simple_lock_t *__ptr)
     60 {
     61 
     62 	*__ptr = __SIMPLELOCK_LOCKED;
     63 }
     64 
     65 static __inline void
     66 __cpu_simple_lock_clear(__cpu_simple_lock_t *__ptr)
     67 {
     68 
     69 	*__ptr = __SIMPLELOCK_UNLOCKED;
     70 }
     71 
     72 #ifdef _KERNEL
     73 
     74 #include <machine/cpufunc.h>
     75 
     76 void	__cpu_simple_lock_init(__cpu_simple_lock_t *);
     77 void	__cpu_simple_lock(__cpu_simple_lock_t *);
     78 int	__cpu_simple_lock_try(__cpu_simple_lock_t *);
     79 void	__cpu_simple_unlock(__cpu_simple_lock_t *);
     80 
     81 #define	SPINLOCK_SPIN_HOOK	/* nothing */
     82 #define	SPINLOCK_BACKOFF_HOOK	x86_pause()
     83 
     84 #else
     85 
     86 static __inline void __cpu_simple_lock_init(__cpu_simple_lock_t *)
     87 	__attribute__((__unused__));
     88 static __inline void __cpu_simple_lock(__cpu_simple_lock_t *)
     89 	__attribute__((__unused__));
     90 static __inline int __cpu_simple_lock_try(__cpu_simple_lock_t *)
     91 	__attribute__((__unused__));
     92 static __inline void __cpu_simple_unlock(__cpu_simple_lock_t *)
     93 	__attribute__((__unused__));
     94 
     95 static __inline void
     96 __cpu_simple_lock_init(__cpu_simple_lock_t *lockp)
     97 {
     98 
     99 	*lockp = __SIMPLELOCK_UNLOCKED;
    100 	__insn_barrier();
    101 }
    102 
    103 static __inline int
    104 __cpu_simple_lock_try(__cpu_simple_lock_t *lockp)
    105 {
    106 	uint8_t val;
    107 
    108 	val = __SIMPLELOCK_LOCKED;
    109 	__asm volatile ("xchgb %0,(%2)" :
    110 	    "=r" (val)
    111 	    :"0" (val), "r" (lockp));
    112 	__insn_barrier();
    113 	return val == __SIMPLELOCK_UNLOCKED;
    114 }
    115 
    116 static __inline void
    117 __cpu_simple_lock(__cpu_simple_lock_t *lockp)
    118 {
    119 
    120 	while (!__cpu_simple_lock_try(lockp))
    121 		/* nothing */;
    122 	__insn_barrier();
    123 }
    124 
    125 /*
    126  * Note on x86 memory ordering
    127  *
    128  * When releasing a lock we must ensure that no stores or loads from within
    129  * the critical section are re-ordered by the CPU to occur outside of it:
    130  * they must have completed and be visible to other processors once the lock
    131  * has been released.
    132  *
    133  * NetBSD usually runs with the kernel mapped (via MTRR) in a WB (write
    134  * back) memory region.  In that case, memory ordering on x86 platforms
    135  * looks like this:
    136  *
    137  * i386		All loads/stores occur in instruction sequence.
    138  *
    139  * i486		All loads/stores occur in instruction sequence.  In
    140  * Pentium	exceptional circumstances, loads can be re-ordered around
    141  *		stores, but for the purposes of releasing a lock it does
    142  *		not matter.  Stores may not be immediately visible to other
    143  *		processors as they can be buffered.  However, since the
    144  *		stores are buffered in order the lock release will always be
    145  *		the last operation in the critical section that becomes
    146  *		visible to other CPUs.
    147  *
    148  * Pentium Pro	The "Intel 64 and IA-32 Architectures Software Developer's
    149  * onwards	Manual" volume 3A (order number 248966) says that (1) "Reads
    150  *		can be carried out speculatively and in any order" and (2)
    151  *		"Reads can pass buffered stores, but the processor is
    152  *		self-consistent.".  This would be a problem for the below,
    153  *		and would mandate a locked instruction cycle or load fence
    154  *		before releasing the simple lock.
    155  *
    156  *		The "Intel Pentium 4 Processor Optimization" guide (order
    157  *		number 253668-022US) says: "Loads can be moved before stores
    158  *		that occurred earlier in the program if they are not
    159  *		predicted to load from the same linear address.".  This is
    160  *		not a problem since the only loads that can be re-ordered
    161  *		take place once the lock has been released via a store.
    162  *
    163  *		The above two documents seem to contradict each other,
    164  *		however with the exception of early steppings of the Pentium
    165  *		Pro, the second document is closer to the truth: a store
    166  *		will always act as a load fence for all loads that precede
    167  *		the store in instruction order.
    168  *
    169  *		Again, note that stores can be buffered and will not always
    170  *		become immediately visible to other CPUs: they are however
    171  *		buffered in order.
    172  *
    173  * AMD64	Stores occur in order and are buffered.  Loads can be
    174  *		reordered, however stores act as load fences, meaning that
    175  *		loads can not be reordered around stores.
    176  */
    177 static __inline void
    178 __cpu_simple_unlock(__cpu_simple_lock_t *lockp)
    179 {
    180 
    181 	__insn_barrier();
    182 	*lockp = __SIMPLELOCK_UNLOCKED;
    183 }
    184 
    185 #endif	/* _KERNEL */
    186 
    187 #endif /* _X86_LOCK_H_ */
    188