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/arch/cris/kernel/semaphore.c

https://bitbucket.org/evzijst/gittest
C | 130 lines | 54 code | 10 blank | 66 comment | 5 complexity | a05c5268d180b3346361405ac89dc029 MD5 | raw file
  1/*
  2 *  Generic semaphore code. Buyer beware. Do your own
  3 * specific changes in <asm/semaphore-helper.h>
  4 */
  5
  6#include <linux/sched.h>
  7#include <linux/init.h>
  8#include <asm/semaphore-helper.h>
  9
 10/*
 11 * Semaphores are implemented using a two-way counter:
 12 * The "count" variable is decremented for each process
 13 * that tries to sleep, while the "waking" variable is
 14 * incremented when the "up()" code goes to wake up waiting
 15 * processes.
 16 *
 17 * Notably, the inline "up()" and "down()" functions can
 18 * efficiently test if they need to do any extra work (up
 19 * needs to do something only if count was negative before
 20 * the increment operation.
 21 *
 22 * waking_non_zero() (from asm/semaphore.h) must execute
 23 * atomically.
 24 *
 25 * When __up() is called, the count was negative before
 26 * incrementing it, and we need to wake up somebody.
 27 *
 28 * This routine adds one to the count of processes that need to
 29 * wake up and exit.  ALL waiting processes actually wake up but
 30 * only the one that gets to the "waking" field first will gate
 31 * through and acquire the semaphore.  The others will go back
 32 * to sleep.
 33 *
 34 * Note that these functions are only called when there is
 35 * contention on the lock, and as such all this is the
 36 * "non-critical" part of the whole semaphore business. The
 37 * critical part is the inline stuff in <asm/semaphore.h>
 38 * where we want to avoid any extra jumps and calls.
 39 */
 40void __up(struct semaphore *sem)
 41{
 42	wake_one_more(sem);
 43	wake_up(&sem->wait);
 44}
 45
 46/*
 47 * Perform the "down" function.  Return zero for semaphore acquired,
 48 * return negative for signalled out of the function.
 49 *
 50 * If called from __down, the return is ignored and the wait loop is
 51 * not interruptible.  This means that a task waiting on a semaphore
 52 * using "down()" cannot be killed until someone does an "up()" on
 53 * the semaphore.
 54 *
 55 * If called from __down_interruptible, the return value gets checked
 56 * upon return.  If the return value is negative then the task continues
 57 * with the negative value in the return register (it can be tested by
 58 * the caller).
 59 *
 60 * Either form may be used in conjunction with "up()".
 61 *
 62 */
 63
 64#define DOWN_VAR				\
 65	struct task_struct *tsk = current;	\
 66	wait_queue_t wait;			\
 67	init_waitqueue_entry(&wait, tsk);
 68
 69#define DOWN_HEAD(task_state)						\
 70									\
 71									\
 72	tsk->state = (task_state);					\
 73	add_wait_queue(&sem->wait, &wait);				\
 74									\
 75	/*								\
 76	 * Ok, we're set up.  sem->count is known to be less than zero	\
 77	 * so we must wait.						\
 78	 *								\
 79	 * We can let go the lock for purposes of waiting.		\
 80	 * We re-acquire it after awaking so as to protect		\
 81	 * all semaphore operations.					\
 82	 *								\
 83	 * If "up()" is called before we call waking_non_zero() then	\
 84	 * we will catch it right away.  If it is called later then	\
 85	 * we will have to go through a wakeup cycle to catch it.	\
 86	 *								\
 87	 * Multiple waiters contend for the semaphore lock to see	\
 88	 * who gets to gate through and who has to wait some more.	\
 89	 */								\
 90	for (;;) {
 91
 92#define DOWN_TAIL(task_state)			\
 93		tsk->state = (task_state);	\
 94	}					\
 95	tsk->state = TASK_RUNNING;		\
 96	remove_wait_queue(&sem->wait, &wait);
 97
 98void __sched __down(struct semaphore * sem)
 99{
100	DOWN_VAR
101	DOWN_HEAD(TASK_UNINTERRUPTIBLE)
102	if (waking_non_zero(sem))
103		break;
104	schedule();
105	DOWN_TAIL(TASK_UNINTERRUPTIBLE)
106}
107
108int __sched __down_interruptible(struct semaphore * sem)
109{
110	int ret = 0;
111	DOWN_VAR
112	DOWN_HEAD(TASK_INTERRUPTIBLE)
113
114	ret = waking_non_zero_interruptible(sem, tsk);
115	if (ret)
116	{
117		if (ret == 1)
118			/* ret != 0 only if we get interrupted -arca */
119			ret = 0;
120		break;
121	}
122	schedule();
123	DOWN_TAIL(TASK_INTERRUPTIBLE)
124	return ret;
125}
126
127int __down_trylock(struct semaphore * sem)
128{
129	return waking_non_zero_trylock(sem);
130}