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![Jörn Engel](/assets/img/avatar_default.png)
Signed-off-by: Jörn Engel <joern@wohnheim.fh-wedel.de> Signed-off-by: Adrian Bunk <bunk@stusta.de>
176 lines
4.7 KiB
C
176 lines
4.7 KiB
C
/*
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* i386 and x86-64 semaphore implementation.
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*
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* (C) Copyright 1999 Linus Torvalds
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*
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* Portions Copyright 1999 Red Hat, Inc.
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*
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* This program is free software; you can redistribute it and/or
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* modify it under the terms of the GNU General Public License
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* as published by the Free Software Foundation; either version
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* 2 of the License, or (at your option) any later version.
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*
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* rw semaphores implemented November 1999 by Benjamin LaHaise <bcrl@kvack.org>
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*/
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#include <linux/sched.h>
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#include <linux/err.h>
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#include <linux/init.h>
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#include <asm/semaphore.h>
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/*
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* Semaphores are implemented using a two-way counter:
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* The "count" variable is decremented for each process
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* that tries to acquire the semaphore, while the "sleeping"
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* variable is a count of such acquires.
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*
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* Notably, the inline "up()" and "down()" functions can
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* efficiently test if they need to do any extra work (up
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* needs to do something only if count was negative before
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* the increment operation.
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*
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* "sleeping" and the contention routine ordering is protected
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* by the spinlock in the semaphore's waitqueue head.
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*
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* Note that these functions are only called when there is
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* contention on the lock, and as such all this is the
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* "non-critical" part of the whole semaphore business. The
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* critical part is the inline stuff in <asm/semaphore.h>
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* where we want to avoid any extra jumps and calls.
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*/
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/*
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* Logic:
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* - only on a boundary condition do we need to care. When we go
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* from a negative count to a non-negative, we wake people up.
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* - when we go from a non-negative count to a negative do we
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* (a) synchronize with the "sleeper" count and (b) make sure
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* that we're on the wakeup list before we synchronize so that
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* we cannot lose wakeup events.
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*/
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fastcall void __up(struct semaphore *sem)
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{
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wake_up(&sem->wait);
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}
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fastcall void __sched __down(struct semaphore * sem)
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{
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struct task_struct *tsk = current;
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DECLARE_WAITQUEUE(wait, tsk);
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unsigned long flags;
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tsk->state = TASK_UNINTERRUPTIBLE;
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spin_lock_irqsave(&sem->wait.lock, flags);
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add_wait_queue_exclusive_locked(&sem->wait, &wait);
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sem->sleepers++;
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for (;;) {
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int sleepers = sem->sleepers;
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/*
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* Add "everybody else" into it. They aren't
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* playing, because we own the spinlock in
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* the wait_queue_head.
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*/
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if (!atomic_add_negative(sleepers - 1, &sem->count)) {
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sem->sleepers = 0;
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break;
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}
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sem->sleepers = 1; /* us - see -1 above */
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spin_unlock_irqrestore(&sem->wait.lock, flags);
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schedule();
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spin_lock_irqsave(&sem->wait.lock, flags);
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tsk->state = TASK_UNINTERRUPTIBLE;
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}
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remove_wait_queue_locked(&sem->wait, &wait);
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wake_up_locked(&sem->wait);
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spin_unlock_irqrestore(&sem->wait.lock, flags);
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tsk->state = TASK_RUNNING;
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}
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fastcall int __sched __down_interruptible(struct semaphore * sem)
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{
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int retval = 0;
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struct task_struct *tsk = current;
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DECLARE_WAITQUEUE(wait, tsk);
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unsigned long flags;
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tsk->state = TASK_INTERRUPTIBLE;
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spin_lock_irqsave(&sem->wait.lock, flags);
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add_wait_queue_exclusive_locked(&sem->wait, &wait);
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sem->sleepers++;
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for (;;) {
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int sleepers = sem->sleepers;
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/*
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* With signals pending, this turns into
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* the trylock failure case - we won't be
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* sleeping, and we* can't get the lock as
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* it has contention. Just correct the count
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* and exit.
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*/
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if (signal_pending(current)) {
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retval = -EINTR;
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sem->sleepers = 0;
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atomic_add(sleepers, &sem->count);
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break;
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}
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/*
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* Add "everybody else" into it. They aren't
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* playing, because we own the spinlock in
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* wait_queue_head. The "-1" is because we're
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* still hoping to get the semaphore.
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*/
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if (!atomic_add_negative(sleepers - 1, &sem->count)) {
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sem->sleepers = 0;
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break;
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}
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sem->sleepers = 1; /* us - see -1 above */
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spin_unlock_irqrestore(&sem->wait.lock, flags);
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schedule();
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spin_lock_irqsave(&sem->wait.lock, flags);
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tsk->state = TASK_INTERRUPTIBLE;
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}
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remove_wait_queue_locked(&sem->wait, &wait);
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wake_up_locked(&sem->wait);
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spin_unlock_irqrestore(&sem->wait.lock, flags);
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tsk->state = TASK_RUNNING;
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return retval;
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}
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/*
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* Trylock failed - make sure we correct for
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* having decremented the count.
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*
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* We could have done the trylock with a
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* single "cmpxchg" without failure cases,
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* but then it wouldn't work on a 386.
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*/
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fastcall int __down_trylock(struct semaphore * sem)
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{
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int sleepers;
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unsigned long flags;
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spin_lock_irqsave(&sem->wait.lock, flags);
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sleepers = sem->sleepers + 1;
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sem->sleepers = 0;
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/*
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* Add "everybody else" and us into it. They aren't
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* playing, because we own the spinlock in the
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* wait_queue_head.
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*/
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if (!atomic_add_negative(sleepers, &sem->count)) {
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wake_up_locked(&sem->wait);
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}
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spin_unlock_irqrestore(&sem->wait.lock, flags);
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return 1;
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}
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