diff --git a/share/mk/src.sys.mk b/share/mk/src.sys.mk
index fff26b477c6..addc115b8ab 100644
--- a/share/mk/src.sys.mk
+++ b/share/mk/src.sys.mk
@@ -34,6 +34,11 @@ __postrcconf_${var}:=	${MK_${var}:U-}${WITHOUT_${var}:Uno:Dyes}${WITH_${var}:Uno
 .endif # SRCCONF
 .endif
 
+# The following should be removed no earlier than LLVM11 being imported into the
+# tree, to ensure we don't regress the build.  LLVM11 and GCC10 will switch the
+# default over to -fno-common, making this redundant.
+CFLAGS+=	-fno-common
+
 # tempting, but bsd.compiler.mk causes problems this early
 # probably need to remove dependence on bsd.own.mk 
 #.include "src.opts.mk"
diff --git a/sys/conf/kern.pre.mk b/sys/conf/kern.pre.mk
index 193d8f6c2fa..918a58b510b 100644
--- a/sys/conf/kern.pre.mk
+++ b/sys/conf/kern.pre.mk
@@ -86,7 +86,7 @@ CFLAGS_PARAM_LARGE_FUNCTION_GROWTH?=1000
 .if ${MACHINE_CPUARCH} == "mips"
 CFLAGS_ARCH_PARAMS?=--param max-inline-insns-single=1000 -DMACHINE_ARCH='"${MACHINE_ARCH}"'
 .endif
-CFLAGS.gcc+= -fno-common -fms-extensions -finline-limit=${INLINE_LIMIT}
+CFLAGS.gcc+= -fms-extensions -finline-limit=${INLINE_LIMIT}
 CFLAGS.gcc+= --param inline-unit-growth=${CFLAGS_PARAM_INLINE_UNIT_GROWTH}
 CFLAGS.gcc+= --param large-function-growth=${CFLAGS_PARAM_LARGE_FUNCTION_GROWTH}
 CFLAGS.gcc+= -fms-extensions
@@ -98,6 +98,10 @@ WERROR?=	-Wno-error
 .else
 WERROR?=	-Werror
 .endif
+# The following should be removed no earlier than LLVM11 being imported into the
+# tree, to ensure we don't regress the build.  LLVM11 and GCC10 will switch the
+# default over to -fno-common, making this redundant.
+CFLAGS+=	-fno-common
 
 # XXX LOCORE means "don't declare C stuff" not "for locore.s".
 ASM_CFLAGS= -x assembler-with-cpp -DLOCORE ${CFLAGS} ${ASM_CFLAGS.${.IMPSRC:T}} 
diff --git a/sys/conf/kmod.mk b/sys/conf/kmod.mk
index 55c8344e7da..e5c72030838 100644
--- a/sys/conf/kmod.mk
+++ b/sys/conf/kmod.mk
@@ -133,6 +133,13 @@ CFLAGS.gcc+= --param large-function-growth=1000
 
 # Disallow common variables, and if we end up with commons from
 # somewhere unexpected, allocate storage for them in the module itself.
+#
+# -fno-common is the default for src builds, but this should be left in place
+# until at least we catch up to GCC10/LLVM11 or otherwise enable -fno-common
+# in <bsd.sys.mk> instead.  For now, we will have duplicate -fno-common in
+# CFLAGS for in-tree module builds as they will also pick it up from
+# share/mk/src.sys.mk, but the following is important for out-of-tree modules
+# (e.g. ports).
 CFLAGS+=	-fno-common
 LDFLAGS+=	-d -warn-common