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Linus Torvalds1da177e2005-04-16 15:20:36 -07001/* rwsem.h: R/W semaphores implemented using XADD/CMPXCHG for i486+
2 *
3 * Written by David Howells (dhowells@redhat.com).
4 *
Dave Jones99122a32008-01-30 13:30:28 +01005 * Derived from asm-x86/semaphore.h
Linus Torvalds1da177e2005-04-16 15:20:36 -07006 *
7 *
8 * The MSW of the count is the negated number of active writers and waiting
9 * lockers, and the LSW is the total number of active locks
10 *
11 * The lock count is initialized to 0 (no active and no waiting lockers).
12 *
13 * When a writer subtracts WRITE_BIAS, it'll get 0xffff0001 for the case of an
14 * uncontended lock. This can be determined because XADD returns the old value.
15 * Readers increment by 1 and see a positive value when uncontended, negative
16 * if there are writers (and maybe) readers waiting (in which case it goes to
17 * sleep).
18 *
19 * The value of WAITING_BIAS supports up to 32766 waiting processes. This can
20 * be extended to 65534 by manually checking the whole MSW rather than relying
21 * on the S flag.
22 *
23 * The value of ACTIVE_BIAS supports up to 65535 active processes.
24 *
25 * This should be totally fair - if anything is waiting, a process that wants a
26 * lock will go to the back of the queue. When the currently active lock is
27 * released, if there's a writer at the front of the queue, then that and only
28 * that will be woken up; if there's a bunch of consequtive readers at the
29 * front, then they'll all be woken up, but no other readers will be.
30 */
31
H. Peter Anvin1965aae2008-10-22 22:26:29 -070032#ifndef _ASM_X86_RWSEM_H
33#define _ASM_X86_RWSEM_H
Linus Torvalds1da177e2005-04-16 15:20:36 -070034
35#ifndef _LINUX_RWSEM_H
36#error "please don't include asm/rwsem.h directly, use linux/rwsem.h instead"
37#endif
38
39#ifdef __KERNEL__
H. Peter Anvin1838ef12010-01-18 14:00:34 -080040#include <asm/asm.h>
Linus Torvalds1da177e2005-04-16 15:20:36 -070041
Linus Torvalds1da177e2005-04-16 15:20:36 -070042/*
H. Peter Anvin1838ef12010-01-18 14:00:34 -080043 * The bias values and the counter type limits the number of
44 * potential readers/writers to 32767 for 32 bits and 2147483647
45 * for 64 bits.
Linus Torvalds1da177e2005-04-16 15:20:36 -070046 */
Joe Perches6e5609a2008-03-23 01:03:21 -070047
H. Peter Anvin1838ef12010-01-18 14:00:34 -080048#ifdef CONFIG_X86_64
49# define RWSEM_ACTIVE_MASK 0xffffffffL
50#else
51# define RWSEM_ACTIVE_MASK 0x0000ffffL
52#endif
53
54#define RWSEM_UNLOCKED_VALUE 0x00000000L
55#define RWSEM_ACTIVE_BIAS 0x00000001L
56#define RWSEM_WAITING_BIAS (-RWSEM_ACTIVE_MASK-1)
Linus Torvalds1da177e2005-04-16 15:20:36 -070057#define RWSEM_ACTIVE_READ_BIAS RWSEM_ACTIVE_BIAS
58#define RWSEM_ACTIVE_WRITE_BIAS (RWSEM_WAITING_BIAS + RWSEM_ACTIVE_BIAS)
Joe Perches6e5609a2008-03-23 01:03:21 -070059
Linus Torvalds1da177e2005-04-16 15:20:36 -070060/*
61 * lock for reading
62 */
63static inline void __down_read(struct rw_semaphore *sem)
64{
Joe Perches6e5609a2008-03-23 01:03:21 -070065 asm volatile("# beginning down_read\n\t"
H. Peter Anvin1838ef12010-01-18 14:00:34 -080066 LOCK_PREFIX _ASM_INC "(%1)\n\t"
Michel Lespinasseb4bcb4c2010-07-20 15:19:45 -070067 /* adds 0x00000001 */
Joe Perches6e5609a2008-03-23 01:03:21 -070068 " jns 1f\n"
69 " call call_rwsem_down_read_failed\n"
70 "1:\n\t"
71 "# ending down_read\n\t"
72 : "+m" (sem->count)
73 : "a" (sem)
74 : "memory", "cc");
Linus Torvalds1da177e2005-04-16 15:20:36 -070075}
76
77/*
78 * trylock for reading -- returns 1 if successful, 0 if contention
79 */
80static inline int __down_read_trylock(struct rw_semaphore *sem)
81{
Thomas Gleixnerbde11ef2011-01-26 20:05:53 +000082 long result, tmp;
Joe Perches6e5609a2008-03-23 01:03:21 -070083 asm volatile("# beginning __down_read_trylock\n\t"
Linus Torvalds59c33fa2010-01-12 16:21:09 -080084 " mov %0,%1\n\t"
Joe Perches6e5609a2008-03-23 01:03:21 -070085 "1:\n\t"
Linus Torvalds59c33fa2010-01-12 16:21:09 -080086 " mov %1,%2\n\t"
87 " add %3,%2\n\t"
Joe Perches6e5609a2008-03-23 01:03:21 -070088 " jle 2f\n\t"
Linus Torvalds59c33fa2010-01-12 16:21:09 -080089 LOCK_PREFIX " cmpxchg %2,%0\n\t"
Joe Perches6e5609a2008-03-23 01:03:21 -070090 " jnz 1b\n\t"
91 "2:\n\t"
92 "# ending __down_read_trylock\n\t"
93 : "+m" (sem->count), "=&a" (result), "=&r" (tmp)
94 : "i" (RWSEM_ACTIVE_READ_BIAS)
95 : "memory", "cc");
96 return result >= 0 ? 1 : 0;
Linus Torvalds1da177e2005-04-16 15:20:36 -070097}
98
99/*
100 * lock for writing
101 */
Ingo Molnar4ea21762006-07-03 00:24:53 -0700102static inline void __down_write_nested(struct rw_semaphore *sem, int subclass)
Linus Torvalds1da177e2005-04-16 15:20:36 -0700103{
Thomas Gleixnerbde11ef2011-01-26 20:05:53 +0000104 long tmp;
Joe Perches6e5609a2008-03-23 01:03:21 -0700105 asm volatile("# beginning down_write\n\t"
Linus Torvalds59c33fa2010-01-12 16:21:09 -0800106 LOCK_PREFIX " xadd %1,(%2)\n\t"
Michel Lespinasseb4bcb4c2010-07-20 15:19:45 -0700107 /* adds 0xffff0001, returns the old value */
Linus Torvalds59c33fa2010-01-12 16:21:09 -0800108 " test %1,%1\n\t"
Joe Perches6e5609a2008-03-23 01:03:21 -0700109 /* was the count 0 before? */
110 " jz 1f\n"
111 " call call_rwsem_down_write_failed\n"
112 "1:\n"
113 "# ending down_write"
114 : "+m" (sem->count), "=d" (tmp)
Michel Lespinasseb4bcb4c2010-07-20 15:19:45 -0700115 : "a" (sem), "1" (RWSEM_ACTIVE_WRITE_BIAS)
Joe Perches6e5609a2008-03-23 01:03:21 -0700116 : "memory", "cc");
Linus Torvalds1da177e2005-04-16 15:20:36 -0700117}
118
Ingo Molnar4ea21762006-07-03 00:24:53 -0700119static inline void __down_write(struct rw_semaphore *sem)
120{
121 __down_write_nested(sem, 0);
122}
123
Linus Torvalds1da177e2005-04-16 15:20:36 -0700124/*
125 * trylock for writing -- returns 1 if successful, 0 if contention
126 */
127static inline int __down_write_trylock(struct rw_semaphore *sem)
128{
Thomas Gleixnerbde11ef2011-01-26 20:05:53 +0000129 long ret = cmpxchg(&sem->count, RWSEM_UNLOCKED_VALUE,
130 RWSEM_ACTIVE_WRITE_BIAS);
Linus Torvalds1da177e2005-04-16 15:20:36 -0700131 if (ret == RWSEM_UNLOCKED_VALUE)
132 return 1;
133 return 0;
134}
135
136/*
137 * unlock after reading
138 */
139static inline void __up_read(struct rw_semaphore *sem)
140{
Thomas Gleixnerbde11ef2011-01-26 20:05:53 +0000141 long tmp;
Joe Perches6e5609a2008-03-23 01:03:21 -0700142 asm volatile("# beginning __up_read\n\t"
Linus Torvalds59c33fa2010-01-12 16:21:09 -0800143 LOCK_PREFIX " xadd %1,(%2)\n\t"
Joe Perches6e5609a2008-03-23 01:03:21 -0700144 /* subtracts 1, returns the old value */
145 " jns 1f\n\t"
Michel Lespinasseb4bcb4c2010-07-20 15:19:45 -0700146 " call call_rwsem_wake\n" /* expects old value in %edx */
Joe Perches6e5609a2008-03-23 01:03:21 -0700147 "1:\n"
148 "# ending __up_read\n"
149 : "+m" (sem->count), "=d" (tmp)
Michel Lespinasseb4bcb4c2010-07-20 15:19:45 -0700150 : "a" (sem), "1" (-RWSEM_ACTIVE_READ_BIAS)
Joe Perches6e5609a2008-03-23 01:03:21 -0700151 : "memory", "cc");
Linus Torvalds1da177e2005-04-16 15:20:36 -0700152}
153
154/*
155 * unlock after writing
156 */
157static inline void __up_write(struct rw_semaphore *sem)
158{
Thomas Gleixnerbde11ef2011-01-26 20:05:53 +0000159 long tmp;
Joe Perches6e5609a2008-03-23 01:03:21 -0700160 asm volatile("# beginning __up_write\n\t"
Linus Torvalds59c33fa2010-01-12 16:21:09 -0800161 LOCK_PREFIX " xadd %1,(%2)\n\t"
Michel Lespinassea751bd82010-07-20 15:19:45 -0700162 /* subtracts 0xffff0001, returns the old value */
163 " jns 1f\n\t"
Michel Lespinasseb4bcb4c2010-07-20 15:19:45 -0700164 " call call_rwsem_wake\n" /* expects old value in %edx */
Joe Perches6e5609a2008-03-23 01:03:21 -0700165 "1:\n\t"
166 "# ending __up_write\n"
Linus Torvalds59c33fa2010-01-12 16:21:09 -0800167 : "+m" (sem->count), "=d" (tmp)
168 : "a" (sem), "1" (-RWSEM_ACTIVE_WRITE_BIAS)
169 : "memory", "cc");
Linus Torvalds1da177e2005-04-16 15:20:36 -0700170}
171
172/*
173 * downgrade write lock to read lock
174 */
175static inline void __downgrade_write(struct rw_semaphore *sem)
176{
H. Peter Anvin1838ef12010-01-18 14:00:34 -0800177 asm volatile("# beginning __downgrade_write\n\t"
178 LOCK_PREFIX _ASM_ADD "%2,(%1)\n\t"
Avi Kivity0d1622d2010-02-13 10:33:12 +0200179 /*
180 * transitions 0xZZZZ0001 -> 0xYYYY0001 (i386)
181 * 0xZZZZZZZZ00000001 -> 0xYYYYYYYY00000001 (x86_64)
182 */
Joe Perches6e5609a2008-03-23 01:03:21 -0700183 " jns 1f\n\t"
184 " call call_rwsem_downgrade_wake\n"
185 "1:\n\t"
186 "# ending __downgrade_write\n"
187 : "+m" (sem->count)
Avi Kivity0d1622d2010-02-13 10:33:12 +0200188 : "a" (sem), "er" (-RWSEM_WAITING_BIAS)
Joe Perches6e5609a2008-03-23 01:03:21 -0700189 : "memory", "cc");
Linus Torvalds1da177e2005-04-16 15:20:36 -0700190}
191
192/*
193 * implement atomic add functionality
194 */
Thomas Gleixnerbde11ef2011-01-26 20:05:53 +0000195static inline void rwsem_atomic_add(long delta, struct rw_semaphore *sem)
Linus Torvalds1da177e2005-04-16 15:20:36 -0700196{
H. Peter Anvin1838ef12010-01-18 14:00:34 -0800197 asm volatile(LOCK_PREFIX _ASM_ADD "%1,%0"
Joe Perches6e5609a2008-03-23 01:03:21 -0700198 : "+m" (sem->count)
H. Peter Anvin1838ef12010-01-18 14:00:34 -0800199 : "er" (delta));
Linus Torvalds1da177e2005-04-16 15:20:36 -0700200}
201
202/*
203 * implement exchange and add functionality
204 */
Thomas Gleixnerbde11ef2011-01-26 20:05:53 +0000205static inline long rwsem_atomic_update(long delta, struct rw_semaphore *sem)
Linus Torvalds1da177e2005-04-16 15:20:36 -0700206{
Thomas Gleixnerbde11ef2011-01-26 20:05:53 +0000207 long tmp = delta;
Linus Torvalds1da177e2005-04-16 15:20:36 -0700208
Joe Perches6e5609a2008-03-23 01:03:21 -0700209 asm volatile(LOCK_PREFIX "xadd %0,%1"
210 : "+r" (tmp), "+m" (sem->count)
211 : : "memory");
Linus Torvalds1da177e2005-04-16 15:20:36 -0700212
Joe Perches6e5609a2008-03-23 01:03:21 -0700213 return tmp + delta;
Linus Torvalds1da177e2005-04-16 15:20:36 -0700214}
215
Linus Torvalds1da177e2005-04-16 15:20:36 -0700216#endif /* __KERNEL__ */
H. Peter Anvin1965aae2008-10-22 22:26:29 -0700217#endif /* _ASM_X86_RWSEM_H */