/* rwsem.c: R/W semaphores: contention handling functions * * Written by David Howells (dhowells@redhat.com). * Derived from arch/i386/kernel/semaphore.c */#include <linux/rwsem.h>#include <linux/sched.h>#include <linux/module.h>struct rwsem_waiter {	struct list_head	list;	struct task_struct	*task;	unsigned int		flags;#define RWSEM_WAITING_FOR_READ	0x00000001#define RWSEM_WAITING_FOR_WRITE	0x00000002};#if RWSEM_DEBUG#undef rwsemtracevoid rwsemtrace(struct rw_semaphore *sem, const char *str){	printk("sem=%p\n",sem);	printk("(sem)=%08lx\n",sem->count);	if (sem->debug)		printk("[%d] %s({%08lx})\n",current->pid,str,sem->count);}#endif/* * handle the lock being released whilst there are processes blocked on it that can now run * - if we come here, then: *   - the 'active part' of the count (&0x0000ffff) reached zero but has been re-incremented *   - the 'waiting part' of the count (&0xffff0000) is negative (and will still be so) *   - there must be someone on the queue * - the spinlock must be held by the caller * - woken process blocks are discarded from the list after having flags zeroised */static inline struct rw_semaphore *__rwsem_do_wake(struct rw_semaphore *sem){	struct rwsem_waiter *waiter;	struct list_head *next;	signed long oldcount;	int woken, loop;	rwsemtrace(sem,"Entering __rwsem_do_wake");	/* only wake someone up if we can transition the active part of the count from 0 -> 1 */ try_again:	oldcount = rwsem_atomic_update(RWSEM_ACTIVE_BIAS,sem) - RWSEM_ACTIVE_BIAS;	if (oldcount & RWSEM_ACTIVE_MASK)		goto undo;	waiter = list_entry(sem->wait_list.next,struct rwsem_waiter,list);	/* try to grant a single write lock if there's a writer at the front of the queue	 * - note we leave the 'active part' of the count incremented by 1 and the waiting part	 *   incremented by 0x00010000	 */	if (!(waiter->flags & RWSEM_WAITING_FOR_WRITE))		goto readers_only;	list_del(&waiter->list);	waiter->flags = 0;	wake_up_process(waiter->task);	goto out;	/* grant an infinite number of read locks to the readers at the front of the queue	 * - note we increment the 'active part' of the count by the number of readers (less one	 *   for the activity decrement we've already done) before waking any processes up	 */ readers_only:	woken = 0;	do {		woken++;		if (waiter->list.next==&sem->wait_list)			break;		waiter = list_entry(waiter->list.next,struct rwsem_waiter,list);	} while (waiter->flags & RWSEM_WAITING_FOR_READ);	loop = woken;	woken *= RWSEM_ACTIVE_BIAS-RWSEM_WAITING_BIAS;	woken -= RWSEM_ACTIVE_BIAS;	rwsem_atomic_add(woken,sem);	next = sem->wait_list.next;	for (; loop>0; loop--) {		waiter = list_entry(next,struct rwsem_waiter,list);		next = waiter->list.next;		waiter->flags = 0;		wake_up_process(waiter->task);	}	sem->wait_list.next = next;	next->prev = &sem->wait_list; out:	rwsemtrace(sem,"Leaving __rwsem_do_wake");	return sem;	/* undo the change to count, but check for a transition 1->0 */ undo:	if (rwsem_atomic_update(-RWSEM_ACTIVE_BIAS,sem)!=0)		goto out;	goto try_again;}/* * wait for a lock to be granted */static inline struct rw_semaphore *rwsem_down_failed_common(struct rw_semaphore *sem,								 struct rwsem_waiter *waiter,								 signed long adjustment){	struct task_struct *tsk = current;	signed long count;	set_task_state(tsk,TASK_UNINTERRUPTIBLE);	/* set up my own style of waitqueue */	spin_lock(&sem->wait_lock);	waiter->task = tsk;	list_add_tail(&waiter->list,&sem->wait_list);	/* note that we're now waiting on the lock, but no longer actively read-locking */	count = rwsem_atomic_update(adjustment,sem);	/* if there are no longer active locks, wake the front queued process(es) up	 * - it might even be this process, since the waker takes a more active part	 */	if (!(count & RWSEM_ACTIVE_MASK))		sem = __rwsem_do_wake(sem);	spin_unlock(&sem->wait_lock);	/* wait to be given the lock */	for (;;) {		if (!waiter->flags)			break;		schedule();		set_task_state(tsk, TASK_UNINTERRUPTIBLE);	}	tsk->state = TASK_RUNNING;	return sem;}/* * wait for the read lock to be granted */struct rw_semaphore *rwsem_down_read_failed(struct rw_semaphore *sem){	struct rwsem_waiter waiter;	rwsemtrace(sem,"Entering rwsem_down_read_failed");	waiter.flags = RWSEM_WAITING_FOR_READ;	rwsem_down_failed_common(sem,&waiter,RWSEM_WAITING_BIAS-RWSEM_ACTIVE_BIAS);	rwsemtrace(sem,"Leaving rwsem_down_read_failed");	return sem;}/* * wait for the write lock to be granted */struct rw_semaphore *rwsem_down_write_failed(struct rw_semaphore *sem){	struct rwsem_waiter waiter;	rwsemtrace(sem,"Entering rwsem_down_write_failed");	waiter.flags = RWSEM_WAITING_FOR_WRITE;	rwsem_down_failed_common(sem,&waiter,-RWSEM_ACTIVE_BIAS);	rwsemtrace(sem,"Leaving rwsem_down_write_failed");	return sem;}/* * handle waking up a waiter on the semaphore * - up_read has decremented the active part of the count if we come here */struct rw_semaphore *rwsem_wake(struct rw_semaphore *sem){	rwsemtrace(sem,"Entering rwsem_wake");	spin_lock(&sem->wait_lock);	/* do nothing if list empty */	if (!list_empty(&sem->wait_list))		sem = __rwsem_do_wake(sem);	spin_unlock(&sem->wait_lock);	rwsemtrace(sem,"Leaving rwsem_wake");	return sem;}EXPORT_SYMBOL_NOVERS(rwsem_down_read_failed);EXPORT_SYMBOL_NOVERS(rwsem_down_write_failed);EXPORT_SYMBOL_NOVERS(rwsem_wake);#if RWSEM_DEBUGEXPORT_SYMBOL(rwsemtrace);#endif