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