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semaphore.c

/*
 *  ARM semaphore implementation, taken from
 *
 *  i386 semaphore implementation.
 *
 *  (C) Copyright 1999 Linus Torvalds
 *  (C) Copyright 2003 Ian Molton (ARM26 mods)
 *
 *  Modified for ARM by Russell King
 *
 * This program is free software; you can redistribute it and/or modify
 * it under the terms of the GNU General Public License version 2 as
 * published by the Free Software Foundation.
 */
#include <linux/module.h>
#include <linux/sched.h>
#include <linux/errno.h>
#include <linux/init.h>

#include <asm/semaphore.h>

/*
 * Semaphores are implemented using a two-way counter:
 * The "count" variable is decremented for each process
 * that tries to acquire the semaphore, while the "sleeping"
 * variable is a count of such acquires.
 *
 * Notably, the inline "up()" and "down()" functions can
 * efficiently test if they need to do any extra work (up
 * needs to do something only if count was negative before
 * the increment operation.
 *
 * "sleeping" and the contention routine ordering is
 * protected by the semaphore spinlock.
 *
 * Note that these functions are only called when there is
 * contention on the lock, and as such all this is the
 * "non-critical" part of the whole semaphore business. The
 * critical part is the inline stuff in <asm/semaphore.h>
 * where we want to avoid any extra jumps and calls.
 */

/*
 * Logic:
 *  - only on a boundary condition do we need to care. When we go
 *    from a negative count to a non-negative, we wake people up.
 *  - when we go from a non-negative count to a negative do we
 *    (a) synchronize with the "sleeper" count and (b) make sure
 *    that we're on the wakeup list before we synchronize so that
 *    we cannot lose wakeup events.
 */

void __up(struct semaphore *sem)
{
      wake_up(&sem->wait);
}

static DEFINE_SPINLOCK(semaphore_lock);

void __sched __down(struct semaphore * sem)
{
      struct task_struct *tsk = current;
      DECLARE_WAITQUEUE(wait, tsk);
      tsk->state = TASK_UNINTERRUPTIBLE;
      add_wait_queue_exclusive(&sem->wait, &wait);

      spin_lock_irq(&semaphore_lock);
      sem->sleepers++;
      for (;;) {
            int sleepers = sem->sleepers;

            /*
             * Add "everybody else" into it. They aren't
             * playing, because we own the spinlock.
             */
            if (!atomic_add_negative(sleepers - 1, &sem->count)) {
                  sem->sleepers = 0;
                  break;
            }
            sem->sleepers = 1;      /* us - see -1 above */
            spin_unlock_irq(&semaphore_lock);

            schedule();
            tsk->state = TASK_UNINTERRUPTIBLE;
            spin_lock_irq(&semaphore_lock);
      }
      spin_unlock_irq(&semaphore_lock);
      remove_wait_queue(&sem->wait, &wait);
      tsk->state = TASK_RUNNING;
      wake_up(&sem->wait);
}

int __sched __down_interruptible(struct semaphore * sem)
{
      int retval = 0;
      struct task_struct *tsk = current;
      DECLARE_WAITQUEUE(wait, tsk);
      tsk->state = TASK_INTERRUPTIBLE;
      add_wait_queue_exclusive(&sem->wait, &wait);

      spin_lock_irq(&semaphore_lock);
      sem->sleepers ++;
      for (;;) {
            int sleepers = sem->sleepers;

            /*
             * With signals pending, this turns into
             * the trylock failure case - we won't be
             * sleeping, and we* can't get the lock as
             * it has contention. Just correct the count
             * and exit.
             */
            if (signal_pending(current)) {
                  retval = -EINTR;
                  sem->sleepers = 0;
                  atomic_add(sleepers, &sem->count);
                  break;
            }

            /*
             * Add "everybody else" into it. They aren't
             * playing, because we own the spinlock. The
             * "-1" is because we're still hoping to get
             * the lock.
             */
            if (!atomic_add_negative(sleepers - 1, &sem->count)) {
                  sem->sleepers = 0;
                  break;
            }
            sem->sleepers = 1;      /* us - see -1 above */
            spin_unlock_irq(&semaphore_lock);

            schedule();
            tsk->state = TASK_INTERRUPTIBLE;
            spin_lock_irq(&semaphore_lock);
      }
      spin_unlock_irq(&semaphore_lock);
      tsk->state = TASK_RUNNING;
      remove_wait_queue(&sem->wait, &wait);
      wake_up(&sem->wait);
      return retval;
}

/*
 * Trylock failed - make sure we correct for
 * having decremented the count.
 *
 * We could have done the trylock with a
 * single "cmpxchg" without failure cases,
 * but then it wouldn't work on a 386.
 */
int __down_trylock(struct semaphore * sem)
{
      int sleepers;
      unsigned long flags;

      spin_lock_irqsave(&semaphore_lock, flags);
      sleepers = sem->sleepers + 1;
      sem->sleepers = 0;

      /*
       * Add "everybody else" and us into it. They aren't
       * playing, because we own the spinlock.
       */
      if (!atomic_add_negative(sleepers, &sem->count))
            wake_up(&sem->wait);

      spin_unlock_irqrestore(&semaphore_lock, flags);
      return 1;
}

/*
 * The semaphore operations have a special calling sequence that
 * allow us to do a simpler in-line version of them. These routines
 * need to convert that sequence back into the C sequence when
 * there is contention on the semaphore.
 *
 * ip contains the semaphore pointer on entry. Save the C-clobbered
 * registers (r0 to r3 and lr), but not ip, as we use it as a return
 * value in some cases..
 */
asm(" .section .sched.text , #alloc, #execinstr \n\
      .align      5                       \n\
      .globl      __down_failed                 \n\
__down_failed:                            \n\
      stmfd sp!, {r0 - r3, lr}            \n\
      mov   r0, ip                        \n\
      bl    __down                        \n\
      ldmfd sp!, {r0 - r3, pc}^           \n\
                                    \n\
      .align      5                       \n\
      .globl      __down_interruptible_failed   \n\
__down_interruptible_failed:              \n\
      stmfd sp!, {r0 - r3, lr}            \n\
      mov   r0, ip                        \n\
      bl    __down_interruptible          \n\
      mov   ip, r0                        \n\
      ldmfd sp!, {r0 - r3, pc}^           \n\
                                    \n\
      .align      5                       \n\
      .globl      __down_trylock_failed         \n\
__down_trylock_failed:                    \n\
      stmfd sp!, {r0 - r3, lr}            \n\
      mov   r0, ip                        \n\
      bl    __down_trylock                \n\
      mov   ip, r0                        \n\
      ldmfd sp!, {r0 - r3, pc}^           \n\
                                    \n\
      .align      5                       \n\
      .globl      __up_wakeup             \n\
__up_wakeup:                              \n\
      stmfd sp!, {r0 - r3, lr}            \n\
      mov   r0, ip                        \n\
      bl    __up                    \n\
      ldmfd sp!, {r0 - r3, pc}^           \n\
      ");

EXPORT_SYMBOL(__down_failed);
EXPORT_SYMBOL(__down_interruptible_failed);
EXPORT_SYMBOL(__down_trylock_failed);
EXPORT_SYMBOL(__up_wakeup);


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