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