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1 | jermar | 1 | /* |
2 | * Reader/Writer locks |
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3 | */ |
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4 | |||
5 | /* |
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6 | * Copyright (C) 2001-2004 Jakub Jermar |
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7 | * All rights reserved. |
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8 | * |
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9 | * Redistribution and use in source and binary forms, with or without |
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10 | * modification, are permitted provided that the following conditions |
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11 | * are met: |
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12 | * |
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13 | * - Redistributions of source code must retain the above copyright |
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14 | * notice, this list of conditions and the following disclaimer. |
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15 | * - Redistributions in binary form must reproduce the above copyright |
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16 | * notice, this list of conditions and the following disclaimer in the |
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17 | * documentation and/or other materials provided with the distribution. |
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18 | * - The name of the author may not be used to endorse or promote products |
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19 | * derived from this software without specific prior written permission. |
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20 | * |
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21 | * THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR |
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22 | * IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES |
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23 | * OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. |
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24 | * IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT, |
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25 | * INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT |
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26 | * NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, |
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27 | * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY |
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28 | * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT |
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29 | * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF |
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30 | * THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE. |
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31 | */ |
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32 | |||
33 | /* |
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34 | * These locks are not recursive. |
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35 | * Neither readers nor writers will suffer starvation. |
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36 | * |
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37 | * If there is a writer followed by a reader waiting for the rwlock |
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38 | * and the writer times out, all leading readers are automatically woken up |
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39 | * and allowed in. |
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40 | */ |
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41 | |||
42 | /* |
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43 | * NOTE ON rwlock_holder_type |
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44 | * This field is set on an attempt to acquire the exclusive mutex |
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45 | * to the respective value depending whether the caller is a reader |
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46 | * or a writer. The field is examined only if the thread had been |
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47 | * previously blocked on the exclusive mutex. Thus it is save |
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48 | * to store the rwlock type in the thread structure, because |
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49 | * each thread can block on only one rwlock at a time. |
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50 | */ |
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51 | |||
52 | #include <synch/synch.h> |
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53 | #include <synch/rwlock.h> |
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54 | #include <synch/spinlock.h> |
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55 | #include <synch/mutex.h> |
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56 | #include <synch/waitq.h> |
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57 | |||
58 | #include <list.h> |
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59 | #include <typedefs.h> |
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60 | #include <arch/asm.h> |
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61 | #include <arch.h> |
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62 | #include <proc/thread.h> |
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63 | #include <panic.h> |
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64 | |||
65 | #define ALLOW_ALL 0 |
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66 | #define ALLOW_READERS_ONLY 1 |
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67 | |||
68 | static void let_others_in(rwlock_t *rwl, int readers_only); |
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69 | static void release_spinlock(void *arg); |
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70 | |||
71 | void rwlock_initialize(rwlock_t *rwl) { |
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72 | spinlock_initialize(&rwl->lock); |
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73 | mutex_initialize(&rwl->exclusive); |
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74 | rwl->readers_in = 0; |
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75 | } |
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76 | |||
77 | int _rwlock_write_lock_timeout(rwlock_t *rwl, __u32 usec, int trylock) |
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78 | { |
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79 | pri_t pri; |
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80 | int rc; |
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81 | |||
82 | pri = cpu_priority_high(); |
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15 | jermar | 83 | spinlock_lock(&THREAD->lock); |
84 | THREAD->rwlock_holder_type = RWLOCK_WRITER; |
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85 | spinlock_unlock(&THREAD->lock); |
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1 | jermar | 86 | cpu_priority_restore(pri); |
87 | |||
88 | /* |
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89 | * Writers take the easy part. |
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90 | * They just need to acquire the exclusive mutex. |
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91 | */ |
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92 | rc = _mutex_lock_timeout(&rwl->exclusive, usec, trylock); |
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93 | if (SYNCH_FAILED(rc)) { |
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94 | |||
95 | /* |
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96 | * Lock operation timed out. |
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97 | * The state of rwl is UNKNOWN at this point. |
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98 | * No claims about its holder can be made. |
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99 | */ |
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100 | |||
101 | pri = cpu_priority_high(); |
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102 | spinlock_lock(&rwl->lock); |
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103 | /* |
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104 | * Now when rwl is locked, we can inspect it again. |
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105 | * If it is held by some readers already, we can let |
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106 | * readers from the head of the wait queue in. |
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107 | */ |
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108 | if (rwl->readers_in) |
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109 | let_others_in(rwl, ALLOW_READERS_ONLY); |
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110 | spinlock_unlock(&rwl->lock); |
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111 | cpu_priority_restore(pri); |
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112 | } |
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113 | |||
114 | return rc; |
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115 | } |
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116 | |||
117 | int _rwlock_read_lock_timeout(rwlock_t *rwl, __u32 usec, int trylock) |
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118 | { |
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119 | int rc; |
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120 | pri_t pri; |
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121 | |||
122 | pri = cpu_priority_high(); |
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15 | jermar | 123 | spinlock_lock(&THREAD->lock); |
124 | THREAD->rwlock_holder_type = RWLOCK_READER; |
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125 | spinlock_unlock(&THREAD->lock); |
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1 | jermar | 126 | |
127 | spinlock_lock(&rwl->lock); |
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128 | |||
129 | /* |
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130 | * Find out whether we can get what we want without blocking. |
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131 | */ |
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132 | rc = mutex_trylock(&rwl->exclusive); |
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133 | if (SYNCH_FAILED(rc)) { |
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134 | |||
135 | /* |
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136 | * 'exclusive' mutex is being held by someone else. |
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137 | * If the holder is a reader and there is no one |
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138 | * else waiting for it, we can enter the critical |
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139 | * section. |
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140 | */ |
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141 | |||
142 | if (rwl->readers_in) { |
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143 | spinlock_lock(&rwl->exclusive.sem.wq.lock); |
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144 | if (list_empty(&rwl->exclusive.sem.wq.head)) { |
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145 | /* |
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146 | * We can enter. |
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147 | */ |
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148 | spinlock_unlock(&rwl->exclusive.sem.wq.lock); |
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149 | goto shortcut; |
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150 | } |
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151 | spinlock_unlock(&rwl->exclusive.sem.wq.lock); |
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152 | } |
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153 | |||
154 | /* |
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155 | * In order to prevent a race condition when a reader |
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156 | * could block another reader at the head of the waitq, |
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157 | * we register a function to unlock rwl->lock |
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158 | * after this thread is put asleep. |
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159 | */ |
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160 | thread_register_call_me(release_spinlock, &rwl->lock); |
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161 | |||
162 | rc = _mutex_lock_timeout(&rwl->exclusive, usec, trylock); |
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163 | switch (rc) { |
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164 | case ESYNCH_WOULD_BLOCK: |
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165 | /* |
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166 | * release_spinlock() wasn't called |
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167 | */ |
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168 | thread_register_call_me(NULL, NULL); |
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169 | spinlock_unlock(&rwl->lock); |
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170 | case ESYNCH_TIMEOUT: |
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171 | /* |
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172 | * The sleep timeouted. |
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173 | * We just restore the cpu priority. |
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174 | */ |
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175 | case ESYNCH_OK_BLOCKED: |
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176 | /* |
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177 | * We were woken with rwl->readers_in already incremented. |
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178 | * Note that this arrangement avoids race condition between |
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179 | * two concurrent readers. (Race is avoided if 'exclusive' is |
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180 | * locked at the same time as 'readers_in' is incremented. |
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181 | * Same time means both events happen atomically when |
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182 | * rwl->lock is held.) |
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183 | */ |
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184 | cpu_priority_restore(pri); |
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185 | break; |
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186 | case ESYNCH_OK_ATOMIC: |
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187 | panic(PANIC "_mutex_lock_timeout()==ESYNCH_OK_ATOMIC"); |
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188 | break; |
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189 | dafault: |
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190 | panic(PANIC "invalid ESYNCH"); |
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191 | break; |
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192 | } |
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193 | return rc; |
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194 | } |
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195 | |||
196 | shortcut: |
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197 | |||
198 | /* |
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199 | * We can increment readers_in only if we didn't go to sleep. |
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200 | * For sleepers, rwlock_let_others_in() will do the job. |
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201 | */ |
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202 | rwl->readers_in++; |
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203 | |||
204 | spinlock_unlock(&rwl->lock); |
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205 | cpu_priority_restore(pri); |
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206 | |||
207 | return ESYNCH_OK_ATOMIC; |
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208 | } |
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209 | |||
210 | void rwlock_write_unlock(rwlock_t *rwl) |
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211 | { |
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212 | pri_t pri; |
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213 | |||
214 | pri = cpu_priority_high(); |
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215 | spinlock_lock(&rwl->lock); |
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216 | let_others_in(rwl, ALLOW_ALL); |
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217 | spinlock_unlock(&rwl->lock); |
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218 | cpu_priority_restore(pri); |
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219 | |||
220 | } |
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221 | |||
222 | void rwlock_read_unlock(rwlock_t *rwl) |
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223 | { |
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224 | pri_t pri; |
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225 | |||
226 | pri = cpu_priority_high(); |
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227 | spinlock_lock(&rwl->lock); |
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228 | if (!--rwl->readers_in) |
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229 | let_others_in(rwl, ALLOW_ALL); |
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230 | spinlock_unlock(&rwl->lock); |
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231 | cpu_priority_restore(pri); |
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232 | } |
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233 | |||
234 | |||
235 | /* |
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236 | * Must be called with rwl->lock locked. |
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237 | * Must be called with cpu_priority_high'ed. |
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238 | */ |
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239 | /* |
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240 | * If readers_only is false: (unlock scenario) |
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241 | * Let the first sleeper on 'exclusive' mutex in, no matter |
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242 | * whether it is a reader or a writer. If there are more leading |
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243 | * readers in line, let each of them in. |
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244 | * |
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245 | * Otherwise: (timeout scenario) |
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246 | * Let all leading readers in. |
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247 | */ |
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248 | void let_others_in(rwlock_t *rwl, int readers_only) |
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249 | { |
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250 | rwlock_type_t type = RWLOCK_NONE; |
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251 | thread_t *t = NULL; |
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252 | int one_more = 1; |
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253 | |||
254 | spinlock_lock(&rwl->exclusive.sem.wq.lock); |
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255 | |||
256 | if (!list_empty(&rwl->exclusive.sem.wq.head)) |
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257 | t = list_get_instance(rwl->exclusive.sem.wq.head.next, thread_t, wq_link); |
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258 | do { |
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259 | if (t) { |
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260 | spinlock_lock(&t->lock); |
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261 | type = t->rwlock_holder_type; |
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262 | spinlock_unlock(&t->lock); |
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263 | } |
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264 | |||
265 | /* |
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266 | * If readers_only is true, we wake all leading readers |
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267 | * if and only if rwl is locked by another reader. |
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268 | * Assumption: readers_only ==> rwl->readers_in |
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269 | */ |
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270 | if (readers_only && (type != RWLOCK_READER)) |
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271 | break; |
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272 | |||
273 | |||
274 | if (type == RWLOCK_READER) { |
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275 | /* |
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276 | * Waking up a reader. |
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277 | * We are responsible for incrementing rwl->readers_in for it. |
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278 | */ |
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279 | rwl->readers_in++; |
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280 | } |
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281 | |||
282 | /* |
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283 | * Only the last iteration through this loop can increment |
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284 | * rwl->exclusive.sem.wq.missed_wakeup's. All preceeding |
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285 | * iterations will wake up a thread. |
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286 | */ |
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287 | /* We call the internal version of waitq_wakeup, which |
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288 | * relies on the fact that the waitq is already locked. |
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289 | */ |
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290 | _waitq_wakeup_unsafe(&rwl->exclusive.sem.wq, WAKEUP_FIRST); |
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291 | |||
292 | t = NULL; |
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293 | if (!list_empty(&rwl->exclusive.sem.wq.head)) { |
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294 | t = list_get_instance(rwl->exclusive.sem.wq.head.next, thread_t, wq_link); |
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295 | if (t) { |
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296 | spinlock_lock(&t->lock); |
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297 | if (t->rwlock_holder_type != RWLOCK_READER) |
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298 | one_more = 0; |
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299 | spinlock_unlock(&t->lock); |
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300 | } |
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301 | } |
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302 | } while ((type == RWLOCK_READER) && t && one_more); |
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303 | |||
304 | spinlock_unlock(&rwl->exclusive.sem.wq.lock); |
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305 | } |
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306 | |||
307 | void release_spinlock(void *arg) |
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308 | { |
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309 | spinlock_unlock((spinlock_t *) arg); |
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310 | } |