Generic semaphore implementation

Semaphores are no longer performance-critical, so a generic C
implementation is better for maintainability, debuggability and
extensibility.  Thanks to Peter Zijlstra for fixing the lockdep
warning.  Thanks to Harvey Harrison for pointing out that the
unlikely() was unnecessary.

Signed-off-by: Matthew Wilcox <willy@linux.intel.com>
Acked-by: Ingo Molnar <mingo@elte.hu>

kernel/semaphore.c

@@ -0,0 +1,187 @@
+/*
+ * Copyright (c) 2008 Intel Corporation
+ * Author: Matthew Wilcox <willy@linux.intel.com>
+ *
+ * Distributed under the terms of the GNU GPL, version 2
+ */
+
+#include <linux/compiler.h>
+#include <linux/kernel.h>
+#include <linux/module.h>
+#include <linux/sched.h>
+#include <linux/semaphore.h>
+#include <linux/spinlock.h>
+
+/*
+ * Some notes on the implementation:
+ *
+ * down_trylock() and up() can be called from interrupt context.
+ * So we have to disable interrupts when taking the lock.
+ *
+ * The ->count variable, if positive, defines how many more tasks can
+ * acquire the semaphore.  If negative, it represents how many tasks are
+ * waiting on the semaphore (*).  If zero, no tasks are waiting, and no more
+ * tasks can acquire the semaphore.
+ *
+ * (*) Except for the window between one task calling up() and the task
+ * sleeping in a __down_common() waking up.  In order to avoid a third task
+ * coming in and stealing the second task's wakeup, we leave the ->count
+ * negative.  If we have a more complex situation, the ->count may become
+ * zero or negative (eg a semaphore with count = 2, three tasks attempt to
+ * acquire it, one sleeps, two finish and call up(), the second task to call
+ * up() notices that the list is empty and just increments count).
+ */
+
+static noinline void __down(struct semaphore *sem);
+static noinline int __down_interruptible(struct semaphore *sem);
+static noinline void __up(struct semaphore *sem);
+
+void down(struct semaphore *sem)
+{
+	unsigned long flags;
+
+	spin_lock_irqsave(&sem->lock, flags);
+	if (unlikely(sem->count-- <= 0))
+		__down(sem);
+	spin_unlock_irqrestore(&sem->lock, flags);
+}
+EXPORT_SYMBOL(down);
+
+int down_interruptible(struct semaphore *sem)
+{
+	unsigned long flags;
+	int result = 0;
+
+	spin_lock_irqsave(&sem->lock, flags);
+	if (unlikely(sem->count-- <= 0))
+		result = __down_interruptible(sem);
+	spin_unlock_irqrestore(&sem->lock, flags);
+
+	return result;
+}
+EXPORT_SYMBOL(down_interruptible);
+
+/**
+ * down_trylock - try to acquire the semaphore, without waiting
+ * @sem: the semaphore to be acquired
+ *
+ * Try to acquire the semaphore atomically.  Returns 0 if the mutex has
+ * been acquired successfully and 1 if it is contended.
+ *
+ * NOTE: This return value is inverted from both spin_trylock and
+ * mutex_trylock!  Be careful about this when converting code.
+ *
+ * Unlike mutex_trylock, this function can be used from interrupt context,
+ * and the semaphore can be released by any task or interrupt.
+ */
+int down_trylock(struct semaphore *sem)
+{
+	unsigned long flags;
+	int count;
+
+	spin_lock_irqsave(&sem->lock, flags);
+	count = sem->count - 1;
+	if (likely(count >= 0))
+		sem->count = count;
+	spin_unlock_irqrestore(&sem->lock, flags);
+
+	return (count < 0);
+}
+EXPORT_SYMBOL(down_trylock);
+
+void up(struct semaphore *sem)
+{
+	unsigned long flags;
+
+	spin_lock_irqsave(&sem->lock, flags);
+	if (likely(sem->count >= 0))
+		sem->count++;
+	else
+		__up(sem);
+	spin_unlock_irqrestore(&sem->lock, flags);
+}
+EXPORT_SYMBOL(up);
+
+/* Functions for the contended case */
+
+struct semaphore_waiter {
+	struct list_head list;
+	struct task_struct *task;
+	int up;
+};
+
+/*
+ * Wake up a process waiting on a semaphore.  We need to call this from both
+ * __up and __down_common as it's possible to race a task into the semaphore
+ * if it comes in at just the right time between two tasks calling up() and
+ * a third task waking up.  This function assumes the wait_list is already
+ * checked for being non-empty.
+ */
+static noinline void __sched __up_down_common(struct semaphore *sem)
+{
+	struct semaphore_waiter *waiter = list_first_entry(&sem->wait_list,
+						struct semaphore_waiter, list);
+	list_del(&waiter->list);
+	waiter->up = 1;
+	wake_up_process(waiter->task);
+}
+
+/*
+ * Because this function is inlined, the 'state' parameter will be constant,
+ * and thus optimised away by the compiler.
+ */
+static inline int __sched __down_common(struct semaphore *sem, long state)
+{
+	int result = 0;
+	struct task_struct *task = current;
+	struct semaphore_waiter waiter;
+
+	list_add_tail(&waiter.list, &sem->wait_list);
+	waiter.task = task;
+	waiter.up = 0;
+
+	for (;;) {
+		if (state == TASK_INTERRUPTIBLE && signal_pending(task))
+			goto interrupted;
+		__set_task_state(task, state);
+		spin_unlock_irq(&sem->lock);
+		schedule();
+		spin_lock_irq(&sem->lock);
+		if (waiter.up)
+			goto woken;
+	}
+
+ interrupted:
+	list_del(&waiter.list);
+	result = -EINTR;
+ woken:
+	/*
+	 * Account for the process which woke us up.  For the case where
+	 * we're interrupted, we need to increment the count on our own
+	 * behalf.  I don't believe we can hit the case where the
+	 * sem->count hits zero, *and* there's a second task sleeping,
+	 * but it doesn't hurt, that's not a commonly exercised path and
+	 * it's not a performance path either.
+	 */
+	if (unlikely((++sem->count >= 0) && !list_empty(&sem->wait_list)))
+		__up_down_common(sem);
+	return result;
+}
+
+static noinline void __sched __down(struct semaphore *sem)
+{
+	__down_common(sem, TASK_UNINTERRUPTIBLE);
+}
+
+static noinline int __sched __down_interruptible(struct semaphore *sem)
+{
+	return __down_common(sem, TASK_INTERRUPTIBLE);
+}
+
+static noinline void __sched __up(struct semaphore *sem)
+{
+	if (unlikely(list_empty(&sem->wait_list)))
+		sem->count++;
+	else
+		__up_down_common(sem);
+}