md: make reconfig_mutex optional for writes to md sysfs files.

Rather than using mddev_lock() to take the reconfig_mutex
when writing to any md sysfs file, we only take mddev_lock()
in the particular _store() functions that require it.
Admittedly this is most, but it isn't all.

This also allows us to remove special-case handling for new_dev_store
(in md_attr_store).
Signed-off-by: NeilBrown <neilb@suse.de>

drivers/md/raid5.c

@@ -5400,21 +5400,25 @@ EXPORT_SYMBOL(raid5_set_cache_size);
 static ssize_t
 raid5_store_stripe_cache_size(struct mddev *mddev, const char *page, size_t len)
 {
-	struct r5conf *conf = mddev->private;
+	struct r5conf *conf;
 	unsigned long new;
 	int err;
 
 	if (len >= PAGE_SIZE)
 		return -EINVAL;
-	if (!conf)
-		return -ENODEV;
-
 	if (kstrtoul(page, 10, &new))
 		return -EINVAL;
-	err = raid5_set_cache_size(mddev, new);
+	err = mddev_lock(mddev);
 	if (err)
 		return err;
-	return len;
+	conf = mddev->private;
+	if (!conf)
+		err = -ENODEV;
+	else
+		err = raid5_set_cache_size(mddev, new);
+	mddev_unlock(mddev);
+
+	return err ?: len;
 }
 
 static struct md_sysfs_entry
@@ -5438,19 +5442,27 @@ raid5_show_preread_threshold(struct mddev *mddev, char *page)
 static ssize_t
 raid5_store_preread_threshold(struct mddev *mddev, const char *page, size_t len)
 {
-	struct r5conf *conf = mddev->private;
+	struct r5conf *conf;
 	unsigned long new;
+	int err;
+
 	if (len >= PAGE_SIZE)
 		return -EINVAL;
-	if (!conf)
-		return -ENODEV;
-
 	if (kstrtoul(page, 10, &new))
 		return -EINVAL;
-	if (new > conf->max_nr_stripes)
-		return -EINVAL;
+
+	err = mddev_lock(mddev);
+	if (err)
+		return err;
+	conf = mddev->private;
+	if (!conf)
+		err = -ENODEV;
+	else if (new > conf->max_nr_stripes)
+		err = -EINVAL;
+	else
 		conf->bypass_threshold = new;
-	return len;
+	mddev_unlock(mddev);
+	return err ?: len;
 }
 
 static struct md_sysfs_entry
@@ -5475,19 +5487,23 @@ raid5_show_skip_copy(struct mddev *mddev, char *page)
 static ssize_t
 raid5_store_skip_copy(struct mddev *mddev, const char *page, size_t len)
 {
-	struct r5conf *conf = mddev->private;
+	struct r5conf *conf;
 	unsigned long new;
+	int err;
+
 	if (len >= PAGE_SIZE)
 		return -EINVAL;
-	if (!conf)
-		return -ENODEV;
-
 	if (kstrtoul(page, 10, &new))
 		return -EINVAL;
 	new = !!new;
-	if (new == conf->skip_copy)
-		return len;
 
+	err = mddev_lock(mddev);
+	if (err)
+		return err;
+	conf = mddev->private;
+	if (!conf)
+		err = -ENODEV;
+	else if (new != conf->skip_copy) {
 		mddev_suspend(mddev);
 		conf->skip_copy = new;
 		if (new)
@@ -5497,7 +5513,9 @@ raid5_store_skip_copy(struct mddev *mddev, const char *page, size_t len)
 			mddev->queue->backing_dev_info.capabilities &=
 				~BDI_CAP_STABLE_WRITES;
 		mddev_resume(mddev);
-	return len;
+	}
+	mddev_unlock(mddev);
+	return err ?: len;
 }
 
 static struct md_sysfs_entry
@@ -5538,7 +5556,7 @@ static int alloc_thread_groups(struct r5conf *conf, int cnt,
 static ssize_t
 raid5_store_group_thread_cnt(struct mddev *mddev, const char *page, size_t len)
 {
-	struct r5conf *conf = mddev->private;
+	struct r5conf *conf;
 	unsigned long new;
 	int err;
 	struct r5worker_group *new_groups, *old_groups;
@@ -5546,15 +5564,16 @@ raid5_store_group_thread_cnt(struct mddev *mddev, const char *page, size_t len)
 
 	if (len >= PAGE_SIZE)
 		return -EINVAL;
-	if (!conf)
-		return -ENODEV;
-
 	if (kstrtoul(page, 10, &new))
 		return -EINVAL;
 
-	if (new == conf->worker_cnt_per_group)
-		return len;
-
+	err = mddev_lock(mddev);
+	if (err)
+		return err;
+	conf = mddev->private;
+	if (!conf)
+		err = -ENODEV;
+	else if (new != conf->worker_cnt_per_group) {
 		mddev_suspend(mddev);
 
 		old_groups = conf->worker_groups;
@@ -5575,12 +5594,11 @@ raid5_store_group_thread_cnt(struct mddev *mddev, const char *page, size_t len)
 				kfree(old_groups[0].workers);
 			kfree(old_groups);
 		}
-
 		mddev_resume(mddev);
+	}
+	mddev_unlock(mddev);
 
-	if (err)
-		return err;
-	return len;
+	return err ?: len;
 }
 
 static struct md_sysfs_entry