i386/linux/linux-2.3.21/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 #include <linux/sched.h>
  11
  12 #include <asm/semaphore.h>
  13
  14 /*
  15  * Semaphores are implemented using a two-way counter:
  16  * The "count" variable is decremented for each process
  17  * that tries to aquire the semaphore, while the "sleeping"
  18  * variable is a count of such aquires.
  19  *
  20  * Notably, the inline "up()" and "down()" functions can
  21  * efficiently test if they need to do any extra work (up
  22  * needs to do something only if count was negative before
  23  * the increment operation.
  24  *
  25  * "sleeping" and the contention routine ordering is
  26  * protected by the semaphore spinlock.
  27  *
  28  * Note that these functions are only called when there is
  29  * contention on the lock, and as such all this is the
  30  * "non-critical" part of the whole semaphore business. The
  31  * critical part is the inline stuff in <asm/semaphore.h>
  32  * where we want to avoid any extra jumps and calls.
  33  */
  34
  35 /*
  36  * Logic:
  37  *  - only on a boundary condition do we need to care. When we go
  38  *    from a negative count to a non-negative, we wake people up.
  39  *  - when we go from a non-negative count to a negative do we
  40  *    (a) synchronize with the "sleeper" count and (b) make sure
  41  *    that we're on the wakeup list before we synchronize so that
  42  *    we cannot lose wakeup events.
  43  */
  44
  45 void __up(struct semaphore *sem)
  46 {
  47         wake_up(&sem->wait);
  48 }
  49
  50 static spinlock_t semaphore_lock = SPIN_LOCK_UNLOCKED;
  51
  52 void __down(struct semaphore * sem)
  53 {
  54         struct task_struct *tsk = current;
  55         DECLARE_WAITQUEUE(wait, tsk);
  56         tsk->state = TASK_UNINTERRUPTIBLE;
  57         add_wait_queue(&sem->wait, &wait);
  58
  59         spin_lock_irq(&semaphore_lock);
  60         sem->sleepers++;
  61         for (;;) {
  62                 int sleepers = sem->sleepers;
  63
  64                 /*
  65                  * Add "everybody else" into it. They aren't
  66                  * playing, because we own the spinlock.
  67                  */
  68                 if (!atomic_add_negative(sleepers - 1, &sem->count)) {
  69                         sem->sleepers = 0;
  70                         wake_up(&sem->wait);
  71                         break;
  72                 }
  73                 sem->sleepers = 1;      /* us - see -1 above */
  74                 spin_unlock_irq(&semaphore_lock);
  75
  76                 schedule();
  77                 tsk->state = TASK_UNINTERRUPTIBLE;
  78                 spin_lock_irq(&semaphore_lock);
  79         }
  80         spin_unlock_irq(&semaphore_lock);
  81         remove_wait_queue(&sem->wait, &wait);
  82         tsk->state = TASK_RUNNING;
  83 }
  84
  85 int __down_interruptible(struct semaphore * sem)
  86 {
  87         int retval;
  88         struct task_struct *tsk = current;
  89         DECLARE_WAITQUEUE(wait, tsk);
  90         tsk->state = TASK_INTERRUPTIBLE;
  91         add_wait_queue(&sem->wait, &wait);
  92
  93         spin_lock_irq(&semaphore_lock);
  94         sem->sleepers ++;
  95         for (;;) {
  96                 int sleepers = sem->sleepers;
  97
  98                 /*
  99                  * With signals pending, this turns into
 100                  * the trylock failure case - we won't be
 101                  * sleeping, and we* can't get the lock as
 102                  * it has contention. Just correct the count
 103                  * and exit.
 104                  */
 105                 retval = -EINTR;
 106                 if (signal_pending(current)) {
 107                         sem->sleepers = 0;
 108                         if (atomic_add_negative(sleepers, &sem->count))
 109                                 break;
 110                         wake_up(&sem->wait);
 111                         break;
 112                 }
 113
 114                 /*
 115                  * Add "everybody else" into it. They aren't
 116                  * playing, because we own the spinlock. The
 117                  * "-1" is because we're still hoping to get
 118                  * the lock.
 119                  */
 120                 if (!atomic_add_negative(sleepers - 1, &sem->count)) {
 121                         wake_up(&sem->wait);
 122                         retval = 0;
 123                         sem->sleepers = 0;
 124                         break;
 125                 }
 126                 sem->sleepers = 1;      /* us - see -1 above */
 127                 spin_unlock_irq(&semaphore_lock);
 128
 129                 schedule();
 130                 tsk->state = TASK_INTERRUPTIBLE;
 131                 spin_lock_irq(&semaphore_lock);
 132         }
 133         spin_unlock_irq(&semaphore_lock);
 134         tsk->state = TASK_RUNNING;
 135         remove_wait_queue(&sem->wait, &wait);
 136         return retval;
 137 }
 138
 139 /*
 140  * Trylock failed - make sure we correct for
 141  * having decremented the count.
 142  *
 143  * We could have done the trylock with a
 144  * single "cmpxchg" without failure cases,
 145  * but then it wouldn't work on a 386.
 146  */
 147 int __down_trylock(struct semaphore * sem)
 148 {
 149         int sleepers;
 150
 151         spin_lock_irq(&semaphore_lock);
 152         sleepers = sem->sleepers + 1;
 153         sem->sleepers = 0;
 154
 155         /*
 156          * Add "everybody else" and us into it. They aren't
 157          * playing, because we own the spinlock.
 158          */
 159         if (!atomic_add_negative(sleepers, &sem->count))
 160                 wake_up(&sem->wait);
 161
 162         spin_unlock_irq(&semaphore_lock);
 163         return 1;
 164 }
 165
 166 /*
 167  * The semaphore operations have a special calling sequence that
 168  * allow us to do a simpler in-line version of them. These routines
 169  * need to convert that sequence back into the C sequence when
 170  * there is contention on the semaphore.
 171  *
 172  * r0 contains the semaphore pointer on entry. Save the C-clobbered
 173  * registers (r0 to r3, ip and lr) except r0 in the cases where it
 174  * is used as a return value..
 175  */
 176 asm(".align     5
 177         .globl  __down_failed
 178 __down_failed:
 179         stmfd   sp!, {r0 - r3, ip, lr}
 180         bl      __down
 181         ldmfd   sp!, {r0 - r3, ip, pc}");
 182
 183 asm(".align     5
 184         .globl  __down_interruptible_failed
 185 __down_interruptible_failed:
 186         stmfd   sp!, {r1 - r3, ip, lr}
 187         bl      __down_interruptible
 188         ldmfd   sp!, {r1 - r3, ip, pc}");
 189
 190 asm(".align     5
 191         .globl  __down_trylock_failed
 192 __down_trylock_failed:
 193         stmfd   sp!, {r1 - r3, ip, lr}
 194         bl      __down_trylock
 195         ldmfd   sp!, {r1 - r3, ip, pc}");
 196
 197 asm(".align     5
 198         .globl  __up_wakeup
 199 __up_wakeup:
 200         stmfd   sp!, {r0 - r3, ip, lr}
 201         bl      __up
 202         ldmfd   sp!, {r0 - r3, ip, pc}");