md/raid5: Wait sync io to finish before changing group cnt

One customer reports a bug: raid5 is hung when changing thread cnt
while resync is running. The stripes are all in conf->handle_list
and new threads can't handle them.

Commit b39f35e ("md: don't quiesce in mddev_suspend()") removes
pers->quiesce from mddev_suspend/resume. Before this patch, mddev_suspend
needs to wait for all ios including sync io to finish. Now it's used
to only wait normal io.

Fix this by calling raid5_quiesce from raid5_store_group_thread_cnt
directly to wait all sync requests to finish before changing the group
cnt.

Fixes: b39f35e ("md: don't quiesce in mddev_suspend()")
Cc: [email protected]
Signed-off-by: Xiao Ni <[email protected]>
Reviewed-by: Yu Kuai <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Song Liu <[email protected]>

Author: XiaoNi87

drivers/md/raid5.c

@@ -7176,6 +7176,8 @@ raid5_store_group_thread_cnt(struct mddev *mddev, const char *page, size_t len)
 	err = mddev_suspend_and_lock(mddev);
 	if (err)
 		return err;
+	raid5_quiesce(mddev, true);
+
 	conf = mddev->private;
 	if (!conf)
 		err = -ENODEV;
@@ -7197,6 +7199,8 @@ raid5_store_group_thread_cnt(struct mddev *mddev, const char *page, size_t len)
 			kfree(old_groups);
 		}
 	}
+
+	raid5_quiesce(mddev, false);
 	mddev_unlock_and_resume(mddev);
 
 	return err ?: len;