patch-2.4.4 linux/lib/rwsem.c

• Lines: 211
• Date: Fri Apr 27 13:58:28 2001
• Orig file: v2.4.3/linux/lib/rwsem.c
• Orig date: Wed Dec 31 16:00:00 1969

diff -u --recursive --new-file v2.4.3/linux/lib/rwsem.c linux/lib/rwsem.c
@@ -0,0 +1,210 @@
+/* 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 rwsemtrace
+void 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,
+								 __s32 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_DEBUG
+EXPORT_SYMBOL(rwsemtrace);
+#endif