fuse: hold i_mutex in fuse_file_fallocate()

Changing size of a file on server and local update (fuse_write_update_size)
should be always protected by inode->i_mutex. Otherwise a race like this is
possible:

1. Process 'A' calls fallocate(2) to extend file (~FALLOC_FL_KEEP_SIZE).
fuse_file_fallocate() sends FUSE_FALLOCATE request to the server.
2. Process 'B' calls ftruncate(2) shrinking the file. fuse_do_setattr()
sends shrinking FUSE_SETATTR request to the server and updates local i_size
by i_size_write(inode, outarg.attr.size).
3. Process 'A' resumes execution of fuse_file_fallocate() and calls
fuse_write_update_size(inode, offset + length). But 'offset + length' was
obsoleted by ftruncate from previous step.

Changed in v2 (thanks Brian and Anand for suggestions):
 - made relation between mutex_lock() and fuse_set_nowrite(inode) more
   explicit and clear.
 - updated patch description to use ftruncate(2) in example

Signed-off-by: Maxim V. Patlasov <MPatlasov@parallels.com>
Reviewed-by: Brian Foster <bfoster@redhat.com>
Signed-off-by: Miklos Szeredi <mszeredi@suse.cz>

fs/fuse/file.c

@@ -2470,12 +2470,15 @@ static long fuse_file_fallocate(struct file *file, int mode, loff_t offset,
 		.mode = mode
 	};
 	int err;
+	bool lock_inode = !(mode & FALLOC_FL_KEEP_SIZE) ||
+			   (mode & FALLOC_FL_PUNCH_HOLE);
 
 	if (fc->no_fallocate)
 		return -EOPNOTSUPP;
 
-	if (mode & FALLOC_FL_PUNCH_HOLE) {
+	if (lock_inode) {
 		mutex_lock(&inode->i_mutex);
+		if (mode & FALLOC_FL_PUNCH_HOLE)
 			fuse_set_nowrite(inode);
 	}
 
@@ -2511,7 +2514,8 @@ static long fuse_file_fallocate(struct file *file, int mode, loff_t offset,
 	fuse_invalidate_attr(inode);
 
 out:
-	if (mode & FALLOC_FL_PUNCH_HOLE) {
+	if (lock_inode) {
+		if (mode & FALLOC_FL_PUNCH_HOLE)
 			fuse_release_nowrite(inode);
 		mutex_unlock(&inode->i_mutex);
 	}