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d59dd4620f
Replace a number of memory barriers with smp_ variants. This means we won't take the unnecessary hit on UP machines. Signed-off-by: Anton Blanchard <anton@samba.org> Signed-off-by: Andrew Morton <akpm@osdl.org> Signed-off-by: Linus Torvalds <torvalds@osdl.org>
345 lines
7.5 KiB
C
345 lines
7.5 KiB
C
/* rwsem-spinlock.c: R/W semaphores: contention handling functions for
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* generic spinlock implementation
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*
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* Copyright (c) 2001 David Howells (dhowells@redhat.com).
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* - Derived partially from idea by Andrea Arcangeli <andrea@suse.de>
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* - Derived also from comments by Linus
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*/
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#include <linux/rwsem.h>
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#include <linux/sched.h>
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#include <linux/module.h>
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struct rwsem_waiter {
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struct list_head list;
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struct task_struct *task;
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unsigned int flags;
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#define RWSEM_WAITING_FOR_READ 0x00000001
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#define RWSEM_WAITING_FOR_WRITE 0x00000002
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};
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#if RWSEM_DEBUG
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void rwsemtrace(struct rw_semaphore *sem, const char *str)
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{
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if (sem->debug)
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printk("[%d] %s({%d,%d})\n",
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current->pid, str, sem->activity,
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list_empty(&sem->wait_list) ? 0 : 1);
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}
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#endif
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/*
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* initialise the semaphore
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*/
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void fastcall init_rwsem(struct rw_semaphore *sem)
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{
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sem->activity = 0;
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spin_lock_init(&sem->wait_lock);
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INIT_LIST_HEAD(&sem->wait_list);
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#if RWSEM_DEBUG
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sem->debug = 0;
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#endif
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}
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/*
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* handle the lock release when processes blocked on it that can now run
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* - if we come here, then:
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* - the 'active count' _reached_ zero
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* - the 'waiting count' is non-zero
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* - the spinlock must be held by the caller
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* - woken process blocks are discarded from the list after having task zeroed
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* - writers are only woken if wakewrite is non-zero
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*/
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static inline struct rw_semaphore *
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__rwsem_do_wake(struct rw_semaphore *sem, int wakewrite)
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{
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struct rwsem_waiter *waiter;
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struct task_struct *tsk;
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int woken;
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rwsemtrace(sem, "Entering __rwsem_do_wake");
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waiter = list_entry(sem->wait_list.next, struct rwsem_waiter, list);
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if (!wakewrite) {
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if (waiter->flags & RWSEM_WAITING_FOR_WRITE)
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goto out;
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goto dont_wake_writers;
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}
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/* if we are allowed to wake writers try to grant a single write lock
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* if there's a writer at the front of the queue
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* - we leave the 'waiting count' incremented to signify potential
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* contention
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*/
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if (waiter->flags & RWSEM_WAITING_FOR_WRITE) {
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sem->activity = -1;
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list_del(&waiter->list);
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tsk = waiter->task;
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/* Don't touch waiter after ->task has been NULLed */
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smp_mb();
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waiter->task = NULL;
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wake_up_process(tsk);
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put_task_struct(tsk);
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goto out;
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}
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/* grant an infinite number of read locks to the front of the queue */
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dont_wake_writers:
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woken = 0;
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while (waiter->flags & RWSEM_WAITING_FOR_READ) {
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struct list_head *next = waiter->list.next;
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list_del(&waiter->list);
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tsk = waiter->task;
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smp_mb();
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waiter->task = NULL;
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wake_up_process(tsk);
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put_task_struct(tsk);
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woken++;
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if (list_empty(&sem->wait_list))
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break;
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waiter = list_entry(next, struct rwsem_waiter, list);
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}
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sem->activity += woken;
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out:
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rwsemtrace(sem, "Leaving __rwsem_do_wake");
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return sem;
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}
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/*
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* wake a single writer
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*/
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static inline struct rw_semaphore *
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__rwsem_wake_one_writer(struct rw_semaphore *sem)
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{
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struct rwsem_waiter *waiter;
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struct task_struct *tsk;
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sem->activity = -1;
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waiter = list_entry(sem->wait_list.next, struct rwsem_waiter, list);
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list_del(&waiter->list);
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tsk = waiter->task;
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smp_mb();
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waiter->task = NULL;
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wake_up_process(tsk);
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put_task_struct(tsk);
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return sem;
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}
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/*
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* get a read lock on the semaphore
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*/
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void fastcall __sched __down_read(struct rw_semaphore *sem)
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{
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struct rwsem_waiter waiter;
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struct task_struct *tsk;
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rwsemtrace(sem, "Entering __down_read");
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spin_lock_irq(&sem->wait_lock);
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if (sem->activity >= 0 && list_empty(&sem->wait_list)) {
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/* granted */
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sem->activity++;
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spin_unlock_irq(&sem->wait_lock);
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goto out;
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}
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tsk = current;
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set_task_state(tsk, TASK_UNINTERRUPTIBLE);
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/* set up my own style of waitqueue */
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waiter.task = tsk;
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waiter.flags = RWSEM_WAITING_FOR_READ;
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get_task_struct(tsk);
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list_add_tail(&waiter.list, &sem->wait_list);
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/* we don't need to touch the semaphore struct anymore */
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spin_unlock_irq(&sem->wait_lock);
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/* wait to be given the lock */
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for (;;) {
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if (!waiter.task)
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break;
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schedule();
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set_task_state(tsk, TASK_UNINTERRUPTIBLE);
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}
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tsk->state = TASK_RUNNING;
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out:
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rwsemtrace(sem, "Leaving __down_read");
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}
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/*
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* trylock for reading -- returns 1 if successful, 0 if contention
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*/
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int fastcall __down_read_trylock(struct rw_semaphore *sem)
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{
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unsigned long flags;
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int ret = 0;
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rwsemtrace(sem, "Entering __down_read_trylock");
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spin_lock_irqsave(&sem->wait_lock, flags);
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if (sem->activity >= 0 && list_empty(&sem->wait_list)) {
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/* granted */
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sem->activity++;
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ret = 1;
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}
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spin_unlock_irqrestore(&sem->wait_lock, flags);
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rwsemtrace(sem, "Leaving __down_read_trylock");
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return ret;
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}
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/*
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* get a write lock on the semaphore
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* - we increment the waiting count anyway to indicate an exclusive lock
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*/
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void fastcall __sched __down_write(struct rw_semaphore *sem)
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{
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struct rwsem_waiter waiter;
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struct task_struct *tsk;
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rwsemtrace(sem, "Entering __down_write");
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spin_lock_irq(&sem->wait_lock);
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if (sem->activity == 0 && list_empty(&sem->wait_list)) {
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/* granted */
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sem->activity = -1;
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spin_unlock_irq(&sem->wait_lock);
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goto out;
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}
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tsk = current;
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set_task_state(tsk, TASK_UNINTERRUPTIBLE);
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/* set up my own style of waitqueue */
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waiter.task = tsk;
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waiter.flags = RWSEM_WAITING_FOR_WRITE;
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get_task_struct(tsk);
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list_add_tail(&waiter.list, &sem->wait_list);
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/* we don't need to touch the semaphore struct anymore */
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spin_unlock_irq(&sem->wait_lock);
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/* wait to be given the lock */
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for (;;) {
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if (!waiter.task)
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break;
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schedule();
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set_task_state(tsk, TASK_UNINTERRUPTIBLE);
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}
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tsk->state = TASK_RUNNING;
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out:
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rwsemtrace(sem, "Leaving __down_write");
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}
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/*
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* trylock for writing -- returns 1 if successful, 0 if contention
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*/
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int fastcall __down_write_trylock(struct rw_semaphore *sem)
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{
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unsigned long flags;
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int ret = 0;
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rwsemtrace(sem, "Entering __down_write_trylock");
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spin_lock_irqsave(&sem->wait_lock, flags);
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if (sem->activity == 0 && list_empty(&sem->wait_list)) {
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/* granted */
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sem->activity = -1;
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ret = 1;
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}
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spin_unlock_irqrestore(&sem->wait_lock, flags);
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rwsemtrace(sem, "Leaving __down_write_trylock");
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return ret;
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}
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/*
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* release a read lock on the semaphore
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*/
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void fastcall __up_read(struct rw_semaphore *sem)
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{
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unsigned long flags;
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rwsemtrace(sem, "Entering __up_read");
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spin_lock_irqsave(&sem->wait_lock, flags);
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if (--sem->activity == 0 && !list_empty(&sem->wait_list))
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sem = __rwsem_wake_one_writer(sem);
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spin_unlock_irqrestore(&sem->wait_lock, flags);
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rwsemtrace(sem, "Leaving __up_read");
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}
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/*
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* release a write lock on the semaphore
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*/
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void fastcall __up_write(struct rw_semaphore *sem)
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{
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unsigned long flags;
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rwsemtrace(sem, "Entering __up_write");
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spin_lock_irqsave(&sem->wait_lock, flags);
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sem->activity = 0;
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if (!list_empty(&sem->wait_list))
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sem = __rwsem_do_wake(sem, 1);
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spin_unlock_irqrestore(&sem->wait_lock, flags);
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rwsemtrace(sem, "Leaving __up_write");
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}
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/*
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* downgrade a write lock into a read lock
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* - just wake up any readers at the front of the queue
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*/
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void fastcall __downgrade_write(struct rw_semaphore *sem)
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{
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unsigned long flags;
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rwsemtrace(sem, "Entering __downgrade_write");
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spin_lock_irqsave(&sem->wait_lock, flags);
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sem->activity = 1;
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if (!list_empty(&sem->wait_list))
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sem = __rwsem_do_wake(sem, 0);
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spin_unlock_irqrestore(&sem->wait_lock, flags);
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rwsemtrace(sem, "Leaving __downgrade_write");
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}
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EXPORT_SYMBOL(init_rwsem);
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EXPORT_SYMBOL(__down_read);
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EXPORT_SYMBOL(__down_read_trylock);
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EXPORT_SYMBOL(__down_write);
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EXPORT_SYMBOL(__down_write_trylock);
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EXPORT_SYMBOL(__up_read);
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EXPORT_SYMBOL(__up_write);
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EXPORT_SYMBOL(__downgrade_write);
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#if RWSEM_DEBUG
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EXPORT_SYMBOL(rwsemtrace);
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#endif
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