arch/arm26/kernel/semaphore.c

   1 /*
   2  *  ARM semaphore implementation, taken from
   3  *
   4  *  i386 semaphore implementation.
   5  *
   6  *  (C) Copyright 1999 Linus Torvalds
   7  *  (C) Copyright 2003 Ian Molton (ARM26 mods)
   8  *
   9  *  Modified for ARM by Russell King
  10  *
  11  * This program is free software; you can redistribute it and/or modify
  12  * it under the terms of the GNU General Public License version 2 as
  13  * published by the Free Software Foundation.
  14  */
  15 #include <linux/module.h>
  16 #include <linux/config.h>
  17 #include <linux/sched.h>
  18 #include <linux/errno.h>
  19 #include <linux/init.h>
  20
  21 #include <asm/semaphore.h>
  22
  23 /*
  24  * Semaphores are implemented using a two-way counter:
  25  * The "count" variable is decremented for each process
  26  * that tries to acquire the semaphore, while the "sleeping"
  27  * variable is a count of such acquires.
  28  *
  29  * Notably, the inline "up()" and "down()" functions can
  30  * efficiently test if they need to do any extra work (up
  31  * needs to do something only if count was negative before
  32  * the increment operation.
  33  *
  34  * "sleeping" and the contention routine ordering is
  35  * protected by the semaphore spinlock.
  36  *
  37  * Note that these functions are only called when there is
  38  * contention on the lock, and as such all this is the
  39  * "non-critical" part of the whole semaphore business. The
  40  * critical part is the inline stuff in <asm/semaphore.h>
  41  * where we want to avoid any extra jumps and calls.
  42  */
  43
  44 /*
  45  * Logic:
  46  *  - only on a boundary condition do we need to care. When we go
  47  *    from a negative count to a non-negative, we wake people up.
  48  *  - when we go from a non-negative count to a negative do we
  49  *    (a) synchronize with the "sleeper" count and (b) make sure
  50  *    that we're on the wakeup list before we synchronize so that
  51  *    we cannot lose wakeup events.
  52  */
  53
  54 void __up(struct semaphore *sem)
  55 {
  56         wake_up(&sem->wait);
  57 }
  58
  59 static DEFINE_SPINLOCK(semaphore_lock);
  60
  61 void __sched __down(struct semaphore * sem)
  62 {
  63         struct task_struct *tsk = current;
  64         DECLARE_WAITQUEUE(wait, tsk);
  65         tsk->state = TASK_UNINTERRUPTIBLE;
  66         add_wait_queue_exclusive(&sem->wait, &wait);
  67
  68         spin_lock_irq(&semaphore_lock);
  69         sem->sleepers++;
  70         for (;;) {
  71                 int sleepers = sem->sleepers;
  72
  73                 /*
  74                  * Add "everybody else" into it. They aren't
  75                  * playing, because we own the spinlock.
  76                  */
  77                 if (!atomic_add_negative(sleepers - 1, &sem->count)) {
  78                         sem->sleepers = 0;
  79                         break;
  80                 }
  81                 sem->sleepers = 1;      /* us - see -1 above */
  82                 spin_unlock_irq(&semaphore_lock);
  83
  84                 schedule();
  85                 tsk->state = TASK_UNINTERRUPTIBLE;
  86                 spin_lock_irq(&semaphore_lock);
  87         }
  88         spin_unlock_irq(&semaphore_lock);
  89         remove_wait_queue(&sem->wait, &wait);
  90         tsk->state = TASK_RUNNING;
  91         wake_up(&sem->wait);
  92 }
  93
  94 int __sched __down_interruptible(struct semaphore * sem)
  95 {
  96         int retval = 0;
  97         struct task_struct *tsk = current;
  98         DECLARE_WAITQUEUE(wait, tsk);
  99         tsk->state = TASK_INTERRUPTIBLE;
 100         add_wait_queue_exclusive(&sem->wait, &wait);
 101
 102         spin_lock_irq(&semaphore_lock);
 103         sem->sleepers ++;
 104         for (;;) {
 105                 int sleepers = sem->sleepers;
 106
 107                 /*
 108                  * With signals pending, this turns into
 109                  * the trylock failure case - we won't be
 110                  * sleeping, and we* can't get the lock as
 111                  * it has contention. Just correct the count
 112                  * and exit.
 113                  */
 114                 if (signal_pending(current)) {
 115                         retval = -EINTR;
 116                         sem->sleepers = 0;
 117                         atomic_add(sleepers, &sem->count);
 118                         break;
 119                 }
 120
 121                 /*
 122                  * Add "everybody else" into it. They aren't
 123                  * playing, because we own the spinlock. The
 124                  * "-1" is because we're still hoping to get
 125                  * the lock.
 126                  */
 127                 if (!atomic_add_negative(sleepers - 1, &sem->count)) {
 128                         sem->sleepers = 0;
 129                         break;
 130                 }
 131                 sem->sleepers = 1;      /* us - see -1 above */
 132                 spin_unlock_irq(&semaphore_lock);
 133
 134                 schedule();
 135                 tsk->state = TASK_INTERRUPTIBLE;
 136                 spin_lock_irq(&semaphore_lock);
 137         }
 138         spin_unlock_irq(&semaphore_lock);
 139         tsk->state = TASK_RUNNING;
 140         remove_wait_queue(&sem->wait, &wait);
 141         wake_up(&sem->wait);
 142         return retval;
 143 }
 144
 145 /*
 146  * Trylock failed - make sure we correct for
 147  * having decremented the count.
 148  *
 149  * We could have done the trylock with a
 150  * single "cmpxchg" without failure cases,
 151  * but then it wouldn't work on a 386.
 152  */
 153 int __down_trylock(struct semaphore * sem)
 154 {
 155         int sleepers;
 156         unsigned long flags;
 157
 158         spin_lock_irqsave(&semaphore_lock, flags);
 159         sleepers = sem->sleepers + 1;
 160         sem->sleepers = 0;
 161
 162         /*
 163          * Add "everybody else" and us into it. They aren't
 164          * playing, because we own the spinlock.
 165          */
 166         if (!atomic_add_negative(sleepers, &sem->count))
 167                 wake_up(&sem->wait);
 168
 169         spin_unlock_irqrestore(&semaphore_lock, flags);
 170         return 1;
 171 }
 172
 173 /*
 174  * The semaphore operations have a special calling sequence that
 175  * allow us to do a simpler in-line version of them. These routines
 176  * need to convert that sequence back into the C sequence when
 177  * there is contention on the semaphore.
 178  *
 179  * ip contains the semaphore pointer on entry. Save the C-clobbered
 180  * registers (r0 to r3 and lr), but not ip, as we use it as a return
 181  * value in some cases..
 182  */
 183 asm("   .section .sched.text , #alloc, #execinstr       \n\
 184         .align  5                               \n\
 185         .globl  __down_failed                   \n\
 186 __down_failed:                                  \n\
 187         stmfd   sp!, {r0 - r3, lr}              \n\
 188         mov     r0, ip                          \n\
 189         bl      __down                          \n\
 190         ldmfd   sp!, {r0 - r3, pc}^             \n\
 191                                                 \n\
 192         .align  5                               \n\
 193         .globl  __down_interruptible_failed     \n\
 194 __down_interruptible_failed:                    \n\
 195         stmfd   sp!, {r0 - r3, lr}              \n\
 196         mov     r0, ip                          \n\
 197         bl      __down_interruptible            \n\
 198         mov     ip, r0                          \n\
 199         ldmfd   sp!, {r0 - r3, pc}^             \n\
 200                                                 \n\
 201         .align  5                               \n\
 202         .globl  __down_trylock_failed           \n\
 203 __down_trylock_failed:                          \n\
 204         stmfd   sp!, {r0 - r3, lr}              \n\
 205         mov     r0, ip                          \n\
 206         bl      __down_trylock                  \n\
 207         mov     ip, r0                          \n\
 208         ldmfd   sp!, {r0 - r3, pc}^             \n\
 209                                                 \n\
 210         .align  5                               \n\
 211         .globl  __up_wakeup                     \n\
 212 __up_wakeup:                                    \n\
 213         stmfd   sp!, {r0 - r3, lr}              \n\
 214         mov     r0, ip                          \n\
 215         bl      __up                            \n\
 216         ldmfd   sp!, {r0 - r3, pc}^             \n\
 217         ");
 218
 219 EXPORT_SYMBOL(__down_failed);
 220 EXPORT_SYMBOL(__down_interruptible_failed);
 221 EXPORT_SYMBOL(__down_trylock_failed);
 222 EXPORT_SYMBOL(__up_wakeup);
 223