File: /usr/src/linux/arch/parisc/kernel/semaphore.c
1 /*
2 * Just taken from alpha implementation.
3 * This can't work well, perhaps.
4 */
5 /*
6 * Generic semaphore code. Buyer beware. Do your own
7 * specific changes in <asm/semaphore-helper.h>
8 */
9
10 #include <linux/sched.h>
11 #include <asm/semaphore-helper.h>
12
13 /*
14 * Semaphores are implemented using a two-way counter:
15 * The "count" variable is decremented for each process
16 * that tries to sleep, while the "waking" variable is
17 * incremented when the "up()" code goes to wake up waiting
18 * processes.
19 *
20 * Notably, the inline "up()" and "down()" functions can
21 * efficiently test if they need to do any extra work (up
22 * needs to do something only if count was negative before
23 * the increment operation.
24 *
25 * waking_non_zero() (from asm/semaphore.h) must execute
26 * atomically.
27 *
28 * When __up() is called, the count was negative before
29 * incrementing it, and we need to wake up somebody.
30 *
31 * This routine adds one to the count of processes that need to
32 * wake up and exit. ALL waiting processes actually wake up but
33 * only the one that gets to the "waking" field first will gate
34 * through and acquire the semaphore. The others will go back
35 * to sleep.
36 *
37 * Note that these functions are only called when there is
38 * contention on the lock, and as such all this is the
39 * "non-critical" part of the whole semaphore business. The
40 * critical part is the inline stuff in <asm/semaphore.h>
41 * where we want to avoid any extra jumps and calls.
42 */
43 void __up(struct semaphore *sem)
44 {
45 wake_one_more(sem);
46 wake_up(&sem->wait);
47 }
48
49 /*
50 * Perform the "down" function. Return zero for semaphore acquired,
51 * return negative for signalled out of the function.
52 *
53 * If called from __down, the return is ignored and the wait loop is
54 * not interruptible. This means that a task waiting on a semaphore
55 * using "down()" cannot be killed until someone does an "up()" on
56 * the semaphore.
57 *
58 * If called from __down_interruptible, the return value gets checked
59 * upon return. If the return value is negative then the task continues
60 * with the negative value in the return register (it can be tested by
61 * the caller).
62 *
63 * Either form may be used in conjunction with "up()".
64 *
65 */
66
67
68 #define DOWN_HEAD(task_state) \
69 \
70 \
71 current->state = (task_state); \
72 add_wait_queue(&sem->wait, &wait); \
73 \
74 /* \
75 * Ok, we're set up. sem->count is known to be less than zero \
76 * so we must wait. \
77 * \
78 * We can let go the lock for purposes of waiting. \
79 * We re-acquire it after awaking so as to protect \
80 * all semaphore operations. \
81 * \
82 * If "up()" is called before we call waking_non_zero() then \
83 * we will catch it right away. If it is called later then \
84 * we will have to go through a wakeup cycle to catch it. \
85 * \
86 * Multiple waiters contend for the semaphore lock to see \
87 * who gets to gate through and who has to wait some more. \
88 */ \
89 for (;;) {
90
91 #define DOWN_TAIL(task_state) \
92 current->state = (task_state); \
93 } \
94 current->state = TASK_RUNNING; \
95 remove_wait_queue(&sem->wait, &wait);
96
97 void __down(struct semaphore * sem)
98 {
99 DECLARE_WAITQUEUE(wait, current);
100
101 DOWN_HEAD(TASK_UNINTERRUPTIBLE)
102 if (waking_non_zero(sem))
103 break;
104 schedule();
105 DOWN_TAIL(TASK_UNINTERRUPTIBLE)
106 }
107
108 int __down_interruptible(struct semaphore * sem)
109 {
110 DECLARE_WAITQUEUE(wait, current);
111 int ret = 0;
112
113 DOWN_HEAD(TASK_INTERRUPTIBLE)
114
115 ret = waking_non_zero_interruptible(sem, current);
116 if (ret)
117 {
118 if (ret == 1)
119 /* ret != 0 only if we get interrupted -arca */
120 ret = 0;
121 break;
122 }
123 schedule();
124 DOWN_TAIL(TASK_INTERRUPTIBLE)
125 return ret;
126 }
127
128 int __down_trylock(struct semaphore * sem)
129 {
130 return waking_non_zero_trylock(sem);
131 }
132