Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 1 | /* |
| 2 | * i386 semaphore implementation. |
| 3 | * |
| 4 | * (C) Copyright 1999 Linus Torvalds |
| 5 | * |
| 6 | * Portions Copyright 1999 Red Hat, Inc. |
| 7 | * |
| 8 | * This program is free software; you can redistribute it and/or |
| 9 | * modify it under the terms of the GNU General Public License |
| 10 | * as published by the Free Software Foundation; either version |
| 11 | * 2 of the License, or (at your option) any later version. |
| 12 | * |
| 13 | * rw semaphores implemented November 1999 by Benjamin LaHaise <bcrl@kvack.org> |
| 14 | */ |
| 15 | #include <linux/config.h> |
| 16 | #include <linux/sched.h> |
| 17 | #include <linux/err.h> |
| 18 | #include <linux/init.h> |
| 19 | #include <asm/semaphore.h> |
| 20 | |
| 21 | /* |
| 22 | * Semaphores are implemented using a two-way counter: |
| 23 | * The "count" variable is decremented for each process |
| 24 | * that tries to acquire the semaphore, while the "sleeping" |
| 25 | * variable is a count of such acquires. |
| 26 | * |
| 27 | * Notably, the inline "up()" and "down()" functions can |
| 28 | * efficiently test if they need to do any extra work (up |
| 29 | * needs to do something only if count was negative before |
| 30 | * the increment operation. |
| 31 | * |
| 32 | * "sleeping" and the contention routine ordering is protected |
| 33 | * by the spinlock in the semaphore's waitqueue head. |
| 34 | * |
| 35 | * Note that these functions are only called when there is |
| 36 | * contention on the lock, and as such all this is the |
| 37 | * "non-critical" part of the whole semaphore business. The |
| 38 | * critical part is the inline stuff in <asm/semaphore.h> |
| 39 | * where we want to avoid any extra jumps and calls. |
| 40 | */ |
| 41 | |
| 42 | /* |
| 43 | * Logic: |
| 44 | * - only on a boundary condition do we need to care. When we go |
| 45 | * from a negative count to a non-negative, we wake people up. |
| 46 | * - when we go from a non-negative count to a negative do we |
| 47 | * (a) synchronize with the "sleeper" count and (b) make sure |
| 48 | * that we're on the wakeup list before we synchronize so that |
| 49 | * we cannot lose wakeup events. |
| 50 | */ |
| 51 | |
| 52 | static fastcall void __attribute_used__ __up(struct semaphore *sem) |
| 53 | { |
| 54 | wake_up(&sem->wait); |
| 55 | } |
| 56 | |
| 57 | static fastcall void __attribute_used__ __sched __down(struct semaphore * sem) |
| 58 | { |
| 59 | struct task_struct *tsk = current; |
| 60 | DECLARE_WAITQUEUE(wait, tsk); |
| 61 | unsigned long flags; |
| 62 | |
| 63 | tsk->state = TASK_UNINTERRUPTIBLE; |
| 64 | spin_lock_irqsave(&sem->wait.lock, flags); |
| 65 | add_wait_queue_exclusive_locked(&sem->wait, &wait); |
| 66 | |
| 67 | sem->sleepers++; |
| 68 | for (;;) { |
| 69 | int sleepers = sem->sleepers; |
| 70 | |
| 71 | /* |
| 72 | * Add "everybody else" into it. They aren't |
| 73 | * playing, because we own the spinlock in |
| 74 | * the wait_queue_head. |
| 75 | */ |
| 76 | if (!atomic_add_negative(sleepers - 1, &sem->count)) { |
| 77 | sem->sleepers = 0; |
| 78 | break; |
| 79 | } |
| 80 | sem->sleepers = 1; /* us - see -1 above */ |
| 81 | spin_unlock_irqrestore(&sem->wait.lock, flags); |
| 82 | |
| 83 | schedule(); |
| 84 | |
| 85 | spin_lock_irqsave(&sem->wait.lock, flags); |
| 86 | tsk->state = TASK_UNINTERRUPTIBLE; |
| 87 | } |
| 88 | remove_wait_queue_locked(&sem->wait, &wait); |
| 89 | wake_up_locked(&sem->wait); |
| 90 | spin_unlock_irqrestore(&sem->wait.lock, flags); |
| 91 | tsk->state = TASK_RUNNING; |
| 92 | } |
| 93 | |
| 94 | static fastcall int __attribute_used__ __sched __down_interruptible(struct semaphore * sem) |
| 95 | { |
| 96 | int retval = 0; |
| 97 | struct task_struct *tsk = current; |
| 98 | DECLARE_WAITQUEUE(wait, tsk); |
| 99 | unsigned long flags; |
| 100 | |
| 101 | tsk->state = TASK_INTERRUPTIBLE; |
| 102 | spin_lock_irqsave(&sem->wait.lock, flags); |
| 103 | add_wait_queue_exclusive_locked(&sem->wait, &wait); |
| 104 | |
| 105 | sem->sleepers++; |
| 106 | for (;;) { |
| 107 | int sleepers = sem->sleepers; |
| 108 | |
| 109 | /* |
| 110 | * With signals pending, this turns into |
| 111 | * the trylock failure case - we won't be |
| 112 | * sleeping, and we* can't get the lock as |
| 113 | * it has contention. Just correct the count |
| 114 | * and exit. |
| 115 | */ |
| 116 | if (signal_pending(current)) { |
| 117 | retval = -EINTR; |
| 118 | sem->sleepers = 0; |
| 119 | atomic_add(sleepers, &sem->count); |
| 120 | break; |
| 121 | } |
| 122 | |
| 123 | /* |
| 124 | * Add "everybody else" into it. They aren't |
| 125 | * playing, because we own the spinlock in |
| 126 | * wait_queue_head. The "-1" is because we're |
| 127 | * still hoping to get the semaphore. |
| 128 | */ |
| 129 | if (!atomic_add_negative(sleepers - 1, &sem->count)) { |
| 130 | sem->sleepers = 0; |
| 131 | break; |
| 132 | } |
| 133 | sem->sleepers = 1; /* us - see -1 above */ |
| 134 | spin_unlock_irqrestore(&sem->wait.lock, flags); |
| 135 | |
| 136 | schedule(); |
| 137 | |
| 138 | spin_lock_irqsave(&sem->wait.lock, flags); |
| 139 | tsk->state = TASK_INTERRUPTIBLE; |
| 140 | } |
| 141 | remove_wait_queue_locked(&sem->wait, &wait); |
| 142 | wake_up_locked(&sem->wait); |
| 143 | spin_unlock_irqrestore(&sem->wait.lock, flags); |
| 144 | |
| 145 | tsk->state = TASK_RUNNING; |
| 146 | return retval; |
| 147 | } |
| 148 | |
| 149 | /* |
| 150 | * Trylock failed - make sure we correct for |
| 151 | * having decremented the count. |
| 152 | * |
| 153 | * We could have done the trylock with a |
| 154 | * single "cmpxchg" without failure cases, |
| 155 | * but then it wouldn't work on a 386. |
| 156 | */ |
| 157 | static fastcall int __attribute_used__ __down_trylock(struct semaphore * sem) |
| 158 | { |
| 159 | int sleepers; |
| 160 | unsigned long flags; |
| 161 | |
| 162 | spin_lock_irqsave(&sem->wait.lock, flags); |
| 163 | sleepers = sem->sleepers + 1; |
| 164 | sem->sleepers = 0; |
| 165 | |
| 166 | /* |
| 167 | * Add "everybody else" and us into it. They aren't |
| 168 | * playing, because we own the spinlock in the |
| 169 | * wait_queue_head. |
| 170 | */ |
| 171 | if (!atomic_add_negative(sleepers, &sem->count)) { |
| 172 | wake_up_locked(&sem->wait); |
| 173 | } |
| 174 | |
| 175 | spin_unlock_irqrestore(&sem->wait.lock, flags); |
| 176 | return 1; |
| 177 | } |
| 178 | |
| 179 | |
| 180 | /* |
| 181 | * The semaphore operations have a special calling sequence that |
| 182 | * allow us to do a simpler in-line version of them. These routines |
| 183 | * need to convert that sequence back into the C sequence when |
| 184 | * there is contention on the semaphore. |
| 185 | * |
| 186 | * %eax contains the semaphore pointer on entry. Save the C-clobbered |
| 187 | * registers (%eax, %edx and %ecx) except %eax whish is either a return |
| 188 | * value or just clobbered.. |
| 189 | */ |
| 190 | asm( |
| 191 | ".section .sched.text\n" |
| 192 | ".align 4\n" |
| 193 | ".globl __down_failed\n" |
| 194 | "__down_failed:\n\t" |
| 195 | #if defined(CONFIG_FRAME_POINTER) |
| 196 | "pushl %ebp\n\t" |
| 197 | "movl %esp,%ebp\n\t" |
| 198 | #endif |
| 199 | "pushl %edx\n\t" |
| 200 | "pushl %ecx\n\t" |
| 201 | "call __down\n\t" |
| 202 | "popl %ecx\n\t" |
| 203 | "popl %edx\n\t" |
| 204 | #if defined(CONFIG_FRAME_POINTER) |
| 205 | "movl %ebp,%esp\n\t" |
| 206 | "popl %ebp\n\t" |
| 207 | #endif |
| 208 | "ret" |
| 209 | ); |
| 210 | |
| 211 | asm( |
| 212 | ".section .sched.text\n" |
| 213 | ".align 4\n" |
| 214 | ".globl __down_failed_interruptible\n" |
| 215 | "__down_failed_interruptible:\n\t" |
| 216 | #if defined(CONFIG_FRAME_POINTER) |
| 217 | "pushl %ebp\n\t" |
| 218 | "movl %esp,%ebp\n\t" |
| 219 | #endif |
| 220 | "pushl %edx\n\t" |
| 221 | "pushl %ecx\n\t" |
| 222 | "call __down_interruptible\n\t" |
| 223 | "popl %ecx\n\t" |
| 224 | "popl %edx\n\t" |
| 225 | #if defined(CONFIG_FRAME_POINTER) |
| 226 | "movl %ebp,%esp\n\t" |
| 227 | "popl %ebp\n\t" |
| 228 | #endif |
| 229 | "ret" |
| 230 | ); |
| 231 | |
| 232 | asm( |
| 233 | ".section .sched.text\n" |
| 234 | ".align 4\n" |
| 235 | ".globl __down_failed_trylock\n" |
| 236 | "__down_failed_trylock:\n\t" |
| 237 | #if defined(CONFIG_FRAME_POINTER) |
| 238 | "pushl %ebp\n\t" |
| 239 | "movl %esp,%ebp\n\t" |
| 240 | #endif |
| 241 | "pushl %edx\n\t" |
| 242 | "pushl %ecx\n\t" |
| 243 | "call __down_trylock\n\t" |
| 244 | "popl %ecx\n\t" |
| 245 | "popl %edx\n\t" |
| 246 | #if defined(CONFIG_FRAME_POINTER) |
| 247 | "movl %ebp,%esp\n\t" |
| 248 | "popl %ebp\n\t" |
| 249 | #endif |
| 250 | "ret" |
| 251 | ); |
| 252 | |
| 253 | asm( |
| 254 | ".section .sched.text\n" |
| 255 | ".align 4\n" |
| 256 | ".globl __up_wakeup\n" |
| 257 | "__up_wakeup:\n\t" |
| 258 | "pushl %edx\n\t" |
| 259 | "pushl %ecx\n\t" |
| 260 | "call __up\n\t" |
| 261 | "popl %ecx\n\t" |
| 262 | "popl %edx\n\t" |
| 263 | "ret" |
| 264 | ); |
| 265 | |
| 266 | /* |
| 267 | * rw spinlock fallbacks |
| 268 | */ |
| 269 | #if defined(CONFIG_SMP) |
| 270 | asm( |
| 271 | ".section .sched.text\n" |
| 272 | ".align 4\n" |
| 273 | ".globl __write_lock_failed\n" |
| 274 | "__write_lock_failed:\n\t" |
| 275 | LOCK "addl $" RW_LOCK_BIAS_STR ",(%eax)\n" |
| 276 | "1: rep; nop\n\t" |
| 277 | "cmpl $" RW_LOCK_BIAS_STR ",(%eax)\n\t" |
| 278 | "jne 1b\n\t" |
| 279 | LOCK "subl $" RW_LOCK_BIAS_STR ",(%eax)\n\t" |
| 280 | "jnz __write_lock_failed\n\t" |
| 281 | "ret" |
| 282 | ); |
| 283 | |
| 284 | asm( |
| 285 | ".section .sched.text\n" |
| 286 | ".align 4\n" |
| 287 | ".globl __read_lock_failed\n" |
| 288 | "__read_lock_failed:\n\t" |
| 289 | LOCK "incl (%eax)\n" |
| 290 | "1: rep; nop\n\t" |
| 291 | "cmpl $1,(%eax)\n\t" |
| 292 | "js 1b\n\t" |
| 293 | LOCK "decl (%eax)\n\t" |
| 294 | "js __read_lock_failed\n\t" |
| 295 | "ret" |
| 296 | ); |
| 297 | #endif |