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/*
* ARM semaphore implementation, taken from
*
* i386 semaphore implementation.
*
* (C) Copyright 1999 Linus Torvalds
*
* Modified for ARM by Russell King
*/
#include <linux/sched.h>
#include <linux/errno.h>
#include <asm/semaphore.h>
/*
* Semaphores are implemented using a two-way counter:
* The "count" variable is decremented for each process
* that tries to aquire the semaphore, while the "sleeping"
* variable is a count of such aquires.
*
* 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 spinlock_t semaphore_lock = SPIN_LOCK_UNLOCKED;
void __down(struct semaphore * sem)
{
struct task_struct *tsk = current;
DECLARE_WAITQUEUE(wait, tsk);
tsk->state = TASK_UNINTERRUPTIBLE;
add_wait_queue(&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 (!atomic_add_negative(sleepers - 1, &sem->count)) {
sem->sleepers = 0;
wake_up(&sem->wait);
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;
}
int __down_interruptible(struct semaphore * sem)
{
int retval;
struct task_struct *tsk = current;
DECLARE_WAITQUEUE(wait, tsk);
tsk->state = TASK_INTERRUPTIBLE;
add_wait_queue(&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.
*/
retval = -EINTR;
if (signal_pending(current)) {
sem->sleepers = 0;
if (atomic_add_negative(sleepers, &sem->count))
break;
wake_up(&sem->wait);
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 (!atomic_add_negative(sleepers - 1, &sem->count)) {
wake_up(&sem->wait);
retval = 0;
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);
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;
spin_lock_irq(&semaphore_lock);
sleepers = sem->sleepers + 1;
sem->sleepers = 0;
/*
* Add "everybody else" and us into it. They aren't
* playing, because we own the spinlock.
*/
if (!atomic_add_negative(sleepers, &sem->count))
wake_up(&sem->wait);
spin_unlock_irq(&semaphore_lock);
return 1;
}
/*
* The semaphore operations have a special calling sequence that
* allow us to do a simpler in-line version of them. These routines
* need to convert that sequence back into the C sequence when
* there is contention on the semaphore.
*
* r0 contains the semaphore pointer on entry. Save the C-clobbered
* registers (r0 to r3, ip and lr) except r0 in the cases where it
* is used as a return value..
*/
asm(".align 5
.globl __down_failed
__down_failed:
stmfd sp!, {r0 - r3, ip, lr}
bl __down
ldmfd sp!, {r0 - r3, ip, pc}");
asm(".align 5
.globl __down_interruptible_failed
__down_interruptible_failed:
stmfd sp!, {r1 - r3, ip, lr}
bl __down_interruptible
ldmfd sp!, {r1 - r3, ip, pc}");
asm(".align 5
.globl __down_trylock_failed
__down_trylock_failed:
stmfd sp!, {r1 - r3, ip, lr}
bl __down_trylock
ldmfd sp!, {r1 - r3, ip, pc}");
asm(".align 5
.globl __up_wakeup
__up_wakeup:
stmfd sp!, {r0 - r3, ip, lr}
bl __up
ldmfd sp!, {r0 - r3, ip, pc}");
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