arch/i386/kernel/semaphore.c

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