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/*
* Copyright (c) 2017 Richard Braun.
*
* 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 3 of the License, or
* (at your option) any later version.
*
* This program is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
* GNU General Public License for more details.
*
* You should have received a copy of the GNU General Public License
* along with this program. If not, see <http://www.gnu.org/licenses/>.
*
*
* Semaphores are resource-counting sleeping synchronization objects.
* They are used to synchronize access to resources and signal events.
*
* The main operations supported by semaphores are locking and unlocking.
* A semaphore is implemented as an atomic integer with an initial value.
* Locking a semaphore means decrementing that integer, whereas unlocking
* means incrementing it. Locking can only succeed if the semaphore value
* is strictly greater than 0.
*
* Semaphores should not be used to implement critical sections. Instead,
* use mutexes, which are similar to binary semaphores but with additional
* restrictions that can improve debugging.
*/
#ifndef _KERN_SEMAPHORE_H
#define _KERN_SEMAPHORE_H
#include <kern/assert.h>
#include <kern/error.h>
#include <kern/macros.h>
#define SEMAPHORE_VALUE_MAX 32768
#include <kern/semaphore_i.h>
struct semaphore;
/*
* Initialize a semaphore.
*/
static inline void
semaphore_init(struct semaphore *semaphore, unsigned int value)
{
assert(value <= SEMAPHORE_VALUE_MAX);
semaphore->value = value;
}
/*
* Attempt to lock a semaphore.
*
* This function may not sleep.
*
* Return 0 on success, ERROR_AGAIN if the semaphore could not be decremented.
*/
static inline int
semaphore_trywait(struct semaphore *semaphore)
{
unsigned int prev;
prev = semaphore_dec(semaphore);
if (unlikely(prev == 0)) {
return ERROR_AGAIN;
}
return 0;
}
/*
* Lock a semaphore.
*
* If the semaphore value doesn't allow locking, the calling thread sleeps
* until the semaphore value is incremented.
*/
static inline void
semaphore_wait(struct semaphore *semaphore)
{
unsigned int prev;
prev = semaphore_dec(semaphore);
if (unlikely(prev == 0)) {
semaphore_wait_slow(semaphore);
}
}
/*
* Unlock a semaphore.
*
* If the semaphore value becomes strictly greater than 0, a thread waiting
* on the semaphore is awaken.
*
* A semaphore may be unlocked from interrupt context.
*/
static inline void
semaphore_post(struct semaphore *semaphore)
{
unsigned int prev;
prev = semaphore_inc(semaphore);
if (unlikely(prev == 0)) {
semaphore_post_slow(semaphore);
}
}
/*
* Get the value of a semaphore.
*/
static inline unsigned int
semaphore_getvalue(const struct semaphore *semaphore)
{
return read_once(semaphore->value);
}
#endif /* _KERN_SEMAPHORE_H */
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