patch-2.4.4 linux/lib/rwsem-spinlock.c

• Lines: 240
• Date: Wed Apr 25 13:31:03 2001
• Orig file: v2.4.3/linux/lib/rwsem-spinlock.c
• Orig date: Wed Dec 31 16:00:00 1969

diff -u --recursive --new-file v2.4.3/linux/lib/rwsem-spinlock.c linux/lib/rwsem-spinlock.c
@@ -0,0 +1,239 @@
+/* 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/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
+void rwsemtrace(struct rw_semaphore *sem, const char *str)
+{
+	if (sem->debug)
+		printk("[%d] %s({%d,%d})\n",
+		       current->pid,str,sem->activity,list_empty(&sem->wait_list)?0:1);
+}
+#endif
+
+/*
+ * initialise the semaphore
+ */
+void init_rwsem(struct rw_semaphore *sem)
+{
+	sem->activity = 0;
+	spin_lock_init(&sem->wait_lock);
+	INIT_LIST_HEAD(&sem->wait_list);
+#if RWSEM_DEBUG
+	sem->debug = 0;
+#endif
+}
+
+/*
+ * handle the lock being released whilst there are 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 flags zeroised
+ */
+static inline struct rw_semaphore *__rwsem_do_wake(struct rw_semaphore *sem)
+{
+	struct rwsem_waiter *waiter;
+	int woken;
+
+	rwsemtrace(sem,"Entering __rwsem_do_wake");
+
+	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
+	 * - we leave the 'waiting count' incremented to signify potential contention
+	 */
+	if (waiter->flags & RWSEM_WAITING_FOR_WRITE) {
+		sem->activity = -1;
+		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 */
+	woken = 0;
+	do {
+		list_del(&waiter->list);
+		waiter->flags = 0;
+		wake_up_process(waiter->task);
+		woken++;
+		if (list_empty(&sem->wait_list))
+			break;
+		waiter = list_entry(sem->wait_list.next,struct rwsem_waiter,list);
+	} while (waiter->flags&RWSEM_WAITING_FOR_READ);
+
+	sem->activity += woken;
+
+ out:
+	rwsemtrace(sem,"Leaving __rwsem_do_wake");
+	return sem;
+}
+
+/*
+ * wake a single writer
+ */
+static inline struct rw_semaphore *__rwsem_wake_one_writer(struct rw_semaphore *sem)
+{
+	struct rwsem_waiter *waiter;
+
+	sem->activity = -1;
+
+	waiter = list_entry(sem->wait_list.next,struct rwsem_waiter,list);
+	list_del(&waiter->list);
+
+	waiter->flags = 0;
+	wake_up_process(waiter->task);
+	return sem;
+}
+
+/*
+ * get a read lock on the semaphore
+ */
+void __down_read(struct rw_semaphore *sem)
+{
+	struct rwsem_waiter waiter;
+	struct task_struct *tsk;
+
+	rwsemtrace(sem,"Entering __down_read");
+
+	spin_lock(&sem->wait_lock);
+
+	if (sem->activity>=0 && list_empty(&sem->wait_list)) {
+		/* granted */
+		sem->activity++;
+		spin_unlock(&sem->wait_lock);
+		goto out;
+	}
+
+	tsk = current;
+	set_task_state(tsk,TASK_UNINTERRUPTIBLE);
+
+	/* set up my own style of waitqueue */
+	waiter.task = tsk;
+	waiter.flags = RWSEM_WAITING_FOR_READ;
+
+	list_add_tail(&waiter.list,&sem->wait_list);
+
+	/* we don't need to touch the semaphore struct anymore */
+	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;
+
+ out:
+	rwsemtrace(sem,"Leaving __down_read");
+}
+
+/*
+ * get a write lock on the semaphore
+ * - note that we increment the waiting count anyway to indicate an exclusive lock
+ */
+void __down_write(struct rw_semaphore *sem)
+{
+	struct rwsem_waiter waiter;
+	struct task_struct *tsk;
+
+	rwsemtrace(sem,"Entering __down_write");
+
+	spin_lock(&sem->wait_lock);
+
+	if (sem->activity==0 && list_empty(&sem->wait_list)) {
+		/* granted */
+		sem->activity = -1;
+		spin_unlock(&sem->wait_lock);
+		goto out;
+	}
+
+	tsk = current;
+	set_task_state(tsk,TASK_UNINTERRUPTIBLE);
+
+	/* set up my own style of waitqueue */
+	waiter.task = tsk;
+	waiter.flags = RWSEM_WAITING_FOR_WRITE;
+
+	list_add_tail(&waiter.list,&sem->wait_list);
+
+	/* we don't need to touch the semaphore struct anymore */
+	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;
+
+ out:
+	rwsemtrace(sem,"Leaving __down_write");
+}
+
+/*
+ * release a read lock on the semaphore
+ */
+void __up_read(struct rw_semaphore *sem)
+{
+	rwsemtrace(sem,"Entering __up_read");
+
+	spin_lock(&sem->wait_lock);
+
+	if (--sem->activity==0 && !list_empty(&sem->wait_list))
+		sem = __rwsem_wake_one_writer(sem);
+
+	spin_unlock(&sem->wait_lock);
+
+	rwsemtrace(sem,"Leaving __up_read");
+}
+
+/*
+ * release a write lock on the semaphore
+ */
+void __up_write(struct rw_semaphore *sem)
+{
+	rwsemtrace(sem,"Entering __up_write");
+
+	spin_lock(&sem->wait_lock);
+
+	sem->activity = 0;
+	if (!list_empty(&sem->wait_list))
+		sem = __rwsem_do_wake(sem);
+
+	spin_unlock(&sem->wait_lock);
+
+	rwsemtrace(sem,"Leaving __up_write");
+}
+
+EXPORT_SYMBOL(init_rwsem);
+EXPORT_SYMBOL(__down_read);
+EXPORT_SYMBOL(__down_write);
+EXPORT_SYMBOL(__up_read);
+EXPORT_SYMBOL(__up_write);
+#if RWSEM_DEBUG
+EXPORT_SYMBOL(rwsemtrace);
+#endif