lib/semaphore-sleepers.c

   1 /*
   2  * i386 and 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/sched.h>
  16 #include <linux/err.h>
  17 #include <linux/init.h>
  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 protected
  32  * by the spinlock in the semaphore's waitqueue head.
  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 fastcall void __up(struct semaphore *sem)
  52 {
  53         wake_up(&sem->wait);
  54 }
  55
  56 fastcall void __sched __down(struct semaphore * sem)
  57 {
  58         struct task_struct *tsk = current;
  59         DECLARE_WAITQUEUE(wait, tsk);
  60         unsigned long flags;
  61
  62         tsk->state = TASK_UNINTERRUPTIBLE;
  63         spin_lock_irqsave(&sem->wait.lock, flags);
  64         add_wait_queue_exclusive_locked(&sem->wait, &wait);
  65
  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 in
  73                  * the wait_queue_head.
  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_irqrestore(&sem->wait.lock, flags);
  81
  82                 schedule();
  83
  84                 spin_lock_irqsave(&sem->wait.lock, flags);
  85                 tsk->state = TASK_UNINTERRUPTIBLE;
  86         }
  87         remove_wait_queue_locked(&sem->wait, &wait);
  88         wake_up_locked(&sem->wait);
  89         spin_unlock_irqrestore(&sem->wait.lock, flags);
  90         tsk->state = TASK_RUNNING;
  91 }
  92
  93 fastcall int __sched __down_interruptible(struct semaphore * sem)
  94 {
  95         int retval = 0;
  96         struct task_struct *tsk = current;
  97         DECLARE_WAITQUEUE(wait, tsk);
  98         unsigned long flags;
  99
 100         tsk->state = TASK_INTERRUPTIBLE;
 101         spin_lock_irqsave(&sem->wait.lock, flags);
 102         add_wait_queue_exclusive_locked(&sem->wait, &wait);
 103
 104         sem->sleepers++;
 105         for (;;) {
 106                 int sleepers = sem->sleepers;
 107
 108                 /*
 109                  * With signals pending, this turns into
 110                  * the trylock failure case - we won't be
 111                  * sleeping, and we* can't get the lock as
 112                  * it has contention. Just correct the count
 113                  * and exit.
 114                  */
 115                 if (signal_pending(current)) {
 116                         retval = -EINTR;
 117                         sem->sleepers = 0;
 118                         atomic_add(sleepers, &sem->count);
 119                         break;
 120                 }
 121
 122                 /*
 123                  * Add "everybody else" into it. They aren't
 124                  * playing, because we own the spinlock in
 125                  * wait_queue_head. The "-1" is because we're
 126                  * still hoping to get the semaphore.
 127                  */
 128                 if (!atomic_add_negative(sleepers - 1, &sem->count)) {
 129                         sem->sleepers = 0;
 130                         break;
 131                 }
 132                 sem->sleepers = 1;      /* us - see -1 above */
 133                 spin_unlock_irqrestore(&sem->wait.lock, flags);
 134
 135                 schedule();
 136
 137                 spin_lock_irqsave(&sem->wait.lock, flags);
 138                 tsk->state = TASK_INTERRUPTIBLE;
 139         }
 140         remove_wait_queue_locked(&sem->wait, &wait);
 141         wake_up_locked(&sem->wait);
 142         spin_unlock_irqrestore(&sem->wait.lock, flags);
 143
 144         tsk->state = TASK_RUNNING;
 145         return retval;
 146 }
 147
 148 /*
 149  * Trylock failed - make sure we correct for
 150  * having decremented the count.
 151  *
 152  * We could have done the trylock with a
 153  * single "cmpxchg" without failure cases,
 154  * but then it wouldn't work on a 386.
 155  */
 156 fastcall int __down_trylock(struct semaphore * sem)
 157 {
 158         int sleepers;
 159         unsigned long flags;
 160
 161         spin_lock_irqsave(&sem->wait.lock, flags);
 162         sleepers = sem->sleepers + 1;
 163         sem->sleepers = 0;
 164
 165         /*
 166          * Add "everybody else" and us into it. They aren't
 167          * playing, because we own the spinlock in the
 168          * wait_queue_head.
 169          */
 170         if (!atomic_add_negative(sleepers, &sem->count)) {
 171                 wake_up_locked(&sem->wait);
 172         }
 173
 174         spin_unlock_irqrestore(&sem->wait.lock, flags);
 175         return 1;
 176 }