--- /dev/null
+/* rwsem-spinlock.c: R/W semaphores: contention handling functions for
+ * generic spinlock implementation
+ *
+ * Copyright (c) 2001 David Howells (dhowells@redhat.com).
+ * - Derived partially from idea by Andrea Arcangeli <andrea@suse.de>
+ * - Derived also from comments by Linus
+ */
+#include <linux/rwsem.h>
+#include <linux/sched.h>
+#include <linux/export.h>
+
+enum rwsem_waiter_type {
+ RWSEM_WAITING_FOR_WRITE,
+ RWSEM_WAITING_FOR_READ
+};
+
+struct rwsem_waiter {
+ struct list_head list;
+ struct task_struct *task;
+ enum rwsem_waiter_type type;
+};
+
+int rwsem_is_locked(struct rw_semaphore *sem)
+{
+ int ret = 1;
+ unsigned long flags;
+
+ if (raw_spin_trylock_irqsave(&sem->wait_lock, flags)) {
+ ret = (sem->count != 0);
+ raw_spin_unlock_irqrestore(&sem->wait_lock, flags);
+ }
+ return ret;
+}
+EXPORT_SYMBOL(rwsem_is_locked);
+
+/*
+ * initialise the semaphore
+ */
+void __init_rwsem(struct rw_semaphore *sem, const char *name,
+ struct lock_class_key *key)
+{
+#ifdef CONFIG_DEBUG_LOCK_ALLOC
+ /*
+ * Make sure we are not reinitializing a held semaphore:
+ */
+ debug_check_no_locks_freed((void *)sem, sizeof(*sem));
+ lockdep_init_map(&sem->dep_map, name, key, 0);
+#endif
+ sem->count = 0;
+ raw_spin_lock_init(&sem->wait_lock);
+ INIT_LIST_HEAD(&sem->wait_list);
+}
+EXPORT_SYMBOL(__init_rwsem);
+
+/*
+ * handle the lock release when processes blocked on it that can now run
+ * - if we come here, then:
+ * - the 'active count' _reached_ zero
+ * - the 'waiting count' is non-zero
+ * - 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 wakewrite is non-zero
+ */
+static inline struct rw_semaphore *
+__rwsem_do_wake(struct rw_semaphore *sem, int wakewrite)
+{
+ struct rwsem_waiter *waiter;
+ struct task_struct *tsk;
+ int woken;
+
+ waiter = list_entry(sem->wait_list.next, struct rwsem_waiter, list);
+
+ if (waiter->type == RWSEM_WAITING_FOR_WRITE) {
+ if (wakewrite)
+ /* Wake up a writer. Note that we do not grant it the
+ * lock - it will have to acquire it when it runs. */
+ wake_up_process(waiter->task);
+ goto out;
+ }
+
+ /* grant an infinite number of read locks to the front of the queue */
+ woken = 0;
+ do {
+ struct list_head *next = waiter->list.next;
+
+ list_del(&waiter->list);
+ tsk = waiter->task;
+ /*
+ * Make sure we do not wakeup the next reader before
+ * setting the nil condition to grant the next reader;
+ * otherwise we could miss the wakeup on the other
+ * side and end up sleeping again. See the pairing
+ * in rwsem_down_read_failed().
+ */
+ smp_mb();
+ waiter->task = NULL;
+ wake_up_process(tsk);
+ put_task_struct(tsk);
+ woken++;
+ if (next == &sem->wait_list)
+ break;
+ waiter = list_entry(next, struct rwsem_waiter, list);
+ } while (waiter->type != RWSEM_WAITING_FOR_WRITE);
+
+ sem->count += woken;
+
+ out:
+ return sem;
+}
+
+/*
+ * wake a single writer
+ */
+static inline struct rw_semaphore *
+__rwsem_wake_one_writer(struct rw_semaphore *sem)
+{
+ struct rwsem_waiter *waiter;
+
+ waiter = list_entry(sem->wait_list.next, struct rwsem_waiter, list);
+ wake_up_process(waiter->task);
+
+ return sem;
+}
+
+/*
+ * get a read lock on the semaphore
+ */
+void __sched __down_read(struct rw_semaphore *sem)
+{
+ struct rwsem_waiter waiter;
+ struct task_struct *tsk;
+ unsigned long flags;
+
+ raw_spin_lock_irqsave(&sem->wait_lock, flags);
+
+ if (sem->count >= 0 && list_empty(&sem->wait_list)) {
+ /* granted */
+ sem->count++;
+ raw_spin_unlock_irqrestore(&sem->wait_lock, flags);
+ goto out;
+ }
+
+ tsk = current;
+ set_task_state(tsk, TASK_UNINTERRUPTIBLE);
+
+ /* set up my own style of waitqueue */
+ waiter.task = tsk;
+ waiter.type = RWSEM_WAITING_FOR_READ;
+ get_task_struct(tsk);
+
+ list_add_tail(&waiter.list, &sem->wait_list);
+
+ /* we don't need to touch the semaphore struct anymore */
+ raw_spin_unlock_irqrestore(&sem->wait_lock, flags);
+
+ /* wait to be given the lock */
+ for (;;) {
+ if (!waiter.task)
+ break;
+ schedule();
+ set_task_state(tsk, TASK_UNINTERRUPTIBLE);
+ }
+
+ __set_task_state(tsk, TASK_RUNNING);
+ out:
+ ;
+}
+
+/*
+ * trylock for reading -- returns 1 if successful, 0 if contention
+ */
+int __down_read_trylock(struct rw_semaphore *sem)
+{
+ unsigned long flags;
+ int ret = 0;
+
+
+ raw_spin_lock_irqsave(&sem->wait_lock, flags);
+
+ if (sem->count >= 0 && list_empty(&sem->wait_list)) {
+ /* granted */
+ sem->count++;
+ ret = 1;
+ }
+
+ raw_spin_unlock_irqrestore(&sem->wait_lock, flags);
+
+ return ret;
+}
+
+/*
+ * get a write lock on the semaphore
+ */
+void __sched __down_write_nested(struct rw_semaphore *sem, int subclass)
+{
+ struct rwsem_waiter waiter;
+ struct task_struct *tsk;
+ unsigned long flags;
+
+ raw_spin_lock_irqsave(&sem->wait_lock, flags);
+
+ /* set up my own style of waitqueue */
+ tsk = current;
+ waiter.task = tsk;
+ waiter.type = RWSEM_WAITING_FOR_WRITE;
+ list_add_tail(&waiter.list, &sem->wait_list);
+
+ /* wait for someone to release the lock */
+ for (;;) {
+ /*
+ * That is the key to support write lock stealing: allows the
+ * task already on CPU to get the lock soon rather than put
+ * itself into sleep and waiting for system woke it or someone
+ * else in the head of the wait list up.
+ */
+ if (sem->count == 0)
+ break;
+ set_task_state(tsk, TASK_UNINTERRUPTIBLE);
+ raw_spin_unlock_irqrestore(&sem->wait_lock, flags);
+ schedule();
+ raw_spin_lock_irqsave(&sem->wait_lock, flags);
+ }
+ /* got the lock */
+ sem->count = -1;
+ list_del(&waiter.list);
+
+ raw_spin_unlock_irqrestore(&sem->wait_lock, flags);
+}
+
+void __sched __down_write(struct rw_semaphore *sem)
+{
+ __down_write_nested(sem, 0);
+}
+
+/*
+ * trylock for writing -- returns 1 if successful, 0 if contention
+ */
+int __down_write_trylock(struct rw_semaphore *sem)
+{
+ unsigned long flags;
+ int ret = 0;
+
+ raw_spin_lock_irqsave(&sem->wait_lock, flags);
+
+ if (sem->count == 0) {
+ /* got the lock */
+ sem->count = -1;
+ ret = 1;
+ }
+
+ raw_spin_unlock_irqrestore(&sem->wait_lock, flags);
+
+ return ret;
+}
+
+/*
+ * release a read lock on the semaphore
+ */
+void __up_read(struct rw_semaphore *sem)
+{
+ unsigned long flags;
+
+ raw_spin_lock_irqsave(&sem->wait_lock, flags);
+
+ if (--sem->count == 0 && !list_empty(&sem->wait_list))
+ sem = __rwsem_wake_one_writer(sem);
+
+ raw_spin_unlock_irqrestore(&sem->wait_lock, flags);
+}
+
+/*
+ * release a write lock on the semaphore
+ */
+void __up_write(struct rw_semaphore *sem)
+{
+ unsigned long flags;
+
+ raw_spin_lock_irqsave(&sem->wait_lock, flags);
+
+ sem->count = 0;
+ if (!list_empty(&sem->wait_list))
+ sem = __rwsem_do_wake(sem, 1);
+
+ raw_spin_unlock_irqrestore(&sem->wait_lock, flags);
+}
+
+/*
+ * downgrade a write lock into a read lock
+ * - just wake up any readers at the front of the queue
+ */
+void __downgrade_write(struct rw_semaphore *sem)
+{
+ unsigned long flags;
+
+ raw_spin_lock_irqsave(&sem->wait_lock, flags);
+
+ sem->count = 1;
+ if (!list_empty(&sem->wait_list))
+ sem = __rwsem_do_wake(sem, 0);
+
+ raw_spin_unlock_irqrestore(&sem->wait_lock, flags);
+}
+
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