[PATCH V2 4/4] btrfs: cleanup barrier_all_devices() to check dev stat flush error

[Anand Jain <anand.jain@oracle.com>]

The objective of this patch is to cleanup barrier_all_devices()
so that the error checking is in a separate loop independent of
of the loop which submits and waits on the device flush requests.

By doing this it helps to further develop patches which would tune
the error-actions as needed.

Here functions such as btrfs_dev_stats_dirty() couldn't be used
because it doesn't monitor the flush errors BTRFS_DEV_STAT_FLUSH_ERRS.

Signed-off-by: Anand Jain <anand.jain@oracle.com>
---
v2: Address Qu review comments viz..
     Add meaningful names, like cp_list (for checkpoint_list head).
     (And actually it does not need a new struct type just to hold
      the head pointer, list node is already named as device_checkpoint).
     Check return value of add_device_checkpoint()
     Check if the device is already added at add_device_checkpoint()
     Rename fini_devices_checkpoint() to rel_devices_checkpoint()

diff --git a/fs/btrfs/disk-io.c b/fs/btrfs/disk-io.c
index 5719e036048b..d0c401884643 100644
--- a/fs/btrfs/disk-io.c
+++ b/fs/btrfs/disk-io.c
@@ -3566,6 +3566,86 @@ static int write_dev_flush(struct btrfs_device *device, int wait)
 	return 0;
 }
 
+struct device_checkpoint {
+	struct list_head cp_node;
+	struct btrfs_device *device;
+	int stat_value_checkpoint;
+};
+
+static int add_device_checkpoint(struct btrfs_device *device,
+				struct list_head *cp_list)
+{
+	struct device_checkpoint *cdev;
+
+	list_for_each_entry(cdev, cp_list, cp_node) {
+		if (cdev->device == device) {
+			cdev->stat_value_checkpoint =
+				btrfs_dev_stat_read(device,
+					BTRFS_DEV_STAT_FLUSH_ERRS);
+			return 0;
+		}
+	}
+
+	cdev = kzalloc(sizeof(struct device_checkpoint), GFP_KERNEL);
+	if (!cdev)
+		return -ENOMEM;
+
+	list_add(&cdev->cp_node, cp_list);
+
+	cdev->device = device;
+	cdev->stat_value_checkpoint =
+		btrfs_dev_stat_read(device, BTRFS_DEV_STAT_FLUSH_ERRS);
+
+	return 0;
+}
+
+static void rel_devices_checkpoint(struct list_head *cp_list)
+{
+	struct device_checkpoint *cdev;
+
+	while(!list_empty(cp_list)) {
+		cdev = list_entry(cp_list->next,
+				struct device_checkpoint, cp_node);
+		list_del(&cdev->cp_node);
+		kfree(cdev);
+	}
+}
+
+static int check_stat_flush(struct btrfs_device *dev,
+				struct list_head *cp_list)
+{
+	int val;
+	struct device_checkpoint *cdev;
+
+	list_for_each_entry(cdev, cp_list, cp_node) {
+		if (cdev->device == dev) {
+			val = btrfs_dev_stat_read(dev,
+				BTRFS_DEV_STAT_FLUSH_ERRS);
+			if (cdev->stat_value_checkpoint != val)
+				return 1;
+		}
+	}
+	return 0;
+}
+
+static int check_barrier_error(struct btrfs_fs_devices *fsdevs,
+				struct list_head *cp_list)
+{
+	int dropouts = 0;
+	struct btrfs_device *dev;
+
+	list_for_each_entry_rcu(dev, &fsdevs->devices, dev_list) {
+		if (!dev->bdev || check_stat_flush(dev, cp_list))
+			dropouts++;
+	}
+
+	if (dropouts >
+		fsdevs->fs_info->num_tolerated_disk_barrier_failures)
+		return -EIO;
+
+	return 0;
+}
+
 /*
  * send an empty flush down to each device in parallel,
  * then wait for them
@@ -3574,8 +3654,8 @@ static int barrier_all_devices(struct btrfs_fs_info *info)
 {
 	struct list_head *head;
 	struct btrfs_device *dev;
-	int dropouts = 0;
 	int ret;
+	static LIST_HEAD(cp_list);
 
 	/* send down all the barriers */
 	head = &info->fs_devices->devices;
@@ -3587,29 +3667,35 @@ static int barrier_all_devices(struct btrfs_fs_info *info)
 		if (!dev->in_fs_metadata || !dev->writeable)
 			continue;
 
+		ret = add_device_checkpoint(dev, &cp_list);
+		if (ret) {
+			rel_devices_checkpoint(&cp_list);
+			return ret;
+		}
 		ret = write_dev_flush(dev, 0);
-		if (ret)
+		if (ret) {
+			rel_devices_checkpoint(&cp_list);
 			return ret;
+		}
 	}
 
 	/* wait for all the barriers */
 	list_for_each_entry_rcu(dev, head, dev_list) {
 		if (dev->missing)
 			continue;
-		if (!dev->bdev) {
-			dropouts++;
+		if (!dev->bdev)
 			continue;
-		}
 		if (!dev->in_fs_metadata || !dev->writeable)
 			continue;
 
-		ret = write_dev_flush(dev, 1);
-		if (ret)
-			dropouts++;
+		write_dev_flush(dev, 1);
 	}
-	if (dropouts > info->num_tolerated_disk_barrier_failures)
-		return -EIO;
-	return 0;
+
+	ret = check_barrier_error(info->fs_devices, &cp_list);
+
+	rel_devices_checkpoint(&cp_list);
+
+	return ret;
 }
 
 int btrfs_get_num_tolerated_disk_barrier_failures(u64 flags)
-- 
2.10.0