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.

2 files changed, 69 insertions, 1 deletions

diff --git a/kernel/Makefile b/kernel/Makefile
index 7fc87d5..b3ff0a8 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 0000000..040d714
--- /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);