/* * Generic semaphore code. Buyer beware. Do your own * specific changes in <asm/semaphore-helper.h> */ #include <linux/config.h> #include <linux/sched.h> #include <linux/err.h> #include <linux/init.h> #include <asm/semaphore-helper.h> #ifndef CONFIG_RMW_INSNS spinlock_t semaphore_wake_lock; #endif /* * Semaphores are implemented using a two-way counter: * The "count" variable is decremented for each process * that tries to sleep, while the "waking" variable is * incremented when the "up()" code goes to wake up waiting * processes. * * Notably, the inline "up()" and "down()" functions can * efficiently test if they need to do any extra work (up * needs to do something only if count was negative before * the increment operation. * * waking_non_zero() (from asm/semaphore.h) must execute * atomically. * * When __up() is called, the count was negative before * incrementing it, and we need to wake up somebody. * * This routine adds one to the count of processes that need to * wake up and exit. ALL waiting processes actually wake up but * only the one that gets to the "waking" field first will gate * through and acquire the semaphore. The others will go back * to sleep. * * Note that these functions are only called when there is * contention on the lock, and as such all this is the * "non-critical" part of the whole semaphore business. The * critical part is the inline stuff in <asm/semaphore.h> * where we want to avoid any extra jumps and calls. */ void __up(struct semaphore *sem) { wake_one_more(sem); wake_up(&sem->wait); } /* * Perform the "down" function. Return zero for semaphore acquired, * return negative for signalled out of the function. * * If called from __down, the return is ignored and the wait loop is * not interruptible. This means that a task waiting on a semaphore * using "down()" cannot be killed until someone does an "up()" on * the semaphore. * * If called from __down_interruptible, the return value gets checked * upon return. If the return value is negative then the task continues * with the negative value in the return register (it can be tested by * the caller). * * Either form may be used in conjunction with "up()". * */ #define DOWN_HEAD(task_state) \ \ \ current->state = (task_state); \ add_wait_queue(&sem->wait, &wait); \ \ /* \ * Ok, we're set up. sem->count is known to be less than zero \ * so we must wait. \ * \ * We can let go the lock for purposes of waiting. \ * We re-acquire it after awaking so as to protect \ * all semaphore operations. \ * \ * If "up()" is called before we call waking_non_zero() then \ * we will catch it right away. If it is called later then \ * we will have to go through a wakeup cycle to catch it. \ * \ * Multiple waiters contend for the semaphore lock to see \ * who gets to gate through and who has to wait some more. \ */ \ for (;;) { #define DOWN_TAIL(task_state) \ current->state = (task_state); \ } \ current->state = TASK_RUNNING; \ remove_wait_queue(&sem->wait, &wait); void __sched __down(struct semaphore * sem) { DECLARE_WAITQUEUE(wait, current); DOWN_HEAD(TASK_UNINTERRUPTIBLE) if (waking_non_zero(sem)) break; schedule(); DOWN_TAIL(TASK_UNINTERRUPTIBLE) } int __sched __down_interruptible(struct semaphore * sem) { DECLARE_WAITQUEUE(wait, current); int ret = 0; DOWN_HEAD(TASK_INTERRUPTIBLE) ret = waking_non_zero_interruptible(sem, current); if (ret) { if (ret == 1) /* ret != 0 only if we get interrupted -arca */ ret = 0; break; } schedule(); DOWN_TAIL(TASK_INTERRUPTIBLE) return ret; } int __down_trylock(struct semaphore * sem) { return waking_non_zero_trylock(sem); }