arch/arm/kernel/semaphore.c

   1 /*
   2  *  ARM semaphore implementation, taken from
   3  *
   4  *  i386 semaphore implementation.
   5  *
   6  *  (C) Copyright 1999 Linus Torvalds
   7  *
   8  *  Modified for ARM by Russell King
   9  *
  10  * This program is free software; you can redistribute it and/or modify
  11  * it under the terms of the GNU General Public License version 2 as
  12  * published by the Free Software Foundation.
  13  */
  14 #include <linux/config.h>
  15 #include <linux/sched.h>
  16
  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  * The semaphore operations have a special calling sequence that
 171  * allow us to do a simpler in-line version of them. These routines
 172  * need to convert that sequence back into the C sequence when
 173  * there is contention on the semaphore.
 174  *
 175  * ip contains the semaphore pointer on entry. Save the C-clobbered
 176  * registers (r0 to r3 and lr), but not ip, as we use it as a return
 177  * value in some cases..
 178  */
 179 #ifdef CONFIG_CPU_26
 180 asm("   .align  5                               \n\
 181         .globl  __down_failed                   \n\
 182 __down_failed:                                  \n\
 183         stmfd   sp!, {r0 - r3, lr}              \n\
 184         mov     r0, ip                          \n\
 185         bl      __down                          \n\
 186         ldmfd   sp!, {r0 - r3, pc}^             \n\
 187                                                 \n\
 188         .align  5                               \n\
 189         .globl  __down_interruptible_failed     \n\
 190 __down_interruptible_failed:                    \n\
 191         stmfd   sp!, {r0 - r3, lr}              \n\
 192         mov     r0, ip                          \n\
 193         bl      __down_interruptible            \n\
 194         mov     ip, r0                          \n\
 195         ldmfd   sp!, {r0 - r3, pc}^             \n\
 196
 197         .align  5                               \n\
 198         .globl  __down_trylock_failed           \n\
 199 __down_trylock_failed:                          \n\
 200         stmfd   sp!, {r0 - r3, lr}              \n\
 201         mov     r0, ip                          \n\
 202         bl      __down_trylock                  \n\
 203         mov     ip, r0                          \n\
 204         ldmfd   sp!, {r0 - r3, pc}^             \n\
 205                                                 \n\
 206         .align  5                               \n\
 207         .globl  __up_wakeup                     \n\
 208 __up_wakeup:                                    \n\
 209         stmfd   sp!, {r0 - r3, lr}              \n\
 210         mov     r0, ip                          \n\
 211         bl      __up                            \n\
 212         ldmfd   sp!, {r0 - r3, pc}^             \n\
 213         ");
 214
 215 #else
 216 /* 32 bit version */
 217 asm("   .align  5                               \n\
 218         .globl  __down_failed                   \n\
 219 __down_failed:                                  \n\
 220         stmfd   sp!, {r0 - r3, lr}              \n\
 221         mov     r0, ip                          \n\
 222         bl      __down                          \n\
 223         ldmfd   sp!, {r0 - r3, pc}              \n\
 224                                                 \n\
 225         .align  5                               \n\
 226         .globl  __down_interruptible_failed     \n\
 227 __down_interruptible_failed:                    \n\
 228         stmfd   sp!, {r0 - r3, lr}              \n\
 229         mov     r0, ip                          \n\
 230         bl      __down_interruptible            \n\
 231         mov     ip, r0                          \n\
 232         ldmfd   sp!, {r0 - r3, pc}              \n\
 233                                                 \n\
 234         .align  5                               \n\
 235         .globl  __down_trylock_failed           \n\
 236 __down_trylock_failed:                          \n\
 237         stmfd   sp!, {r0 - r3, lr}              \n\
 238         mov     r0, ip                          \n\
 239         bl      __down_trylock                  \n\
 240         mov     ip, r0                          \n\
 241         ldmfd   sp!, {r0 - r3, pc}              \n\
 242                                                 \n\
 243         .align  5                               \n\
 244         .globl  __up_wakeup                     \n\
 245 __up_wakeup:                                    \n\
 246         stmfd   sp!, {r0 - r3, lr}              \n\
 247         mov     r0, ip                          \n\
 248         bl      __up                            \n\
 249         ldmfd   sp!, {r0 - r3, pc}              \n\
 250         ");
 251
 252 #endif