nfs: take i_mutex during direct I/O reads

We'll need the i_mutex to prevent i_dio_count from incrementing while
truncate is waiting for it to reach zero, and protects against having
the pagecache repopulated after we flushed it.
Signed-off-by: NChristoph Hellwig <hch@lst.de>
Signed-off-by: NTrond Myklebust <trond.myklebust@primarydata.com>

fs/nfs/direct.c

@@ -500,16 +500,17 @@ ssize_t nfs_file_direct_read(struct kiocb *iocb, const struct iovec *iov,
 	if (!count)
 		goto out;
 
+	mutex_lock(&inode->i_mutex);
 	result = nfs_sync_mapping(mapping);
 	if (result)
-		goto out;
+		goto out_unlock;
 
 	task_io_account_read(count);
 
 	result = -ENOMEM;
 	dreq = nfs_direct_req_alloc();
 	if (dreq == NULL)
-		goto out;
+		goto out_unlock;
 
 	dreq->inode = inode;
 	dreq->bytes_left = iov_length(iov, nr_segs);
@@ -525,13 +526,22 @@ ssize_t nfs_file_direct_read(struct kiocb *iocb, const struct iovec *iov,
 
 	NFS_I(inode)->read_io += iov_length(iov, nr_segs);
 	result = nfs_direct_read_schedule_iovec(dreq, iov, nr_segs, pos, uio);
+
+	mutex_unlock(&inode->i_mutex);
+
 	if (!result) {
 		result = nfs_direct_wait(dreq);
 		if (result > 0)
 			iocb->ki_pos = pos + result;
 	}
+
+	nfs_direct_req_release(dreq);
+	return result;
+
 out_release:
 	nfs_direct_req_release(dreq);
+out_unlock:
+	mutex_unlock(&inode->i_mutex);
 out:
 	return result;
 }