[PATCH] add wait queue function callback support

This adds support for wait queue function callbacks, which are used by
aio to build async read / write operations on top of existing wait
queues at points that would normally block a process.

include/linux/wait.h

@@ -19,13 +19,17 @@
 #include <asm/page.h>
 #include <asm/processor.h>
 
+typedef struct __wait_queue wait_queue_t;
+typedef int (*wait_queue_func_t)(wait_queue_t *wait, unsigned mode, int sync);
+extern int default_wake_function(wait_queue_t *wait, unsigned mode, int sync);
+
 struct __wait_queue {
 	unsigned int flags;
 #define WQ_FLAG_EXCLUSIVE	0x01
 	struct task_struct * task;
+	wait_queue_func_t func;
 	struct list_head task_list;
 };
-typedef struct __wait_queue wait_queue_t;
 
 struct __wait_queue_head {
 	spinlock_t lock;
@@ -40,6 +44,7 @@ typedef struct __wait_queue_head wait_queue_head_t;
 
 #define __WAITQUEUE_INITIALIZER(name, tsk) {				\
 	task:		tsk,						\
+	func:		default_wake_function,				\
 	task_list:	{ NULL, NULL } }
 
 #define DECLARE_WAITQUEUE(name, tsk)					\
@@ -62,6 +67,15 @@ static inline void init_waitqueue_entry(wait_queue_t *q, struct task_struct *p)
 {
 	q->flags = 0;
 	q->task = p;
+	q->func = default_wake_function;
+}
+
+static inline void init_waitqueue_func_entry(wait_queue_t *q,
+					wait_queue_func_t func)
+{
+	q->flags = 0;
+	q->task = NULL;
+	q->func = func;
 }
 
 static inline int waitqueue_active(wait_queue_head_t *q)
@@ -89,6 +103,22 @@ static inline void __remove_wait_queue(wait_queue_head_t *head,
 	list_del(&old->task_list);
 }
 
+#define add_wait_queue_cond(q, wait, cond) \
+	({							\
+		unsigned long flags;				\
+		int _raced = 0;					\
+		spin_lock_irqsave(&(q)->lock, flags);	\
+		(wait)->flags = 0;				\
+		__add_wait_queue((q), (wait));			\
+		rmb();						\
+		if (!(cond)) {					\
+			_raced = 1;				\
+			__remove_wait_queue((q), (wait));	\
+		}						\
+		spin_lock_irqrestore(&(q)->lock, flags);	\
+		_raced;						\
+	})
+
 #endif /* __KERNEL__ */
 
 #endif

kernel/sched.c

@@ -905,6 +905,12 @@ asmlinkage void preempt_schedule(void)
 }
 #endif /* CONFIG_PREEMPT */
 
+int default_wake_function(wait_queue_t *curr, unsigned mode, int sync)
+{
+	task_t *p = curr->task;
+	return ((p->state & mode) && try_to_wake_up(p, sync));
+}
+
 /*
  * The core wakeup function.  Non-exclusive wakeups (nr_exclusive == 0) just
  * wake everything up.  If it's an exclusive wakeup (nr_exclusive == small +ve
@@ -916,17 +922,16 @@ asmlinkage void preempt_schedule(void)
  */
 static inline void __wake_up_common(wait_queue_head_t *q, unsigned int mode, int nr_exclusive, int sync)
 {
-	struct list_head *tmp;
-	unsigned int state;
-	wait_queue_t *curr;
-	task_t *p;
+	struct list_head *tmp, *next;
 
-	list_for_each(tmp, &q->task_list) {
+	list_for_each_safe(tmp, next, &q->task_list) {
+		wait_queue_t *curr;
+		unsigned flags;
 		curr = list_entry(tmp, wait_queue_t, task_list);
-		p = curr->task;
-		state = p->state;
-		if ((state & mode) && try_to_wake_up(p, sync) &&
-			((curr->flags & WQ_FLAG_EXCLUSIVE) && !--nr_exclusive))
+		flags = curr->flags;
+		if (curr->func(curr, mode, sync) &&
+		    (flags & WQ_FLAG_EXCLUSIVE) &&
+		    !--nr_exclusive)
 			break;
 	}
 }