From 20c076ba05a7e966902ec9289e93f5a5f44e456c Mon Sep 17 00:00:00 2001
From: Thomas Gleixner <tglx@linutronix.de>
Date: Mon, 12 Dec 2011 12:29:04 +0100
Subject: [PATCH 305/414] wait-simple: Simple waitqueue implementation

wait_queue is a swiss army knife and in most of the cases the
complexity is not needed. For RT waitqueues are a constant source of
trouble as we can't convert the head lock to a raw spinlock due to
fancy and long lasting callbacks.

Provide a slim version, which allows RT to replace wait queues. This
should go mainline as well, as it lowers memory consumption and
runtime overhead.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

smp_mb() added by Steven Rostedt to fix a race condition with swait
wakeups vs adding items to the list.
---
 include/linux/wait-simple.h | 235 ++++++++++++++++++++++++++++++++++++++++++++
 kernel/Makefile             |   2 +-
 kernel/wait-simple.c        |  68 +++++++++++++
 3 files changed, 304 insertions(+), 1 deletion(-)
 create mode 100644 include/linux/wait-simple.h
 create mode 100644 kernel/wait-simple.c

diff --git a/include/linux/wait-simple.h b/include/linux/wait-simple.h
new file mode 100644
index 000000000000..a9f24692cbe1
--- /dev/null
+++ b/include/linux/wait-simple.h
@@ -0,0 +1,235 @@
+#ifndef _LINUX_WAIT_SIMPLE_H
+#define _LINUX_WAIT_SIMPLE_H
+
+#include <linux/spinlock.h>
+#include <linux/list.h>
+
+#include <asm/current.h>
+
+struct swaiter {
+	struct task_struct	*task;
+	struct list_head	node;
+};
+
+#define DEFINE_SWAITER(name)					\
+	struct swaiter name = {					\
+		.task	= current,				\
+		.node	= LIST_HEAD_INIT((name).node),		\
+	}
+
+struct swait_head {
+	raw_spinlock_t		lock;
+	struct list_head	list;
+};
+
+#define DEFINE_SWAIT_HEAD(name)					\
+	struct swait_head name = {				\
+		.lock	= __RAW_SPIN_LOCK_UNLOCKED(name.lock),	\
+		.list	= LIST_HEAD_INIT((name).list),		\
+	}
+
+extern void __init_swait_head(struct swait_head *h, struct lock_class_key *key);
+
+#define init_swait_head(swh)					\
+	do {							\
+		static struct lock_class_key __key;		\
+								\
+		__init_swait_head((swh), &__key);		\
+	} while (0)
+
+/*
+ * Waiter functions
+ */
+static inline bool swaiter_enqueued(struct swaiter *w)
+{
+	return w->task != NULL;
+}
+
+extern void swait_prepare(struct swait_head *head, struct swaiter *w, int state);
+extern void swait_finish(struct swait_head *head, struct swaiter *w);
+
+/*
+ * Adds w to head->list. Must be called with head->lock locked.
+ */
+static inline void __swait_enqueue(struct swait_head *head, struct swaiter *w)
+{
+	list_add(&w->node, &head->list);
+	/* We can't let the condition leak before the setting of head */
+	smp_mb();
+}
+
+/*
+ * Removes w from head->list. Must be called with head->lock locked.
+ */
+static inline void __swait_dequeue(struct swaiter *w)
+{
+	list_del_init(&w->node);
+}
+
+/*
+ * Check whether a head has waiters enqueued
+ */
+static inline bool swait_head_has_waiters(struct swait_head *h)
+{
+	/* Make sure the condition is visible before checking list_empty() */
+	smp_mb();
+	return !list_empty(&h->list);
+}
+
+/*
+ * Wakeup functions
+ */
+extern int __swait_wake(struct swait_head *head, unsigned int state);
+
+static inline int swait_wake(struct swait_head *head)
+{
+	return swait_head_has_waiters(head) ?
+		__swait_wake(head, TASK_NORMAL) : 0;
+}
+
+static inline int swait_wake_interruptible(struct swait_head *head)
+{
+	return swait_head_has_waiters(head) ?
+		__swait_wake(head, TASK_INTERRUPTIBLE) : 0;
+}
+
+/*
+ * Event API
+ */
+
+#define __swait_event(wq, condition)					\
+do {									\
+	DEFINE_SWAITER(__wait);						\
+									\
+	for (;;) {							\
+		swait_prepare(&wq, &__wait, TASK_UNINTERRUPTIBLE);	\
+		if (condition)						\
+			break;						\
+		schedule();						\
+	}								\
+	swait_finish(&wq, &__wait);					\
+} while (0)
+
+/**
+ * swait_event - sleep until a condition gets true
+ * @wq: the waitqueue to wait on
+ * @condition: a C expression for the event to wait for
+ *
+ * The process is put to sleep (TASK_UNINTERRUPTIBLE) until the
+ * @condition evaluates to true. The @condition is checked each time
+ * the waitqueue @wq is woken up.
+ *
+ * wake_up() has to be called after changing any variable that could
+ * change the result of the wait condition.
+ */
+#define swait_event(wq, condition)					\
+do {									\
+	if (condition)							\
+		break;							\
+	__swait_event(wq, condition);					\
+} while (0)
+
+#define __swait_event_interruptible(wq, condition, ret)			\
+do {									\
+	DEFINE_SWAITER(__wait);						\
+									\
+	for (;;) {							\
+		swait_prepare(&wq, &__wait, TASK_INTERRUPTIBLE);	\
+		if (condition)						\
+			break;						\
+		if (signal_pending(current)) {				\
+			ret = -ERESTARTSYS;				\
+			break;						\
+		}							\
+		schedule();						\
+	}								\
+	swait_finish(&wq, &__wait);					\
+} while (0)
+
+#define __swait_event_interruptible_timeout(wq, condition, ret)		\
+do {									\
+	DEFINE_SWAITER(__wait);						\
+									\
+	for (;;) {							\
+		swait_prepare(&wq, &__wait, TASK_INTERRUPTIBLE);	\
+		if (condition)						\
+			break;						\
+		if (signal_pending(current)) {				\
+			ret = -ERESTARTSYS;				\
+			break;						\
+		}							\
+		ret = schedule_timeout(ret);				\
+		if (!ret)						\
+			break;						\
+	}								\
+	swait_finish(&wq, &__wait);					\
+} while (0)
+
+/**
+ * swait_event_interruptible - sleep until a condition gets true
+ * @wq: the waitqueue to wait on
+ * @condition: a C expression for the event to wait for
+ *
+ * The process is put to sleep (TASK_INTERRUPTIBLE) until the
+ * @condition evaluates to true. The @condition is checked each time
+ * the waitqueue @wq is woken up.
+ *
+ * wake_up() has to be called after changing any variable that could
+ * change the result of the wait condition.
+ */
+#define swait_event_interruptible(wq, condition)			\
+({									\
+	int __ret = 0;							\
+	if (!(condition))						\
+		__swait_event_interruptible(wq, condition, __ret);	\
+	__ret;								\
+})
+
+#define swait_event_interruptible_timeout(wq, condition, timeout)	\
+({									\
+	int __ret = timeout;						\
+	if (!(condition))						\
+		__swait_event_interruptible_timeout(wq, condition, __ret);	\
+	__ret;								\
+})
+
+#define __swait_event_timeout(wq, condition, ret)			\
+do {									\
+	DEFINE_SWAITER(__wait);						\
+									\
+	for (;;) {							\
+		swait_prepare(&wq, &__wait, TASK_UNINTERRUPTIBLE);	\
+		if (condition)						\
+			break;						\
+		ret = schedule_timeout(ret);				\
+		if (!ret)						\
+			break;						\
+	}								\
+	swait_finish(&wq, &__wait);					\
+} while (0)
+
+/**
+ * swait_event_timeout - sleep until a condition gets true or a timeout elapses
+ * @wq: the waitqueue to wait on
+ * @condition: a C expression for the event to wait for
+ * @timeout: timeout, in jiffies
+ *
+ * The process is put to sleep (TASK_UNINTERRUPTIBLE) until the
+ * @condition evaluates to true. The @condition is checked each time
+ * the waitqueue @wq is woken up.
+ *
+ * wake_up() has to be called after changing any variable that could
+ * change the result of the wait condition.
+ *
+ * The function returns 0 if the @timeout elapsed, and the remaining
+ * jiffies if the condition evaluated to true before the timeout elapsed.
+ */
+#define swait_event_timeout(wq, condition, timeout)			\
+({									\
+	long __ret = timeout;						\
+	if (!(condition))						\
+		__swait_event_timeout(wq, condition, __ret);		\
+	__ret;								\
+})
+
+#endif
diff --git a/kernel/Makefile b/kernel/Makefile
index 7fc87d50baee..b3ff0a8dcb98 100644
--- a/kernel/Makefile
+++ b/kernel/Makefile
@@ -10,7 +10,7 @@ obj-y     = fork.o exec_domain.o panic.o \
 	    kthread.o wait.o sys_ni.o posix-cpu-timers.o \
 	    hrtimer.o nsproxy.o srcu.o semaphore.o \
 	    notifier.o ksysfs.o cred.o reboot.o \
-	    async.o range.o groups.o lglock.o smpboot.o
+	    async.o range.o groups.o lglock.o smpboot.o wait-simple.o
 
 ifdef CONFIG_FUNCTION_TRACER
 # Do not trace debug files and internal ftrace files
diff --git a/kernel/wait-simple.c b/kernel/wait-simple.c
new file mode 100644
index 000000000000..040d7146e4df
--- /dev/null
+++ b/kernel/wait-simple.c
@@ -0,0 +1,68 @@
+/*
+ * Simple waitqueues without fancy flags and callbacks
+ *
+ * (C) 2011 Thomas Gleixner <tglx@linutronix.de>
+ *
+ * Based on kernel/wait.c
+ *
+ * For licencing details see kernel-base/COPYING
+ */
+#include <linux/init.h>
+#include <linux/export.h>
+#include <linux/sched.h>
+#include <linux/wait-simple.h>
+
+void __init_swait_head(struct swait_head *head, struct lock_class_key *key)
+{
+	raw_spin_lock_init(&head->lock);
+	lockdep_set_class(&head->lock, key);
+	INIT_LIST_HEAD(&head->list);
+}
+EXPORT_SYMBOL(__init_swait_head);
+
+void swait_prepare(struct swait_head *head, struct swaiter *w, int state)
+{
+	unsigned long flags;
+
+	raw_spin_lock_irqsave(&head->lock, flags);
+	w->task = current;
+	if (list_empty(&w->node))
+		__swait_enqueue(head, w);
+	set_current_state(state);
+	raw_spin_unlock_irqrestore(&head->lock, flags);
+}
+EXPORT_SYMBOL(swait_prepare);
+
+void swait_finish(struct swait_head *head, struct swaiter *w)
+{
+	unsigned long flags;
+
+	__set_current_state(TASK_RUNNING);
+	if (w->task) {
+		raw_spin_lock_irqsave(&head->lock, flags);
+		__swait_dequeue(w);
+		raw_spin_unlock_irqrestore(&head->lock, flags);
+	}
+}
+EXPORT_SYMBOL(swait_finish);
+
+int __swait_wake(struct swait_head *head, unsigned int state)
+{
+	struct swaiter *curr, *next;
+	unsigned long flags;
+	int woken = 0;
+
+	raw_spin_lock_irqsave(&head->lock, flags);
+
+	list_for_each_entry_safe(curr, next, &head->list, node) {
+		if (wake_up_state(curr->task, state)) {
+			__swait_dequeue(curr);
+			curr->task = NULL;
+			woken++;
+		}
+	}
+
+	raw_spin_unlock_irqrestore(&head->lock, flags);
+	return woken;
+}
+EXPORT_SYMBOL(__swait_wake);
-- 
2.10.2

