Commit | Line | Data |
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1da177e4 LT |
1 | /* |
2 | * Semaphore implementation Copyright (c) 2001 Matthew Wilcox, Hewlett-Packard | |
3 | */ | |
4 | ||
5 | #include <linux/sched.h> | |
6 | #include <linux/spinlock.h> | |
7 | #include <linux/errno.h> | |
8 | #include <linux/init.h> | |
9 | ||
10 | /* | |
11 | * Semaphores are complex as we wish to avoid using two variables. | |
12 | * `count' has multiple roles, depending on its value. If it is positive | |
13 | * or zero, there are no waiters. The functions here will never be | |
14 | * called; see <asm/semaphore.h> | |
15 | * | |
16 | * When count is -1 it indicates there is at least one task waiting | |
17 | * for the semaphore. | |
18 | * | |
19 | * When count is less than that, there are '- count - 1' wakeups | |
20 | * pending. ie if it has value -3, there are 2 wakeups pending. | |
21 | * | |
22 | * Note that these functions are only called when there is contention | |
23 | * on the lock, and as such all this is the "non-critical" part of the | |
24 | * whole semaphore business. The critical part is the inline stuff in | |
25 | * <asm/semaphore.h> where we want to avoid any extra jumps and calls. | |
26 | */ | |
27 | void __up(struct semaphore *sem) | |
28 | { | |
29 | sem->count--; | |
30 | wake_up(&sem->wait); | |
31 | } | |
32 | ||
33 | #define wakers(count) (-1 - count) | |
34 | ||
35 | #define DOWN_HEAD \ | |
36 | int ret = 0; \ | |
37 | DECLARE_WAITQUEUE(wait, current); \ | |
38 | \ | |
39 | /* Note that someone is waiting */ \ | |
40 | if (sem->count == 0) \ | |
41 | sem->count = -1; \ | |
42 | \ | |
43 | /* protected by the sentry still -- use unlocked version */ \ | |
44 | wait.flags = WQ_FLAG_EXCLUSIVE; \ | |
45 | __add_wait_queue_tail(&sem->wait, &wait); \ | |
46 | lost_race: \ | |
47 | spin_unlock_irq(&sem->sentry); \ | |
48 | ||
49 | #define DOWN_TAIL \ | |
50 | spin_lock_irq(&sem->sentry); \ | |
51 | if (wakers(sem->count) == 0 && ret == 0) \ | |
52 | goto lost_race; /* Someone stole our wakeup */ \ | |
53 | __remove_wait_queue(&sem->wait, &wait); \ | |
54 | current->state = TASK_RUNNING; \ | |
55 | if (!waitqueue_active(&sem->wait) && (sem->count < 0)) \ | |
56 | sem->count = wakers(sem->count); | |
57 | ||
58 | #define UPDATE_COUNT \ | |
59 | sem->count += (sem->count < 0) ? 1 : - 1; | |
60 | ||
61 | ||
62 | void __sched __down(struct semaphore * sem) | |
63 | { | |
64 | DOWN_HEAD | |
65 | ||
66 | for(;;) { | |
67 | set_task_state(current, TASK_UNINTERRUPTIBLE); | |
68 | /* we can _read_ this without the sentry */ | |
69 | if (sem->count != -1) | |
70 | break; | |
71 | schedule(); | |
72 | } | |
73 | ||
74 | DOWN_TAIL | |
75 | UPDATE_COUNT | |
76 | } | |
77 | ||
78 | int __sched __down_interruptible(struct semaphore * sem) | |
79 | { | |
80 | DOWN_HEAD | |
81 | ||
82 | for(;;) { | |
83 | set_task_state(current, TASK_INTERRUPTIBLE); | |
84 | /* we can _read_ this without the sentry */ | |
85 | if (sem->count != -1) | |
86 | break; | |
87 | ||
88 | if (signal_pending(current)) { | |
89 | ret = -EINTR; | |
90 | break; | |
91 | } | |
92 | schedule(); | |
93 | } | |
94 | ||
95 | DOWN_TAIL | |
96 | ||
97 | if (!ret) { | |
98 | UPDATE_COUNT | |
99 | } | |
100 | ||
101 | return ret; | |
102 | } |