btrfs: open code fc_mount() to avoid releasing s_umount rw_sempahore

[CURRENT BEHAVIOR]
Currently inside btrfs_get_tree_subvol(), we call fc_mount() to grab a
tree, then re-lock s_umount inside btrfs_reconfigure_for_mount() to
avoid race with remount.

However fc_mount() itself is just doing two things:

1. Call vfs_get_tree()
2. Release s_umount then call vfs_create_mount()

[ENHANCEMENT]
Instead of calling fc_mount(), we can open-code it with vfs_get_tree()
first.
This provides a benefit that, since we have the full control of
s_umount, we do not need to re-lock that rw_sempahore when calling
btrfs_reconfigure_for_mount(), meaning less race between RO/RW remount.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Reviewed-by: Qu Wenruo <wqu@suse.com>
[ Rework the subject and commit message, refactor the error handling ]
Signed-off-by: Qu Wenruo <wqu@suse.com>
Tested-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

diff --git a/fs/btrfs/super.c b/fs/btrfs/super.c
index a8dfbb84c15c4ed76c90a2a94ac87428268bbaa2..2d0d8c6e77b45601ee4fdca877f46a01f6a0bbaf 100644 (file)
--- a/fs/btrfs/super.c
+++ b/fs/btrfs/super.c
@@ -1996,17 +1996,13 @@ error:
  * btrfs or not, setting the whole super block RO.  To make per-subvolume mounting
  * work with different options work we need to keep backward compatibility.
  */
-static int btrfs_reconfigure_for_mount(struct fs_context *fc, struct vfsmount *mnt)
+static int btrfs_reconfigure_for_mount(struct fs_context *fc)
 {
        int ret = 0;
 
-       if (fc->sb_flags & SB_RDONLY)
-               return ret;
-
-       down_write(&mnt->mnt_sb->s_umount);
-       if (!(fc->sb_flags & SB_RDONLY) && (mnt->mnt_sb->s_flags & SB_RDONLY))
+       if (!(fc->sb_flags & SB_RDONLY) && (fc->root->d_sb->s_flags & SB_RDONLY))
                ret = btrfs_reconfigure(fc);
-       up_write(&mnt->mnt_sb->s_umount);
+
        return ret;
 }
 
@@ -2059,17 +2055,18 @@ static int btrfs_get_tree_subvol(struct fs_context *fc)
        security_free_mnt_opts(&fc->security);
        fc->security = NULL;
 
-       mnt = fc_mount(dup_fc);
-       if (IS_ERR(mnt)) {
-               put_fs_context(dup_fc);
-               return PTR_ERR(mnt);
-       }
-       ret = btrfs_reconfigure_for_mount(dup_fc, mnt);
+       ret = vfs_get_tree(dup_fc);
+       if (ret)
+               goto error;
+
+       ret = btrfs_reconfigure_for_mount(dup_fc);
+       up_write(&dup_fc->root->d_sb->s_umount);
+       if (ret)
+               goto error;
+       mnt = vfs_create_mount(dup_fc);
        put_fs_context(dup_fc);
-       if (ret) {
-               mntput(mnt);
-               return ret;
-       }
+       if (IS_ERR(mnt))
+               return PTR_ERR(mnt);
 
        /*
         * This free's ->subvol_name, because if it isn't set we have to
@@ -2083,6 +2080,9 @@ static int btrfs_get_tree_subvol(struct fs_context *fc)
 
        fc->root = dentry;
        return 0;
+error:
+       put_fs_context(dup_fc);
+       return ret;
 }
 
 static int btrfs_get_tree(struct fs_context *fc)