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1 | jermar | 1 | /* |
2071 | jermar | 2 | * Copyright (c) 2001-2004 Jakub Jermar |
1 | jermar | 3 | * All rights reserved. |
4 | * |
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5 | * Redistribution and use in source and binary forms, with or without |
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6 | * modification, are permitted provided that the following conditions |
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7 | * are met: |
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8 | * |
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9 | * - Redistributions of source code must retain the above copyright |
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10 | * notice, this list of conditions and the following disclaimer. |
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11 | * - Redistributions in binary form must reproduce the above copyright |
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12 | * notice, this list of conditions and the following disclaimer in the |
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13 | * documentation and/or other materials provided with the distribution. |
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14 | * - The name of the author may not be used to endorse or promote products |
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15 | * derived from this software without specific prior written permission. |
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16 | * |
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17 | * THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR |
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18 | * IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES |
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19 | * OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. |
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20 | * IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT, |
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21 | * INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT |
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22 | * NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, |
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23 | * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY |
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24 | * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT |
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25 | * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF |
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26 | * THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE. |
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27 | */ |
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28 | |||
1757 | jermar | 29 | /** @addtogroup sync |
1702 | cejka | 30 | * @{ |
31 | */ |
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32 | |||
1248 | jermar | 33 | /** |
1702 | cejka | 34 | * @file |
1248 | jermar | 35 | * @brief Reader/Writer locks. |
557 | jermar | 36 | * |
37 | * A reader/writer lock can be held by multiple readers at a time. |
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38 | * Or it can be exclusively held by a sole writer at a time. |
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1248 | jermar | 39 | * |
1 | jermar | 40 | * These locks are not recursive. |
1757 | jermar | 41 | * Because a technique called direct hand-off is used and because |
42 | * waiting takes place in a single wait queue, neither readers |
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1248 | jermar | 43 | * nor writers will suffer starvation. |
1 | jermar | 44 | * |
45 | * If there is a writer followed by a reader waiting for the rwlock |
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46 | * and the writer times out, all leading readers are automatically woken up |
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47 | * and allowed in. |
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48 | */ |
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49 | |||
50 | /* |
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51 | * NOTE ON rwlock_holder_type |
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52 | * This field is set on an attempt to acquire the exclusive mutex |
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53 | * to the respective value depending whether the caller is a reader |
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54 | * or a writer. The field is examined only if the thread had been |
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55 | * previously blocked on the exclusive mutex. Thus it is save |
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56 | * to store the rwlock type in the thread structure, because |
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57 | * each thread can block on only one rwlock at a time. |
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58 | */ |
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59 | |||
60 | #include <synch/rwlock.h> |
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61 | #include <synch/spinlock.h> |
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62 | #include <synch/mutex.h> |
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63 | #include <synch/waitq.h> |
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385 | jermar | 64 | #include <synch/synch.h> |
788 | jermar | 65 | #include <adt/list.h> |
1 | jermar | 66 | #include <arch/asm.h> |
67 | #include <arch.h> |
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68 | #include <proc/thread.h> |
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69 | #include <panic.h> |
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70 | |||
71 | #define ALLOW_ALL 0 |
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72 | #define ALLOW_READERS_ONLY 1 |
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73 | |||
74 | static void let_others_in(rwlock_t *rwl, int readers_only); |
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75 | static void release_spinlock(void *arg); |
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76 | |||
385 | jermar | 77 | /** Initialize reader/writer lock |
78 | * |
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79 | * Initialize reader/writer lock. |
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80 | * |
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81 | * @param rwl Reader/Writer lock. |
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82 | */ |
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1 | jermar | 83 | void rwlock_initialize(rwlock_t *rwl) { |
557 | jermar | 84 | spinlock_initialize(&rwl->lock, "rwlock_t"); |
3191 | svoboda | 85 | mutex_initialize(&rwl->exclusive, MUTEX_PASSIVE); |
1 | jermar | 86 | rwl->readers_in = 0; |
87 | } |
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88 | |||
385 | jermar | 89 | /** Acquire reader/writer lock for reading |
90 | * |
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91 | * Acquire reader/writer lock for reading. |
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92 | * Timeout and willingness to block may be specified. |
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93 | * |
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94 | * @param rwl Reader/Writer lock. |
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95 | * @param usec Timeout in microseconds. |
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1502 | jermar | 96 | * @param flags Specify mode of operation. |
385 | jermar | 97 | * |
98 | * For exact description of possible combinations of |
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1502 | jermar | 99 | * usec and flags, see comment for waitq_sleep_timeout(). |
385 | jermar | 100 | * |
101 | * @return See comment for waitq_sleep_timeout(). |
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102 | */ |
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1780 | jermar | 103 | int _rwlock_write_lock_timeout(rwlock_t *rwl, uint32_t usec, int flags) |
1 | jermar | 104 | { |
413 | jermar | 105 | ipl_t ipl; |
1 | jermar | 106 | int rc; |
107 | |||
413 | jermar | 108 | ipl = interrupts_disable(); |
15 | jermar | 109 | spinlock_lock(&THREAD->lock); |
110 | THREAD->rwlock_holder_type = RWLOCK_WRITER; |
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111 | spinlock_unlock(&THREAD->lock); |
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413 | jermar | 112 | interrupts_restore(ipl); |
1 | jermar | 113 | |
114 | /* |
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115 | * Writers take the easy part. |
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116 | * They just need to acquire the exclusive mutex. |
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117 | */ |
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1502 | jermar | 118 | rc = _mutex_lock_timeout(&rwl->exclusive, usec, flags); |
1 | jermar | 119 | if (SYNCH_FAILED(rc)) { |
120 | |||
121 | /* |
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1502 | jermar | 122 | * Lock operation timed out or was interrupted. |
1 | jermar | 123 | * The state of rwl is UNKNOWN at this point. |
124 | * No claims about its holder can be made. |
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125 | */ |
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126 | |||
413 | jermar | 127 | ipl = interrupts_disable(); |
1 | jermar | 128 | spinlock_lock(&rwl->lock); |
129 | /* |
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130 | * Now when rwl is locked, we can inspect it again. |
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131 | * If it is held by some readers already, we can let |
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132 | * readers from the head of the wait queue in. |
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133 | */ |
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134 | if (rwl->readers_in) |
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135 | let_others_in(rwl, ALLOW_READERS_ONLY); |
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136 | spinlock_unlock(&rwl->lock); |
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413 | jermar | 137 | interrupts_restore(ipl); |
1 | jermar | 138 | } |
139 | |||
140 | return rc; |
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141 | } |
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142 | |||
385 | jermar | 143 | /** Acquire reader/writer lock for writing |
144 | * |
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145 | * Acquire reader/writer lock for writing. |
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146 | * Timeout and willingness to block may be specified. |
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147 | * |
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148 | * @param rwl Reader/Writer lock. |
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149 | * @param usec Timeout in microseconds. |
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1502 | jermar | 150 | * @param flags Select mode of operation. |
385 | jermar | 151 | * |
152 | * For exact description of possible combinations of |
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1502 | jermar | 153 | * usec and flags, see comment for waitq_sleep_timeout(). |
385 | jermar | 154 | * |
155 | * @return See comment for waitq_sleep_timeout(). |
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156 | */ |
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1780 | jermar | 157 | int _rwlock_read_lock_timeout(rwlock_t *rwl, uint32_t usec, int flags) |
1 | jermar | 158 | { |
159 | int rc; |
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413 | jermar | 160 | ipl_t ipl; |
1 | jermar | 161 | |
413 | jermar | 162 | ipl = interrupts_disable(); |
15 | jermar | 163 | spinlock_lock(&THREAD->lock); |
164 | THREAD->rwlock_holder_type = RWLOCK_READER; |
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165 | spinlock_unlock(&THREAD->lock); |
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1 | jermar | 166 | |
167 | spinlock_lock(&rwl->lock); |
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168 | |||
169 | /* |
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170 | * Find out whether we can get what we want without blocking. |
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171 | */ |
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172 | rc = mutex_trylock(&rwl->exclusive); |
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173 | if (SYNCH_FAILED(rc)) { |
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174 | |||
175 | /* |
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176 | * 'exclusive' mutex is being held by someone else. |
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177 | * If the holder is a reader and there is no one |
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178 | * else waiting for it, we can enter the critical |
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179 | * section. |
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180 | */ |
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181 | |||
182 | if (rwl->readers_in) { |
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183 | spinlock_lock(&rwl->exclusive.sem.wq.lock); |
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184 | if (list_empty(&rwl->exclusive.sem.wq.head)) { |
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185 | /* |
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186 | * We can enter. |
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187 | */ |
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188 | spinlock_unlock(&rwl->exclusive.sem.wq.lock); |
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189 | goto shortcut; |
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190 | } |
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191 | spinlock_unlock(&rwl->exclusive.sem.wq.lock); |
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192 | } |
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193 | |||
194 | /* |
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195 | * In order to prevent a race condition when a reader |
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196 | * could block another reader at the head of the waitq, |
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197 | * we register a function to unlock rwl->lock |
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198 | * after this thread is put asleep. |
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199 | */ |
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623 | jermar | 200 | #ifdef CONFIG_SMP |
1 | jermar | 201 | thread_register_call_me(release_spinlock, &rwl->lock); |
623 | jermar | 202 | #else |
203 | thread_register_call_me(release_spinlock, NULL); |
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204 | #endif |
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1 | jermar | 205 | |
1502 | jermar | 206 | rc = _mutex_lock_timeout(&rwl->exclusive, usec, flags); |
1 | jermar | 207 | switch (rc) { |
1888 | jermar | 208 | case ESYNCH_WOULD_BLOCK: |
209 | /* |
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210 | * release_spinlock() wasn't called |
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211 | */ |
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212 | thread_register_call_me(NULL, NULL); |
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213 | spinlock_unlock(&rwl->lock); |
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214 | case ESYNCH_TIMEOUT: |
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215 | case ESYNCH_INTERRUPTED: |
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216 | /* |
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217 | * The sleep timed out. |
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218 | * We just restore interrupt priority level. |
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219 | */ |
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220 | case ESYNCH_OK_BLOCKED: |
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221 | /* |
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2087 | jermar | 222 | * We were woken with rwl->readers_in already |
223 | * incremented. |
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224 | * |
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225 | * Note that this arrangement avoids race condition |
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226 | * between two concurrent readers. (Race is avoided if |
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227 | * 'exclusive' is locked at the same time as |
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228 | * 'readers_in' is incremented. Same time means both |
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229 | * events happen atomically when rwl->lock is held.) |
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1888 | jermar | 230 | */ |
231 | interrupts_restore(ipl); |
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232 | break; |
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233 | case ESYNCH_OK_ATOMIC: |
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3149 | svoboda | 234 | panic("_mutex_lock_timeout() == ESYNCH_OK_ATOMIC\n"); |
1888 | jermar | 235 | break; |
236 | default: |
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237 | panic("invalid ESYNCH\n"); |
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238 | break; |
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1 | jermar | 239 | } |
240 | return rc; |
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241 | } |
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242 | |||
243 | shortcut: |
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244 | |||
245 | /* |
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246 | * We can increment readers_in only if we didn't go to sleep. |
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247 | * For sleepers, rwlock_let_others_in() will do the job. |
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248 | */ |
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249 | rwl->readers_in++; |
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250 | |||
251 | spinlock_unlock(&rwl->lock); |
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413 | jermar | 252 | interrupts_restore(ipl); |
1 | jermar | 253 | |
254 | return ESYNCH_OK_ATOMIC; |
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255 | } |
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256 | |||
385 | jermar | 257 | /** Release reader/writer lock held by writer |
258 | * |
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259 | * Release reader/writer lock held by writer. |
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260 | * Handoff reader/writer lock ownership directly |
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261 | * to waiting readers or a writer. |
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262 | * |
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263 | * @param rwl Reader/Writer lock. |
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264 | */ |
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1 | jermar | 265 | void rwlock_write_unlock(rwlock_t *rwl) |
266 | { |
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413 | jermar | 267 | ipl_t ipl; |
1 | jermar | 268 | |
413 | jermar | 269 | ipl = interrupts_disable(); |
1 | jermar | 270 | spinlock_lock(&rwl->lock); |
271 | let_others_in(rwl, ALLOW_ALL); |
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272 | spinlock_unlock(&rwl->lock); |
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413 | jermar | 273 | interrupts_restore(ipl); |
1 | jermar | 274 | |
275 | } |
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276 | |||
385 | jermar | 277 | /** Release reader/writer lock held by reader |
278 | * |
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279 | * Release reader/writer lock held by reader. |
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280 | * Handoff reader/writer lock ownership directly |
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281 | * to a waiting writer or don't do anything if more |
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282 | * readers poses the lock. |
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283 | * |
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284 | * @param rwl Reader/Writer lock. |
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285 | */ |
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1 | jermar | 286 | void rwlock_read_unlock(rwlock_t *rwl) |
287 | { |
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413 | jermar | 288 | ipl_t ipl; |
1 | jermar | 289 | |
413 | jermar | 290 | ipl = interrupts_disable(); |
1 | jermar | 291 | spinlock_lock(&rwl->lock); |
292 | if (!--rwl->readers_in) |
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293 | let_others_in(rwl, ALLOW_ALL); |
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294 | spinlock_unlock(&rwl->lock); |
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413 | jermar | 295 | interrupts_restore(ipl); |
1 | jermar | 296 | } |
297 | |||
298 | |||
557 | jermar | 299 | /** Direct handoff of reader/writer lock ownership. |
385 | jermar | 300 | * |
301 | * Direct handoff of reader/writer lock ownership |
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302 | * to waiting readers or a writer. |
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303 | * |
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1 | jermar | 304 | * Must be called with rwl->lock locked. |
413 | jermar | 305 | * Must be called with interrupts_disable()'d. |
385 | jermar | 306 | * |
307 | * @param rwl Reader/Writer lock. |
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308 | * @param readers_only See the description below. |
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309 | * |
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1 | jermar | 310 | * If readers_only is false: (unlock scenario) |
311 | * Let the first sleeper on 'exclusive' mutex in, no matter |
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312 | * whether it is a reader or a writer. If there are more leading |
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313 | * readers in line, let each of them in. |
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314 | * |
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315 | * Otherwise: (timeout scenario) |
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316 | * Let all leading readers in. |
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317 | */ |
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318 | void let_others_in(rwlock_t *rwl, int readers_only) |
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319 | { |
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320 | rwlock_type_t type = RWLOCK_NONE; |
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321 | thread_t *t = NULL; |
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557 | jermar | 322 | bool one_more = true; |
1 | jermar | 323 | |
324 | spinlock_lock(&rwl->exclusive.sem.wq.lock); |
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325 | |||
326 | if (!list_empty(&rwl->exclusive.sem.wq.head)) |
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2087 | jermar | 327 | t = list_get_instance(rwl->exclusive.sem.wq.head.next, thread_t, |
328 | wq_link); |
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1 | jermar | 329 | do { |
330 | if (t) { |
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331 | spinlock_lock(&t->lock); |
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332 | type = t->rwlock_holder_type; |
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333 | spinlock_unlock(&t->lock); |
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334 | } |
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335 | |||
336 | /* |
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337 | * If readers_only is true, we wake all leading readers |
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338 | * if and only if rwl is locked by another reader. |
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339 | * Assumption: readers_only ==> rwl->readers_in |
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340 | */ |
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341 | if (readers_only && (type != RWLOCK_READER)) |
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342 | break; |
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343 | |||
344 | |||
345 | if (type == RWLOCK_READER) { |
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346 | /* |
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347 | * Waking up a reader. |
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2087 | jermar | 348 | * We are responsible for incrementing rwl->readers_in |
349 | * for it. |
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1 | jermar | 350 | */ |
351 | rwl->readers_in++; |
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352 | } |
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353 | |||
354 | /* |
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355 | * Only the last iteration through this loop can increment |
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356 | * rwl->exclusive.sem.wq.missed_wakeup's. All preceeding |
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357 | * iterations will wake up a thread. |
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358 | */ |
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359 | /* We call the internal version of waitq_wakeup, which |
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360 | * relies on the fact that the waitq is already locked. |
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361 | */ |
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362 | _waitq_wakeup_unsafe(&rwl->exclusive.sem.wq, WAKEUP_FIRST); |
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363 | |||
364 | t = NULL; |
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365 | if (!list_empty(&rwl->exclusive.sem.wq.head)) { |
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2087 | jermar | 366 | t = list_get_instance(rwl->exclusive.sem.wq.head.next, |
367 | thread_t, wq_link); |
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1 | jermar | 368 | if (t) { |
369 | spinlock_lock(&t->lock); |
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370 | if (t->rwlock_holder_type != RWLOCK_READER) |
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557 | jermar | 371 | one_more = false; |
1 | jermar | 372 | spinlock_unlock(&t->lock); |
373 | } |
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374 | } |
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375 | } while ((type == RWLOCK_READER) && t && one_more); |
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376 | |||
377 | spinlock_unlock(&rwl->exclusive.sem.wq.lock); |
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378 | } |
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379 | |||
385 | jermar | 380 | /** Release spinlock callback |
381 | * |
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382 | * This is a callback function invoked from the scheduler. |
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383 | * The callback is registered in _rwlock_read_lock_timeout(). |
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384 | * |
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385 | * @param arg Spinlock. |
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386 | */ |
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1 | jermar | 387 | void release_spinlock(void *arg) |
388 | { |
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389 | spinlock_unlock((spinlock_t *) arg); |
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390 | } |
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1702 | cejka | 391 | |
1757 | jermar | 392 | /** @} |
1702 | cejka | 393 | */ |