File: /usr/src/linux/arch/m68k/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/config.h>
7 #include <linux/sched.h>
8 #include <asm/semaphore-helper.h>
9
10 #ifndef CONFIG_RMW_INSNS
11 spinlock_t semaphore_wake_lock;
12 #endif
13
14 /*
15 * Semaphores are implemented using a two-way counter:
16 * The "count" variable is decremented for each process
17 * that tries to sleep, while the "waking" variable is
18 * incremented when the "up()" code goes to wake up waiting
19 * processes.
20 *
21 * Notably, the inline "up()" and "down()" functions can
22 * efficiently test if they need to do any extra work (up
23 * needs to do something only if count was negative before
24 * the increment operation.
25 *
26 * waking_non_zero() (from asm/semaphore.h) must execute
27 * atomically.
28 *
29 * When __up() is called, the count was negative before
30 * incrementing it, and we need to wake up somebody.
31 *
32 * This routine adds one to the count of processes that need to
33 * wake up and exit. ALL waiting processes actually wake up but
34 * only the one that gets to the "waking" field first will gate
35 * through and acquire the semaphore. The others will go back
36 * to sleep.
37 *
38 * Note that these functions are only called when there is
39 * contention on the lock, and as such all this is the
40 * "non-critical" part of the whole semaphore business. The
41 * critical part is the inline stuff in <asm/semaphore.h>
42 * where we want to avoid any extra jumps and calls.
43 */
44 void __up(struct semaphore *sem)
45 {
46 wake_one_more(sem);
47 wake_up(&sem->wait);
48 }
49
50 /*
51 * Perform the "down" function. Return zero for semaphore acquired,
52 * return negative for signalled out of the function.
53 *
54 * If called from __down, the return is ignored and the wait loop is
55 * not interruptible. This means that a task waiting on a semaphore
56 * using "down()" cannot be killed until someone does an "up()" on
57 * the semaphore.
58 *
59 * If called from __down_interruptible, the return value gets checked
60 * upon return. If the return value is negative then the task continues
61 * with the negative value in the return register (it can be tested by
62 * the caller).
63 *
64 * Either form may be used in conjunction with "up()".
65 *
66 */
67
68
69 #define DOWN_HEAD(task_state) \
70 \
71 \
72 current->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 current->state = (task_state); \
94 } \
95 current->state = TASK_RUNNING; \
96 remove_wait_queue(&sem->wait, &wait);
97
98 void __down(struct semaphore * sem)
99 {
100 DECLARE_WAITQUEUE(wait, current);
101
102 DOWN_HEAD(TASK_UNINTERRUPTIBLE)
103 if (waking_non_zero(sem))
104 break;
105 schedule();
106 DOWN_TAIL(TASK_UNINTERRUPTIBLE)
107 }
108
109 int __down_interruptible(struct semaphore * sem)
110 {
111 DECLARE_WAITQUEUE(wait, current);
112 int ret = 0;
113
114 DOWN_HEAD(TASK_INTERRUPTIBLE)
115
116 ret = waking_non_zero_interruptible(sem, current);
117 if (ret)
118 {
119 if (ret == 1)
120 /* ret != 0 only if we get interrupted -arca */
121 ret = 0;
122 break;
123 }
124 schedule();
125 DOWN_TAIL(TASK_INTERRUPTIBLE)
126 return ret;
127 }
128
129 int __down_trylock(struct semaphore * sem)
130 {
131 return waking_non_zero_trylock(sem);
132 }
133