diff --git a/yosys.spec b/yosys.spec
index 98621b6..685714c 100644
--- a/yosys.spec
+++ b/yosys.spec
@@ -7,7 +7,7 @@
 
 Name:           yosys
 Version:        0.9
-Release:        14.%{snapdate}git%{shortcommit0}%{?dist}
+Release:        15.%{snapdate}git%{shortcommit0}%{?dist}
 Summary:        Yosys Open SYnthesis Suite, including Verilog synthesizer
 License:        ISC and MIT
 URL:            http://www.clifford.at/yosys/
@@ -119,7 +119,8 @@ done
 # disable LTO to allow building for f33 rawhide (BZ 1865657):
 %define _lto_cflags %{nil}
 %set_build_flags
-make %{?_smp_mflags} PREFIX="%{_prefix}" ABCEXTERNAL=%{_bindir}/abc PRETTY=0 all manual
+make %{?_smp_mflags} PREFIX="%{_prefix}" ABCEXTERNAL=%{_bindir}/abc PRETTY=0 all
+make -j1 PREFIX="%{_prefix}" ABCEXTERNAL=%{_bindir}/abc PRETTY=0 manual
 
 %global man_date "`stat -c %y debian/man/yosys-smtbmc.txt | awk '{ print $1 }'`"
 txt2man -d %{man_date} -t YOSYS-SMTBMC debian/man/yosys-smtbmc.txt >yosys-smtbmc.1
@@ -169,6 +170,9 @@ make test ABCEXTERNAL=%{_bindir}/abc SEED=314159265359
 
 
 %changelog
+* Sun Sep 05 2021 Gabriel Somlo <gsomlo@gmail.com> - 0.9.15.20210904git50be8fd
+- use single thread to avoid race condition in pdf doc build
+
 * Sat Sep 04 2021 Gabriel Somlo <gsomlo@gmail.com> - 0.9.14.20210904git50be8fd
 - update snapshot