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Diffstat (limited to 'arch/arm/kernel/semaphore.c')
| -rw-r--r-- | arch/arm/kernel/semaphore.c | 220 |
1 files changed, 220 insertions, 0 deletions
diff --git a/arch/arm/kernel/semaphore.c b/arch/arm/kernel/semaphore.c new file mode 100644 index 0000000..ac423e3 --- /dev/null +++ b/arch/arm/kernel/semaphore.c @@ -0,0 +1,220 @@ +/* + * ARM semaphore implementation, taken from + * + * i386 semaphore implementation. + * + * (C) Copyright 1999 Linus Torvalds + * + * Modified for ARM by Russell King + * + * This program is free software; you can redistribute it and/or modify + * it under the terms of the GNU General Public License version 2 as + * published by the Free Software Foundation. + */ +#include <linux/module.h> +#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 (!atomic_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; + atomic_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 (!atomic_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. + * + * 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(&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 (!atomic_add_negative(sleepers, &sem->count)) + wake_up(&sem->wait); + + spin_unlock_irqrestore(&semaphore_lock, flags); + 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. + * + * ip contains the semaphore pointer on entry. Save the C-clobbered + * registers (r0 to r3 and lr), but not ip, as we use it as a return + * value in some cases.. + */ +asm(" .section .sched.text,\"ax\" \n\ + .align 5 \n\ + .globl __down_failed \n\ +__down_failed: \n\ + stmfd sp!, {r0 - r3, lr} \n\ + mov r0, ip \n\ + bl __down \n\ + ldmfd sp!, {r0 - r3, pc} \n\ + \n\ + .align 5 \n\ + .globl __down_interruptible_failed \n\ +__down_interruptible_failed: \n\ + stmfd sp!, {r0 - r3, lr} \n\ + mov r0, ip \n\ + bl __down_interruptible \n\ + mov ip, r0 \n\ + ldmfd sp!, {r0 - r3, pc} \n\ + \n\ + .align 5 \n\ + .globl __down_trylock_failed \n\ +__down_trylock_failed: \n\ + stmfd sp!, {r0 - r3, lr} \n\ + mov r0, ip \n\ + bl __down_trylock \n\ + mov ip, r0 \n\ + ldmfd sp!, {r0 - r3, pc} \n\ + \n\ + .align 5 \n\ + .globl __up_wakeup \n\ +__up_wakeup: \n\ + stmfd sp!, {r0 - r3, lr} \n\ + mov r0, ip \n\ + bl __up \n\ + ldmfd sp!, {r0 - r3, pc} \n\ + "); + +EXPORT_SYMBOL(__down_failed); +EXPORT_SYMBOL(__down_interruptible_failed); +EXPORT_SYMBOL(__down_trylock_failed); +EXPORT_SYMBOL(__up_wakeup); |