nilfs2: use kthread_create and kthread_stop for the log writer thread

By using kthread_create() and kthread_stop() to start and stop the log
writer thread, eliminate custom thread start and stop helpers, as well as
the wait queue "sc_wait_task" on the "nilfs_sc_info" struct and
NILFS_SEGCTOR_QUIT flag that exist only to implement them.

Also, update the kernel doc comments of the changed functions as
appropriate.

Link: https://lkml.kernel.org/r/20240826174116.5008-8-konishi.ryusuke@gmail.comSigned-off-by: Ryusuke Konishi <konishi.ryusuke@gmail.com>
Cc: Huang Xiaojia <huangxiaojia2@huawei.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

fs/nilfs2/segment.c

@@ -2628,11 +2628,15 @@ static int nilfs_segctor_flush_mode(struct nilfs_sc_info *sci)
 }
 
 /**
- * nilfs_segctor_thread - main loop of the segment constructor thread.
+ * nilfs_segctor_thread - main loop of the log writer thread
  * @arg: pointer to a struct nilfs_sc_info.
  *
- * nilfs_segctor_thread() initializes a timer and serves as a daemon
- * to execute segment constructions.
+ * nilfs_segctor_thread() is the main loop function of the log writer kernel
+ * thread, which determines whether log writing is necessary, and if so,
+ * performs the log write in the background, or waits if not.  It is also
+ * used to decide the background writeback of the superblock.
+ *
+ * Return: Always 0.
  */
 static int nilfs_segctor_thread(void *arg)
 {
@@ -2640,11 +2644,6 @@ static int nilfs_segctor_thread(void *arg)
 	struct the_nilfs *nilfs = sci->sc_super->s_fs_info;
 	int timeout = 0;
 
-	timer_setup(&sci->sc_timer, nilfs_construction_timeout, 0);
-
-	/* start sync. */
-	sci->sc_task = current;
-	wake_up(&sci->sc_wait_task); /* for nilfs_segctor_start_thread() */
 	nilfs_info(sci->sc_super,
 		   "segctord starting. Construction interval = %lu seconds, CP frequency < %lu seconds",
 		   sci->sc_interval / HZ, sci->sc_mjcp_freq / HZ);
@@ -2655,7 +2654,7 @@ static int nilfs_segctor_thread(void *arg)
 	for (;;) {
 		int mode;
 
-		if (sci->sc_state & NILFS_SEGCTOR_QUIT)
+		if (kthread_should_stop())
 			goto end_thread;
 
 		if (timeout || sci->sc_seq_request != sci->sc_seq_done)
@@ -2709,41 +2708,10 @@ static int nilfs_segctor_thread(void *arg)
 	/* end sync. */
 	sci->sc_task = NULL;
 	timer_shutdown_sync(&sci->sc_timer);
-	wake_up(&sci->sc_wait_task); /* for nilfs_segctor_kill_thread() */
 	spin_unlock(&sci->sc_state_lock);
 	return 0;
 }
 
-static int nilfs_segctor_start_thread(struct nilfs_sc_info *sci)
-{
-	struct task_struct *t;
-
-	t = kthread_run(nilfs_segctor_thread, sci, "segctord");
-	if (IS_ERR(t)) {
-		int err = PTR_ERR(t);
-
-		nilfs_err(sci->sc_super, "error %d creating segctord thread",
-			  err);
-		return err;
-	}
-	wait_event(sci->sc_wait_task, sci->sc_task != NULL);
-	return 0;
-}
-
-static void nilfs_segctor_kill_thread(struct nilfs_sc_info *sci)
-	__acquires(&sci->sc_state_lock)
-	__releases(&sci->sc_state_lock)
-{
-	sci->sc_state |= NILFS_SEGCTOR_QUIT;
-
-	while (sci->sc_task) {
-		wake_up(&sci->sc_wait_daemon);
-		spin_unlock(&sci->sc_state_lock);
-		wait_event(sci->sc_wait_task, sci->sc_task == NULL);
-		spin_lock(&sci->sc_state_lock);
-	}
-}
-
 /*
  * Setup & clean-up functions
  */
@@ -2764,7 +2732,6 @@ static struct nilfs_sc_info *nilfs_segctor_new(struct super_block *sb,
 
 	init_waitqueue_head(&sci->sc_wait_request);
 	init_waitqueue_head(&sci->sc_wait_daemon);
-	init_waitqueue_head(&sci->sc_wait_task);
 	spin_lock_init(&sci->sc_state_lock);
 	INIT_LIST_HEAD(&sci->sc_dirty_files);
 	INIT_LIST_HEAD(&sci->sc_segbufs);
@@ -2819,8 +2786,12 @@ static void nilfs_segctor_destroy(struct nilfs_sc_info *sci)
 
 	up_write(&nilfs->ns_segctor_sem);
 
+	if (sci->sc_task) {
+		wake_up(&sci->sc_wait_daemon);
+		kthread_stop(sci->sc_task);
+	}
+
 	spin_lock(&sci->sc_state_lock);
-	nilfs_segctor_kill_thread(sci);
 	flag = ((sci->sc_state & NILFS_SEGCTOR_COMMIT) || sci->sc_flush_request
 		|| sci->sc_seq_request != sci->sc_seq_done);
 	spin_unlock(&sci->sc_state_lock);
@@ -2868,14 +2839,15 @@ static void nilfs_segctor_destroy(struct nilfs_sc_info *sci)
  * This allocates a log writer object, initializes it, and starts the
  * log writer.
  *
- * Return Value: On success, 0 is returned. On error, one of the following
- * negative error code is returned.
- *
- * %-ENOMEM - Insufficient memory available.
+ * Return: 0 on success, or the following negative error code on failure.
+ * * %-EINTR	- Log writer thread creation failed due to interruption.
+ * * %-ENOMEM	- Insufficient memory available.
  */
 int nilfs_attach_log_writer(struct super_block *sb, struct nilfs_root *root)
 {
 	struct the_nilfs *nilfs = sb->s_fs_info;
+	struct nilfs_sc_info *sci;
+	struct task_struct *t;
 	int err;
 
 	if (nilfs->ns_writer) {
@@ -2888,15 +2860,23 @@ int nilfs_attach_log_writer(struct super_block *sb, struct nilfs_root *root)
 		return 0;
 	}
 
-	nilfs->ns_writer = nilfs_segctor_new(sb, root);
-	if (!nilfs->ns_writer)
+	sci = nilfs_segctor_new(sb, root);
+	if (unlikely(!sci))
 		return -ENOMEM;
 
-	err = nilfs_segctor_start_thread(nilfs->ns_writer);
-	if (unlikely(err))
+	nilfs->ns_writer = sci;
+	t = kthread_create(nilfs_segctor_thread, sci, "segctord");
+	if (IS_ERR(t)) {
+		err = PTR_ERR(t);
+		nilfs_err(sb, "error %d creating segctord thread", err);
 		nilfs_detach_log_writer(sb);
+		return err;
+	}
+	sci->sc_task = t;
+	timer_setup(&sci->sc_timer, nilfs_construction_timeout, 0);
 
-	return err;
+	wake_up_process(sci->sc_task);
+	return 0;
 }
 
 /**

fs/nilfs2/segment.h

@@ -105,7 +105,6 @@ struct nilfs_segsum_pointer {
  * @sc_flush_request: inode bitmap of metadata files to be flushed
  * @sc_wait_request: Client request queue
  * @sc_wait_daemon: Daemon wait queue
- * @sc_wait_task: Start/end wait queue to control segctord task
  * @sc_seq_request: Request counter
  * @sc_seq_accepted: Accepted request count
  * @sc_seq_done: Completion counter
@@ -158,7 +157,6 @@ struct nilfs_sc_info {
 
 	wait_queue_head_t	sc_wait_request;
 	wait_queue_head_t	sc_wait_daemon;
-	wait_queue_head_t	sc_wait_task;
 
 	__u32			sc_seq_request;
 	__u32			sc_seq_accepted;
@@ -191,7 +189,6 @@ enum {
 };
 
 /* sc_state */
-#define NILFS_SEGCTOR_QUIT	    0x0001  /* segctord is being destroyed */
 #define NILFS_SEGCTOR_COMMIT	    0x0004  /* committed transaction exists */
 
 /*