arch/x86_64/kernel/semaphore.c

   1 /*
   2  * x86_64 semaphore implementation.
   3  *
   4  * (C) Copyright 1999 Linus Torvalds
   5  *
   6  * Portions Copyright 1999 Red Hat, Inc.
   7  *
   8  *      This program is free software; you can redistribute it and/or
   9  *      modify it under the terms of the GNU General Public License
  10  *      as published by the Free Software Foundation; either version
  11  *      2 of the License, or (at your option) any later version.
  12  *
  13  * rw semaphores implemented November 1999 by Benjamin LaHaise <bcrl@kvack.org>
  14  */
  15 #include <linux/config.h>
  16 #include <linux/sched.h>
  17 #include <linux/init.h>
  18 #include <asm/errno.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 protected
  34  * by the spinlock in the semaphore's waitqueue head.
  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 void __sched __down(struct semaphore * sem)
  59 {
  60         struct task_struct *tsk = current;
  61         DECLARE_WAITQUEUE(wait, tsk);
  62         unsigned long flags;
  63
  64         tsk->state = TASK_UNINTERRUPTIBLE;
  65         spin_lock_irqsave(&sem->wait.lock, flags);
  66         add_wait_queue_exclusive_locked(&sem->wait, &wait);
  67
  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 in
  75                  * the wait_queue_head.
  76                  */
  77                 if (!atomic_add_negative(sleepers - 1, &sem->count)) {
  78                         sem->sleepers = 0;
  79                         break;
  80                 }
  81                 sem->sleepers = 1;      /* us - see -1 above */
  82                 spin_unlock_irqrestore(&sem->wait.lock, flags);
  83
  84                 schedule();
  85
  86                 spin_lock_irqsave(&sem->wait.lock, flags);
  87                 tsk->state = TASK_UNINTERRUPTIBLE;
  88         }
  89         remove_wait_queue_locked(&sem->wait, &wait);
  90         wake_up_locked(&sem->wait);
  91         spin_unlock_irqrestore(&sem->wait.lock, flags);
  92         tsk->state = TASK_RUNNING;
  93 }
  94
  95 int __sched __down_interruptible(struct semaphore * sem)
  96 {
  97         int retval = 0;
  98         struct task_struct *tsk = current;
  99         DECLARE_WAITQUEUE(wait, tsk);
 100         unsigned long flags;
 101
 102         tsk->state = TASK_INTERRUPTIBLE;
 103         spin_lock_irqsave(&sem->wait.lock, flags);
 104         add_wait_queue_exclusive_locked(&sem->wait, &wait);
 105
 106         sem->sleepers++;
 107         for (;;) {
 108                 int sleepers = sem->sleepers;
 109
 110                 /*
 111                  * With signals pending, this turns into
 112                  * the trylock failure case - we won't be
 113                  * sleeping, and we* can't get the lock as
 114                  * it has contention. Just correct the count
 115                  * and exit.
 116                  */
 117                 if (signal_pending(current)) {
 118                         retval = -EINTR;
 119                         sem->sleepers = 0;
 120                         atomic_add(sleepers, &sem->count);
 121                         break;
 122                 }
 123
 124                 /*
 125                  * Add "everybody else" into it. They aren't
 126                  * playing, because we own the spinlock in
 127                  * wait_queue_head. The "-1" is because we're
 128                  * still hoping to get the semaphore.
 129                  */
 130                 if (!atomic_add_negative(sleepers - 1, &sem->count)) {
 131                         sem->sleepers = 0;
 132                         break;
 133                 }
 134                 sem->sleepers = 1;      /* us - see -1 above */
 135                 spin_unlock_irqrestore(&sem->wait.lock, flags);
 136
 137                 schedule();
 138
 139                 spin_lock_irqsave(&sem->wait.lock, flags);
 140                 tsk->state = TASK_INTERRUPTIBLE;
 141         }
 142         remove_wait_queue_locked(&sem->wait, &wait);
 143         wake_up_locked(&sem->wait);
 144         spin_unlock_irqrestore(&sem->wait.lock, flags);
 145
 146         tsk->state = TASK_RUNNING;
 147         return retval;
 148 }
 149
 150 /*
 151  * Trylock failed - make sure we correct for
 152  * having decremented the count.
 153  *
 154  * We could have done the trylock with a
 155  * single "cmpxchg" without failure cases,
 156  * but then it wouldn't work on a 386.
 157  */
 158 int __down_trylock(struct semaphore * sem)
 159 {
 160         int sleepers;
 161         unsigned long flags;
 162
 163         spin_lock_irqsave(&sem->wait.lock, flags);
 164         sleepers = sem->sleepers + 1;
 165         sem->sleepers = 0;
 166
 167         /*
 168          * Add "everybody else" and us into it. They aren't
 169          * playing, because we own the spinlock in the
 170          * wait_queue_head.
 171          */
 172         if (!atomic_add_negative(sleepers, &sem->count)) {
 173                 wake_up_locked(&sem->wait);
 174         }
 175
 176         spin_unlock_irqrestore(&sem->wait.lock, flags);
 177         return 1;
 178 }
 179
 180