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