9p/trans_fd: avoid sending req to a cancelled conn

When a connection is cancelled by p9_conn_cancel(), all requests on it
should be cancelled---mark req->status as REQ_STATUS_ERROR. However,
because a race over m->err between p9_conn_cancel() and p9_fd_request(),
p9_fd_request might see the old value of m->err, think that the connection
is NOT cancelled, and then add new requests to this cancelled connection.

Fixing this issue by lock-protecting the check on m->err.

Signed-off-by: Sishuai Gong <[e-mail]>
Message-ID: <[e-mail]>
Signed-off-by: Dominique Martinet <[e-mail]>
Reviewed-by: Christian Schoenebeck <[e-mail]>

net/9p/trans_fd.c

···
 
 	p9_debug(P9_DEBUG_TRANS, "mux %p task %p tcall %p id %d\n",
 		 m, current, &req->tc, req->tc.id);
-	if (m->err < 0)
-		return m->err;
 
 	spin_lock(&m->req_lock);
+
+	if (m->err < 0) {
+		spin_unlock(&m->req_lock);
+		return m->err;
+	}
+
 	WRITE_ONCE(req->status, REQ_STATUS_UNSENT);
 	list_add_tail(&req->req_list, &m->unsent_req_list);
 	spin_unlock(&m->req_lock);