FreeBSD/Linux Kernel Cross Reference
sys/lib/semaphore-sleepers.c

     /*
      * i386 and x86-64 semaphore implementation.
      *
      * (C) Copyright 1999 Linus Torvalds
      *
      * Portions Copyright 1999 Red Hat, Inc.
      *
      *      This program is free software; you can redistribute it and/or
      *      modify it under the terms of the GNU General Public License
     *      as published by the Free Software Foundation; either version
     *      2 of the License, or (at your option) any later version.
     *
     * rw semaphores implemented November 1999 by Benjamin LaHaise <bcrl@kvack.org>
     */
    #include <linux/sched.h>
    #include <linux/err.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 spinlock in the semaphore's waitqueue head.
     *
     * 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);
    }
    
    void __sched __down(struct semaphore *sem)
    {
            struct task_struct *tsk = current;
            DECLARE_WAITQUEUE(wait, tsk);
            unsigned long flags;
    
            tsk->state = TASK_UNINTERRUPTIBLE;
            spin_lock_irqsave(&sem->wait.lock, flags);
            add_wait_queue_exclusive_locked(&sem->wait, &wait);
    
            sem->sleepers++;
            for (;;) {
                    int sleepers = sem->sleepers;
    
                    /*
                     * Add "everybody else" into it. They aren't
                     * playing, because we own the spinlock in
                     * the wait_queue_head.
                     */
                    if (!atomic_add_negative(sleepers - 1, &sem->count)) {
                            sem->sleepers = 0;
                            break;
                    }
                    sem->sleepers = 1;      /* us - see -1 above */
                    spin_unlock_irqrestore(&sem->wait.lock, flags);
    
                    schedule();
    
                    spin_lock_irqsave(&sem->wait.lock, flags);
                    tsk->state = TASK_UNINTERRUPTIBLE;
            }
            remove_wait_queue_locked(&sem->wait, &wait);
            wake_up_locked(&sem->wait);
            spin_unlock_irqrestore(&sem->wait.lock, flags);
            tsk->state = TASK_RUNNING;
    }
    
    int __sched __down_interruptible(struct semaphore *sem)
    {
            int retval = 0;
            struct task_struct *tsk = current;
            DECLARE_WAITQUEUE(wait, tsk);
            unsigned long flags;
    
           tsk->state = TASK_INTERRUPTIBLE;
           spin_lock_irqsave(&sem->wait.lock, flags);
           add_wait_queue_exclusive_locked(&sem->wait, &wait);
   
           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 in
                    * wait_queue_head. The "-1" is because we're
                    * still hoping to get the semaphore.
                    */
                   if (!atomic_add_negative(sleepers - 1, &sem->count)) {
                           sem->sleepers = 0;
                           break;
                   }
                   sem->sleepers = 1;      /* us - see -1 above */
                   spin_unlock_irqrestore(&sem->wait.lock, flags);
   
                   schedule();
   
                   spin_lock_irqsave(&sem->wait.lock, flags);
                   tsk->state = TASK_INTERRUPTIBLE;
           }
           remove_wait_queue_locked(&sem->wait, &wait);
           wake_up_locked(&sem->wait);
           spin_unlock_irqrestore(&sem->wait.lock, flags);
   
           tsk->state = TASK_RUNNING;
           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(&sem->wait.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 in the
            * wait_queue_head.
            */
           if (!atomic_add_negative(sleepers, &sem->count)) {
                   wake_up_locked(&sem->wait);
           }
   
           spin_unlock_irqrestore(&sem->wait.lock, flags);
           return 1;
   }

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