block: introduce init_wait_func()

There is already a macro DEFINE_WAIT_FUNC() to declare a wait_queue_entry
with a specified waking function. But there is not a counterpart for
initializing one wait_queue_entry with a specified waking function. So
introducing init_wait_func() for this, which also could be used in iocost
and rq-qos. Using default_wake_function() in rq_qos_wait() to wake up
waiters, which could remove ->task field from rq_qos_wait_data.

Cc: Ingo Molnar <mingo@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Muchun Song <songmuchun@bytedance.com>
Acked-by: Tejun Heo <tj@kernel.org>
Link: https://lore.kernel.org/r/20250208090416.38642-1-songmuchun@bytedance.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>

Author: Muchun Song

block/blk-iocost.c

@@ -2718,8 +2718,7 @@ static void ioc_rqos_throttle(struct rq_qos *rqos, struct bio *bio)
 	 * All waiters are on iocg->waitq and the wait states are
 	 * synchronized using waitq.lock.
 	 */
-	init_waitqueue_func_entry(&wait.wait, iocg_wake_fn);
-	wait.wait.private = current;
+	init_wait_func(&wait.wait, iocg_wake_fn);
 	wait.bio = bio;
 	wait.abs_cost = abs_cost;
 	wait.committed = false;	/* will be set true by waker */

block/blk-rq-qos.c

@@ -196,7 +196,6 @@ bool rq_depth_scale_down(struct rq_depth *rqd, bool hard_throttle)
 
 struct rq_qos_wait_data {
 	struct wait_queue_entry wq;
-	struct task_struct *task;
 	struct rq_wait *rqw;
 	acquire_inflight_cb_t *cb;
 	void *private_data;
@@ -218,7 +217,12 @@ static int rq_qos_wake_function(struct wait_queue_entry *curr,
 		return -1;
 
 	data->got_token = true;
-	wake_up_process(data->task);
+	/*
+	 * autoremove_wake_function() removes the wait entry only when it
+	 * actually changed the task state. We want the wait always removed.
+	 * Remove explicitly and use default_wake_function().
+	 */
+	default_wake_function(curr, mode, wake_flags, key);
 	list_del_init_careful(&curr->entry);
 	return 1;
 }
@@ -244,11 +248,6 @@ void rq_qos_wait(struct rq_wait *rqw, void *private_data,
 		 cleanup_cb_t *cleanup_cb)
 {
 	struct rq_qos_wait_data data = {
-		.wq = {
-			.func	= rq_qos_wake_function,
-			.entry	= LIST_HEAD_INIT(data.wq.entry),
-		},
-		.task = current,
 		.rqw = rqw,
 		.cb = acquire_inflight_cb,
 		.private_data = private_data,
@@ -259,6 +258,7 @@ void rq_qos_wait(struct rq_wait *rqw, void *private_data,
 	if (!has_sleeper && acquire_inflight_cb(rqw, private_data))
 		return;
 
+	init_wait_func(&data.wq, rq_qos_wake_function);
 	has_sleeper = !prepare_to_wait_exclusive(&rqw->wait, &data.wq,
 						 TASK_UNINTERRUPTIBLE);
 	do {

include/linux/wait.h

@@ -1207,14 +1207,16 @@ int autoremove_wake_function(struct wait_queue_entry *wq_entry, unsigned mode, i
 
 #define DEFINE_WAIT(name) DEFINE_WAIT_FUNC(name, autoremove_wake_function)
 
-#define init_wait(wait)								\
+#define init_wait_func(wait, function)						\
 	do {									\
 		(wait)->private = current;					\
-		(wait)->func = autoremove_wake_function;			\
+		(wait)->func = function;					\
 		INIT_LIST_HEAD(&(wait)->entry);					\
 		(wait)->flags = 0;						\
 	} while (0)
 
+#define init_wait(wait)	init_wait_func(wait, autoremove_wake_function)
+
 typedef int (*task_call_f)(struct task_struct *p, void *arg);
 extern int task_call_func(struct task_struct *p, task_call_f func, void *arg);