File: /usr/src/linux/arch/mips/kernel/semaphore.c
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 <asm/semaphore-helper.h>
8
9 /*
10 * Semaphores are implemented using a two-way counter:
11 * The "count" variable is decremented for each process
12 * that tries to sleep, while the "waking" variable is
13 * incremented when the "up()" code goes to wake up waiting
14 * processes.
15 *
16 * Notably, the inline "up()" and "down()" functions can
17 * efficiently test if they need to do any extra work (up
18 * needs to do something only if count was negative before
19 * the increment operation.
20 *
21 * waking_non_zero() (from asm/semaphore.h) must execute
22 * atomically.
23 *
24 * When __up() is called, the count was negative before
25 * incrementing it, and we need to wake up somebody.
26 *
27 * This routine adds one to the count of processes that need to
28 * wake up and exit. ALL waiting processes actually wake up but
29 * only the one that gets to the "waking" field first will gate
30 * through and acquire the semaphore. The others will go back
31 * to sleep.
32 *
33 * Note that these functions are only called when there is
34 * contention on the lock, and as such all this is the
35 * "non-critical" part of the whole semaphore business. The
36 * critical part is the inline stuff in <asm/semaphore.h>
37 * where we want to avoid any extra jumps and calls.
38 */
39 void __up(struct semaphore *sem)
40 {
41 wake_one_more(sem);
42 wake_up(&sem->wait);
43 }
44
45 /*
46 * Perform the "down" function. Return zero for semaphore acquired,
47 * return negative for signalled out of the function.
48 *
49 * If called from __down, the return is ignored and the wait loop is
50 * not interruptible. This means that a task waiting on a semaphore
51 * using "down()" cannot be killed until someone does an "up()" on
52 * the semaphore.
53 *
54 * If called from __down_interruptible, the return value gets checked
55 * upon return. If the return value is negative then the task continues
56 * with the negative value in the return register (it can be tested by
57 * the caller).
58 *
59 * Either form may be used in conjunction with "up()".
60 *
61 */
62
63 #define DOWN_VAR \
64 struct task_struct *tsk = current; \
65 wait_queue_t wait; \
66 init_waitqueue_entry(&wait, tsk);
67
68 #define DOWN_HEAD(task_state) \
69 \
70 \
71 tsk->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 tsk->state = (task_state); \
93 } \
94 tsk->state = TASK_RUNNING; \
95 remove_wait_queue(&sem->wait, &wait);
96
97 void __down(struct semaphore * sem)
98 {
99 DOWN_VAR
100 DOWN_HEAD(TASK_UNINTERRUPTIBLE)
101 if (waking_non_zero(sem))
102 break;
103 schedule();
104 DOWN_TAIL(TASK_UNINTERRUPTIBLE)
105 }
106
107 int __down_interruptible(struct semaphore * sem)
108 {
109 int ret = 0;
110 DOWN_VAR
111 DOWN_HEAD(TASK_INTERRUPTIBLE)
112
113 ret = waking_non_zero_interruptible(sem, tsk);
114 if (ret)
115 {
116 if (ret == 1)
117 /* ret != 0 only if we get interrupted -arca */
118 ret = 0;
119 break;
120 }
121 schedule();
122 DOWN_TAIL(TASK_INTERRUPTIBLE)
123 return ret;
124 }
125
126 int __down_trylock(struct semaphore * sem)
127 {
128 return waking_non_zero_trylock(sem);
129 }
130