Btrfs: introduce btrfs_{start, end}_nocow_write() for each subvolume

[linux-2.6-block.git] / fs / btrfs / extent-tree.c

diff --git a/fs/btrfs/extent-tree.c b/fs/btrfs/extent-tree.c
index 19ea8ad70c671661da0b1c5677e48be4f14086a5..6b821c64b37b715f2ad1d6e4e23ae0e1cddf04ad 100644 (file)
--- a/fs/btrfs/extent-tree.c
+++ b/fs/btrfs/extent-tree.c
@@ -8938,3 +8938,38 @@ int btrfs_trim_fs(struct btrfs_root *root, struct fstrim_range *range)
        range->len = trimmed;
        return ret;
 }
+
+/*
+ * btrfs_{start,end}_write() is similar to mnt_{want, drop}_write(),
+ * they are used to prevent the some tasks writing data into the page cache
+ * by nocow before the subvolume is snapshoted, but flush the data into
+ * the disk after the snapshot creation.
+ */
+void btrfs_end_nocow_write(struct btrfs_root *root)
+{
+       percpu_counter_dec(&root->subv_writers->counter);
+       /*
+        * Make sure counter is updated before we wake up
+        * waiters.
+        */
+       smp_mb();
+       if (waitqueue_active(&root->subv_writers->wait))
+               wake_up(&root->subv_writers->wait);
+}
+
+int btrfs_start_nocow_write(struct btrfs_root *root)
+{
+       if (unlikely(atomic_read(&root->will_be_snapshoted)))
+               return 0;
+
+       percpu_counter_inc(&root->subv_writers->counter);
+       /*
+        * Make sure counter is updated before we check for snapshot creation.
+        */
+       smp_mb();
+       if (unlikely(atomic_read(&root->will_be_snapshoted))) {
+               btrfs_end_nocow_write(root);
+               return 0;
+       }
+       return 1;
+}