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/*
* Generic semaphore code. Buyer beware. Do your own
* specific changes in <asm/semaphore-helper.h>
*/
#include <linux/sched.h>
#include <asm/semaphore-helper.h>
/*
* Semaphores are implemented using a two-way counter:
* The "count" variable is decremented for each process
* that tries to sleep, while the "waking" variable is
* incremented when the "up()" code goes to wake up waiting
* processes.
*
* 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.
*
* waking_non_zero() (from asm/semaphore.h) must execute
* atomically.
*
* When __up() is called, the count was negative before
* incrementing it, and we need to wake up somebody.
*
* This routine adds one to the count of processes that need to
* wake up and exit. ALL waiting processes actually wake up but
* only the one that gets to the "waking" field first will gate
* through and acquire the semaphore. The others will go back
* to sleep.
*
* 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.
*/
void __up(struct semaphore *sem)
{
wake_one_more(sem);
wake_up(&sem->wait);
}
/*
* Perform the "down" function. Return zero for semaphore acquired,
* return negative for signalled out of the function.
*
* If called from __down, the return is ignored and the wait loop is
* not interruptible. This means that a task waiting on a semaphore
* using "down()" cannot be killed until someone does an "up()" on
* the semaphore.
*
* If called from __down_interruptible, the return value gets checked
* upon return. If the return value is negative then the task continues
* with the negative value in the return register (it can be tested by
* the caller).
*
* Either form may be used in conjunction with "up()".
*
*/
#define DOWN_VAR \
struct task_struct *tsk = current; \
wait_queue_t wait; \
init_waitqueue_entry(&wait, tsk);
#define DOWN_HEAD(task_state) \
\
\
tsk->state = (task_state); \
add_wait_queue(&sem->wait, &wait); \
\
/* \
* Ok, we're set up. sem->count is known to be less than zero \
* so we must wait. \
* \
* We can let go the lock for purposes of waiting. \
* We re-acquire it after awaking so as to protect \
* all semaphore operations. \
* \
* If "up()" is called before we call waking_non_zero() then \
* we will catch it right away. If it is called later then \
* we will have to go through a wakeup cycle to catch it. \
* \
* Multiple waiters contend for the semaphore lock to see \
* who gets to gate through and who has to wait some more. \
*/ \
for (;;) {
#define DOWN_TAIL(task_state) \
tsk->state = (task_state); \
} \
tsk->state = TASK_RUNNING; \
remove_wait_queue(&sem->wait, &wait);
void __sched __down(struct semaphore * sem)
{
DOWN_VAR
DOWN_HEAD(TASK_UNINTERRUPTIBLE)
if (waking_non_zero(sem))
break;
schedule();
DOWN_TAIL(TASK_UNINTERRUPTIBLE)
}
int __sched __down_interruptible(struct semaphore * sem)
{
int ret = 0;
DOWN_VAR
DOWN_HEAD(TASK_INTERRUPTIBLE)
ret = waking_non_zero_interruptible(sem, tsk);
if (ret)
{
if (ret == 1)
/* ret != 0 only if we get interrupted -arca */
ret = 0;
break;
}
schedule();
DOWN_TAIL(TASK_INTERRUPTIBLE)
return ret;
}
int __down_trylock(struct semaphore * sem)
{
return waking_non_zero_trylock(sem);
}
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