GFS2: Ensure uptodate inode size when using O_APPEND

The VFS reads the inode size during generic_file_aio_write() but
with no locking around it. In order to get the expected result
from O_APPEND opens, this patch updated the inode size before
calling generic_file_aio_write()

There is of course still a race here, in that there is nothing to
prevent another node coming in and extending the file in the
mean time. On the other hand, when used with file locking this
will ensure that the expected results are obtained.

Signed-off-by: Steven Whitehouse <swhiteho@redhat.com>

fs/gfs2/file.c

@@ -569,6 +569,40 @@ static int gfs2_fsync(struct file *file, struct dentry *dentry, int datasync)
 	return ret;
 }
 
+/**
+ * gfs2_file_aio_write - Perform a write to a file
+ * @iocb: The io context
+ * @iov: The data to write
+ * @nr_segs: Number of @iov segments
+ * @pos: The file position
+ *
+ * We have to do a lock/unlock here to refresh the inode size for
+ * O_APPEND writes, otherwise we can land up writing at the wrong
+ * offset. There is still a race, but provided the app is using its
+ * own file locking, this will make O_APPEND work as expected.
+ *
+ */
+
+static ssize_t gfs2_file_aio_write(struct kiocb *iocb, const struct iovec *iov,
+				   unsigned long nr_segs, loff_t pos)
+{
+	struct file *file = iocb->ki_filp;
+
+	if (file->f_flags & O_APPEND) {
+		struct dentry *dentry = file->f_dentry;
+		struct gfs2_inode *ip = GFS2_I(dentry->d_inode);
+		struct gfs2_holder gh;
+		int ret;
+
+		ret = gfs2_glock_nq_init(ip->i_gl, LM_ST_SHARED, 0, &gh);
+		if (ret)
+			return ret;
+		gfs2_glock_dq_uninit(&gh);
+	}
+
+	return generic_file_aio_write(iocb, iov, nr_segs, pos);
+}
+
 #ifdef CONFIG_GFS2_FS_LOCKING_DLM
 
 /**
@@ -711,7 +745,7 @@ const struct file_operations gfs2_file_fops = {
 	.read		= do_sync_read,
 	.aio_read	= generic_file_aio_read,
 	.write		= do_sync_write,
-	.aio_write	= generic_file_aio_write,
+	.aio_write	= gfs2_file_aio_write,
 	.unlocked_ioctl	= gfs2_ioctl,
 	.mmap		= gfs2_mmap,
 	.open		= gfs2_open,
@@ -741,7 +775,7 @@ const struct file_operations gfs2_file_fops_nolock = {
 	.read		= do_sync_read,
 	.aio_read	= generic_file_aio_read,
 	.write		= do_sync_write,
-	.aio_write	= generic_file_aio_write,
+	.aio_write	= gfs2_file_aio_write,
 	.unlocked_ioctl	= gfs2_ioctl,
 	.mmap		= gfs2_mmap,
 	.open		= gfs2_open,