/* * 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; }