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/err.h>
8#include <linux/init.h>
9#include <asm/semaphore-helper.h>
10
11#ifndef CONFIG_RMW_INSNS
12spinlock_t semaphore_wake_lock;
13#endif
14
15/*
16 * Semaphores are implemented using a two-way counter:
17 * The "count" variable is decremented for each process
18 * that tries to sleep, while the "waking" variable is
19 * incremented when the "up()" code goes to wake up waiting
20 * processes.
21 *
22 * Notably, the inline "up()" and "down()" functions can
23 * efficiently test if they need to do any extra work (up
24 * needs to do something only if count was negative before
25 * the increment operation.
26 *
27 * waking_non_zero() (from asm/semaphore.h) must execute
28 * atomically.
29 *
30 * When __up() is called, the count was negative before
31 * incrementing it, and we need to wake up somebody.
32 *
33 * This routine adds one to the count of processes that need to
34 * wake up and exit.  ALL waiting processes actually wake up but
35 * only the one that gets to the "waking" field first will gate
36 * through and acquire the semaphore.  The others will go back
37 * to sleep.
38 *
39 * Note that these functions are only called when there is
40 * contention on the lock, and as such all this is the
41 * "non-critical" part of the whole semaphore business. The
42 * critical part is the inline stuff in <asm/semaphore.h>
43 * where we want to avoid any extra jumps and calls.
44 */
45void __up(struct semaphore *sem)
46{
47	wake_one_more(sem);
48	wake_up(&sem->wait);
49}
50
51/*
52 * Perform the "down" function.  Return zero for semaphore acquired,
53 * return negative for signalled out of the function.
54 *
55 * If called from __down, the return is ignored and the wait loop is
56 * not interruptible.  This means that a task waiting on a semaphore
57 * using "down()" cannot be killed until someone does an "up()" on
58 * the semaphore.
59 *
60 * If called from __down_interruptible, the return value gets checked
61 * upon return.  If the return value is negative then the task continues
62 * with the negative value in the return register (it can be tested by
63 * the caller).
64 *
65 * Either form may be used in conjunction with "up()".
66 *
67 */
68
69
70#define DOWN_HEAD(task_state)						\
71									\
72									\
73	current->state = (task_state);					\
74	add_wait_queue(&sem->wait, &wait);				\
75									\
76	/*								\
77	 * Ok, we're set up.  sem->count is known to be less than zero	\
78	 * so we must wait.						\
79	 *								\
80	 * We can let go the lock for purposes of waiting.		\
81	 * We re-acquire it after awaking so as to protect		\
82	 * all semaphore operations.					\
83	 *								\
84	 * If "up()" is called before we call waking_non_zero() then	\
85	 * we will catch it right away.  If it is called later then	\
86	 * we will have to go through a wakeup cycle to catch it.	\
87	 *								\
88	 * Multiple waiters contend for the semaphore lock to see	\
89	 * who gets to gate through and who has to wait some more.	\
90	 */								\
91	for (;;) {
92
93#define DOWN_TAIL(task_state)			\
94		current->state = (task_state);	\
95	}					\
96	current->state = TASK_RUNNING;		\
97	remove_wait_queue(&sem->wait, &wait);
98
99void __sched __down(struct semaphore * sem)
100{
101	DECLARE_WAITQUEUE(wait, current);
102
103	DOWN_HEAD(TASK_UNINTERRUPTIBLE)
104	if (waking_non_zero(sem))
105		break;
106	schedule();
107	DOWN_TAIL(TASK_UNINTERRUPTIBLE)
108}
109
110int __sched __down_interruptible(struct semaphore * sem)
111{
112	DECLARE_WAITQUEUE(wait, current);
113	int ret = 0;
114
115	DOWN_HEAD(TASK_INTERRUPTIBLE)
116
117	ret = waking_non_zero_interruptible(sem, current);
118	if (ret)
119	{
120		if (ret == 1)
121			/* ret != 0 only if we get interrupted -arca */
122			ret = 0;
123		break;
124	}
125	schedule();
126	DOWN_TAIL(TASK_INTERRUPTIBLE)
127	return ret;
128}
129
130int __down_trylock(struct semaphore * sem)
131{
132	return waking_non_zero_trylock(sem);
133}
134