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