1 files changed, 268 insertions, 0 deletions
diff --git a/lib/rwsem.c b/lib/rwsem.c
new file mode 100644
index 0000000..7644089
--- /dev/null
+++ b/lib/rwsem.c
@@ -0,0 +1,268 @@
+/* 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 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 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;
+	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;
+		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_DEBUG
+EXPORT_SYMBOL(rwsemtrace);
+#endif