diff --git a/spec/cspec/c/kernel.mk b/spec/cspec/c/kernel.mk
index f429cf5fdf..a1f8e9e3f9 100644
--- a/spec/cspec/c/kernel.mk
+++ b/spec/cspec/c/kernel.mk
@@ -66,7 +66,11 @@ ifndef ISABELLE_PROCESS
   export ISABELLE_PROCESS:=${ISABELLE_HOME}/bin/isabelle-process
 endif
 
-KERNEL_DEPS := $(shell find ${SOURCE_ROOT} -type f)
+# __pycache__ directories are the only in-tree products of the kernel build.
+# But since they are generated after the cmake setup, they would cause unnecessary
+# kernel rebuilds if we treated them as dependencies of the kernel build.
+# We avoid this by excluding __pycache__ directories from the kernel dependencies.
+KERNEL_DEPS := $(shell find ${SOURCE_ROOT} -name __pycache__ -prune -o -type f -print)
 
 # Top level rule for rebuilding kernel_all.c_pp
 ${KERNEL_BUILD_ROOT}/kernel_all.c_pp: ${KERNEL_BUILD_ROOT}/.cmake_done