[PATCH v2] md: split MD_RECOVERY_NEEDED out of mddev_resume

[Yu Kuai <yukuai1@xxxxxxxxxxxxxxx>]

From: Yu Kuai <yukuai3@xxxxxxxxxx>

New mddev_resume() calls are added to synchronize IO with array
reconfiguration, however, this introduces a performance regression while
adding it in md_start_sync():

1) someone sets MD_RECOVERY_NEEDED first;
2) daemon thread grabs reconfig_mutex, then clears MD_RECOVERY_NEEDED and
   queues a new sync work;
3) daemon thread releases reconfig_mutex;
4) in md_start_sync
   a) check that there are spares that can be added/removed, then suspend
      the array;
   b) remove_and_add_spares may not be called, or called without really
      add/remove spares;
   c) resume the array, then set MD_RECOVERY_NEEDED again!

Loop between 2 - 4, then mddev_suspend() will be called quite often, for
consequence, normal IO will be quite slow.

Fix this problem by don't set MD_RECOVERY_NEEDED again in md_start_sync(),
hence the loop will be broken.

Fixes: bc08041b32ab ("md: suspend array in md_start_sync() if array need reconfiguration")
Suggested-by: Song Liu <song@xxxxxxxxxx>
Reported-by: Janpieter Sollie <janpieter.sollie@xxxxxxxxx>
Closes: https://bugzilla.kernel.org/show_bug.cgi?id=218200
Signed-off-by: Yu Kuai <yukuai3@xxxxxxxxxx>
---
Changes in v2:
 - use a new approch as suggested by Song Liu;

 drivers/md/md.c | 18 ++++++++++++++----
 1 file changed, 14 insertions(+), 4 deletions(-)

diff --git a/drivers/md/md.c b/drivers/md/md.c
index bc9d67af1961..49540db8a210 100644
--- a/drivers/md/md.c
+++ b/drivers/md/md.c
@@ -490,7 +490,7 @@ int mddev_suspend(struct mddev *mddev, bool interruptible)
 }
 EXPORT_SYMBOL_GPL(mddev_suspend);
 
-void mddev_resume(struct mddev *mddev)
+static void __mddev_resume(struct mddev *mddev, bool recovery_needed)
 {
 	lockdep_assert_not_held(&mddev->reconfig_mutex);
 
@@ -507,12 +507,18 @@ void mddev_resume(struct mddev *mddev)
 	percpu_ref_resurrect(&mddev->active_io);
 	wake_up(&mddev->sb_wait);
 
-	set_bit(MD_RECOVERY_NEEDED, &mddev->recovery);
+	if (recovery_needed)
+		set_bit(MD_RECOVERY_NEEDED, &mddev->recovery);
 	md_wakeup_thread(mddev->thread);
 	md_wakeup_thread(mddev->sync_thread); /* possibly kick off a reshape */
 
 	mutex_unlock(&mddev->suspend_mutex);
 }
+
+void mddev_resume(struct mddev *mddev)
+{
+	return __mddev_resume(mddev, true);
+}
 EXPORT_SYMBOL_GPL(mddev_resume);
 
 /*
@@ -9389,7 +9395,9 @@ static void md_start_sync(struct work_struct *ws)
 		goto not_running;
 	}
 
-	suspend ? mddev_unlock_and_resume(mddev) : mddev_unlock(mddev);
+	mddev_unlock(mddev);
+	if (suspend)
+		__mddev_resume(mddev, false);
 	md_wakeup_thread(mddev->sync_thread);
 	sysfs_notify_dirent_safe(mddev->sysfs_action);
 	md_new_event();
@@ -9401,7 +9409,9 @@ static void md_start_sync(struct work_struct *ws)
 	clear_bit(MD_RECOVERY_REQUESTED, &mddev->recovery);
 	clear_bit(MD_RECOVERY_CHECK, &mddev->recovery);
 	clear_bit(MD_RECOVERY_RUNNING, &mddev->recovery);
-	suspend ? mddev_unlock_and_resume(mddev) : mddev_unlock(mddev);
+	mddev_unlock(mddev);
+	if (suspend)
+		__mddev_resume(mddev, false);
 
 	wake_up(&resync_wait);
 	if (test_and_clear_bit(MD_RECOVERY_RECOVER, &mddev->recovery) &&
-- 
2.39.2