/* $Id: semaphore.c,v 1.7 2001/04/18 21:06:05 davem Exp $ */ /* sparc32 semaphore implementation, based on i386 version */ #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 (!atomic24_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; atomic24_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 (!atomic24_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. */ 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 (!atomic24_add_negative(sleepers, &sem->count)) wake_up(&sem->wait); spin_unlock_irqrestore(&semaphore_lock, flags); return 1; }