diff --git a/external/Makefile b/external/Makefile
index 48bca5479..71b9d81ef 100644
--- a/external/Makefile
+++ b/external/Makefile
@@ -1,3 +1,10 @@
+SUBMODULES =					\
+	external/libsodium			\
+	external/libwally-core			\
+	external/jsmn				\
+	external/libbase58			\
+	external/libbacktrace
+
 LIBSODIUM_HEADERS := external/libsodium/src/libsodium/include/sodium.h
 LIBWALLY_HEADERS := external/libwally-core/include/wally_bip32.h		\
 			external/libwally-core/include/wally_core.h	\
@@ -21,20 +28,19 @@ EXTERNAL_INCLUDE_FLAGS :=					\
 
 EXTERNAL_LDLIBS := -Lexternal $(patsubst lib%.a,-l%,$(notdir $(EXTERNAL_LIBS)))
 
-# Might exist, but need updating.  Nuke and rebuild.
-submodcheck-%: FORCE
-	if git submodule status external/$* | grep -q '^[-+]'; then rm -rf external/$*; git submodule update --init external/$*; fi
+submodcheck: FORCE
+	@tools/refresh-submodules.sh $(SUBMODULES)
 
 # We build libsodium, since Ubuntu xenial has one too old.
 external/libsodium.a: external/libsodium/src/libsodium/libsodium.la
 	$(MAKE) -C external/libsodium DESTDIR=$$(pwd)/external install-exec
 
-external/libsodium/src/libsodium/include/sodium.h: submodcheck-libsodium
+external/libsodium/src/libsodium/include/sodium.h: submodcheck
 
 external/libsodium/src/libsodium/libsodium.la: external/libsodium/src/libsodium/include/sodium.h
 	cd external/libsodium && ./autogen.sh && ./configure CC="$(CC)" --enable-static=yes --host="$(MAKE_HOST)" --build="$(BUILD)" --enable-shared=no --enable-tests=no --prefix=/ --libdir=/ && $(MAKE)
 
-$(LIBWALLY_HEADERS) $(LIBSECP_HEADERS): submodcheck-libwally-core
+$(LIBWALLY_HEADERS) $(LIBSECP_HEADERS): submodcheck
 
 # libsecp included in libwally.
 # Wildcards here are magic.  See http://stackoverflow.com/questions/2973445/gnu-makefile-rule-generating-a-few-targets-from-a-single-source-file
@@ -44,7 +50,7 @@ external/libsecp256k1.% external/libwallycore.%: external/libwally-core/src/secp
 external/libwally-core/src/libwallycore.% external/libwally-core/src/secp256k1/libsecp256k1.%: $(LIBWALLY_HEADERS) $(LIBSECP_HEADERS)
 	cd external/libwally-core && ./tools/autogen.sh && ./configure CC="$(CC)" --enable-static=yes --host="$(MAKE_HOST)" --build="$(BUILD)" --enable-module-recovery --enable-shared=no --prefix=/ --libdir=/ && $(MAKE)
 
-external/jsmn/jsmn.h: submodcheck-jsmn
+external/jsmn/jsmn.h: submodcheck
 
 # If we tell Make that the above builds both, it runs it twice in
 # parallel.  So we lie :(
@@ -61,7 +67,7 @@ LIBBASE58_SRC := external/libbase58/base58.c
 
 $(LIBBASE58_SRC): $(LIBBASE58_HEADERS)
 
-$(LIBBASE58_HEADERS): submodcheck-libbase58
+$(LIBBASE58_HEADERS): submodcheck
 
 # Can't be inside submodule, as that makes git think it's dirty.
 external/base58.o: $(LIBBASE58_SRC) Makefile
@@ -70,7 +76,7 @@ external/base58.o: $(LIBBASE58_SRC) Makefile
 external/libbase58.a: external/base58.o
 	$(AR) rc $@ $<
 
-external/libbacktrace/backtrace.h: submodcheck-libbacktrace
+external/libbacktrace/backtrace.h: submodcheck
 
 # Need separate build dir: changes inside submodule make git think it's dirty.
 external/libbacktrace.a: external/libbacktrace/backtrace.h
diff --git a/tools/refresh-submodules.sh b/tools/refresh-submodules.sh
new file mode 100755
index 000000000..c34784c32
--- /dev/null
+++ b/tools/refresh-submodules.sh
@@ -0,0 +1,30 @@
+#! /bin/sh
+
+if [ $# = 0 ]; then
+    echo "Usage: $0 <submoduledir1>..." >&2
+    exit 1
+fi
+
+# git submodule can't run in parallel.  Really.
+echo $$ > .refresh-submodules.$$
+if ! mv -n .refresh-submodules.$$ .refresh-submodules; then
+    rm -f .refresh-submodules.$$
+    exit 0
+fi
+trap "rm -f .refresh-submodules" EXIT
+
+# Be a little careful here, since we do rm -rf!
+for m in "$@"; do
+    if ! grep -q "path = $m\$" .gitmodules; then
+	echo "$m is not a submodule!" >&2
+	exit 1
+    fi
+done
+
+# git submodule can segfault.  Really.
+if [ "$(git submodule status "$@" | grep -c '^ ')" != $# ]; then
+    echo Reinitializing submodules "$@" ...
+    git submodule sync "$@"
+    rm -rf "$@"
+    git submodule update --init "$@"
+fi