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