/* 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/init.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 release when processes blocked on it that can now run * - if we come here from up_xxxx(), then: *   - the 'active part' of count (&0x0000ffff) reached 0 (but may have changed) *   - the 'waiting part' of count (&0xffff0000) is -ve (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 task zeroed * - writers are only woken if downgrading is false */static inline struct rw_semaphore *__rwsem_do_wake(struct rw_semaphore *sem, int downgrading){	struct rwsem_waiter *waiter;	struct task_struct *tsk;	struct list_head *next;	signed long oldcount, woken, loop; 	rwsemtrace(sem, "Entering __rwsem_do_wake"); 	if (downgrading)		goto dont_wake_writers; 	/* if we came through an up_xxxx() call, we only 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; 	/* We must be careful not to touch 'waiter' after we set ->task = NULL.	 * It is an allocated on the waiter's stack and may become invalid at	 * any time after that point (due to a wakeup from another source).	 */	list_del(&waiter->list);	tsk = waiter->task;	smp_mb();	waiter->task = NULL;	wake_up_process(tsk);	put_task_struct(tsk);	goto out; 	/* don't want to wake any writers */ dont_wake_writers:	waiter = list_entry(sem->wait_list.next, struct rwsem_waiter, list);	if (waiter->flags & RWSEM_WAITING_FOR_WRITE)		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 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;	if (!downgrading)		/* we'd already done one increment earlier */		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;		tsk = waiter->task;		smp_mb();		waiter->task = NULL;		wake_up_process(tsk);		put_task_struct(tsk);	} 	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_irq(&sem->wait_lock);	waiter->task = tsk;	get_task_struct(tsk); 	list_add_tail(&waiter->list, &sem->wait_list); 	/* we're now waiting on the lock, but no longer actively read-locking */	count = rwsem_atomic_update(adjustment, sem); 	/* if there are no active locks, wake the front queued process(es) up */	if (!(count & RWSEM_ACTIVE_MASK))		sem = __rwsem_do_wake(sem, 0); 	spin_unlock_irq(&sem->wait_lock); 	/* wait to be given the lock */	for (;;) {		if (!waiter->task)			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 fastcall __sched *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 fastcall __sched *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/up_write has decremented the active part of count if we come here */struct rw_semaphore fastcall *rwsem_wake(struct rw_semaphore *sem){	unsigned long flags; 	rwsemtrace(sem, "Entering rwsem_wake"); 	spin_lock_irqsave(&sem->wait_lock, flags); 	/* do nothing if list empty */	if (!list_empty(&sem->wait_list))		sem = __rwsem_do_wake(sem, 0); 	spin_unlock_irqrestore(&sem->wait_lock, flags); 	rwsemtrace(sem, "Leaving rwsem_wake"); 	return sem;} /* * downgrade a write lock into a read lock * - caller incremented waiting part of count and discovered it still negative * - just wake up any readers at the front of the queue */struct rw_semaphore fastcall *rwsem_downgrade_wake(struct rw_semaphore *sem){	unsigned long flags; 	rwsemtrace(sem, "Entering rwsem_downgrade_wake"); 	spin_lock_irqsave(&sem->wait_lock, flags); 	/* do nothing if list empty */	if (!list_empty(&sem->wait_list))		sem = __rwsem_do_wake(sem, 1); 	spin_unlock_irqrestore(&sem->wait_lock, flags); 	rwsemtrace(sem, "Leaving rwsem_downgrade_wake");	return sem;} EXPORT_SYMBOL(rwsem_down_read_failed);EXPORT_SYMBOL(rwsem_down_write_failed);EXPORT_SYMBOL(rwsem_wake);EXPORT_SYMBOL(rwsem_downgrade_wake);#if RWSEM_DEBUGEXPORT_SYMBOL(rwsemtrace);#endif