From c1a95fda2a40ae8c7aad3fa44fa7718a3710eb2d Mon Sep 17 00:00:00 2001 From: Nicolas Pitre Date: Fri, 22 Jan 2016 13:41:57 -0500 Subject: [PATCH] kbuild: add fine grained build dependencies for exported symbols Like with kconfig options, we now have the ability to compile in and out individual EXPORT_SYMBOL() declarations based on the content of include/generated/autoksyms.h. However we don't want the entire world to be rebuilt whenever that file is touched. Let's apply the same build dependency trick used for CONFIG_* symbols where the time stamp of empty files whose paths matching those symbols is used to trigger fine grained rebuilds. In our case the key is the symbol name passed to EXPORT_SYMBOL(). However, unlike config options, we cannot just use fixdep to parse the source code for EXPORT_SYMBOL(ksym) because several variants exist and parsing them all in a separate tool, and keeping it in synch, is not trivially maintainable. Furthermore, there are variants such as EXPORT_SYMBOL_GPL(pci_user_read_config_##size); that are instanciated via a macro for which we can't easily determine the actual exported symbol name(s) short of actually running the preprocessor on them. Storing the symbol name string in a special ELF section doesn't work for targets that output assembly or preprocessed source. So the best way is really to leverage the preprocessor by having it output actual symbol names anchored by a special sequence that can be easily filtered out. Then the list of symbols is simply fed to fixdep to be merged with the other dependencies. That implies the preprocessor is executed twice for each source file. A previous attempt relied on a warning pragma for each EXPORT_SYMBOL() instance that was filtered apart from stderr by the build system with a sed script during the actual compilation pass. Unfortunately the preprocessor/compiler diagnostic output isn't stable between versions and this solution, although more efficient, was deemed too fragile. Because of the lowercasing performed by fixdep, there might be name collisions triggering spurious rebuilds for similar symbols. But this shouldn't be a big issue in practice. (This is the case for CONFIG_* symbols and I didn't want to be different here, whatever the original reason for doing so.) To avoid needless build overhead, the exported symbol name gathering is performed only when CONFIG_TRIM_UNUSED_KSYMS is selected. Signed-off-by: Nicolas Pitre Acked-by: Rusty Russell --- include/linux/export.h | 13 ++++++++++++- scripts/Kbuild.include | 27 +++++++++++++++++++++++++++ scripts/basic/fixdep.c | 1 + 3 files changed, 40 insertions(+), 1 deletion(-) diff --git a/include/linux/export.h b/include/linux/export.h index 77afdb2a2506..2f9ccbe6a639 100644 --- a/include/linux/export.h +++ b/include/linux/export.h @@ -65,7 +65,18 @@ extern struct module __this_module; __attribute__((section("___ksymtab" sec "+" #sym), unused)) \ = { (unsigned long)&sym, __kstrtab_##sym } -#ifdef CONFIG_TRIM_UNUSED_KSYMS +#if defined(__KSYM_DEPS__) + +/* + * For fine grained build dependencies, we want to tell the build system + * about each possible exported symbol even if they're not actually exported. + * We use a string pattern that is unlikely to be valid code that the build + * system filters out from the preprocessor output (see ksym_dep_filter + * in scripts/Kbuild.include). + */ +#define __EXPORT_SYMBOL(sym, sec) === __KSYM_##sym === + +#elif defined(CONFIG_TRIM_UNUSED_KSYMS) #include #include diff --git a/scripts/Kbuild.include b/scripts/Kbuild.include index 80ca538bfba9..a09927e02713 100644 --- a/scripts/Kbuild.include +++ b/scripts/Kbuild.include @@ -258,12 +258,39 @@ if_changed_dep = $(if $(strip $(any-prereq) $(arg-check) ), \ @set -e; \ $(cmd_and_fixdep), @:) +ifndef CONFIG_TRIM_UNUSED_KSYMS + cmd_and_fixdep = \ $(echo-cmd) $(cmd_$(1)); \ scripts/basic/fixdep $(depfile) $@ '$(make-cmd)' > $(dot-target).tmp;\ rm -f $(depfile); \ mv -f $(dot-target).tmp $(dot-target).cmd; +else + +# Filter out exported kernel symbol names from the preprocessor output. +# See also __KSYM_DEPS__ in include/linux/export.h. +# We disable the depfile generation here, so as not to overwrite the existing +# depfile while fixdep is parsing it. +flags_nodeps = $(filter-out -Wp$(comma)-M%, $($(1))) +ksym_dep_filter = \ + case "$(1)" in \ + cc_*_c) $(CPP) $(call flags_nodeps,c_flags) -D__KSYM_DEPS__ $< ;; \ + as_*_S) $(CPP) $(call flags_nodeps,a_flags) -D__KSYM_DEPS__ $< ;; \ + boot*|build*|*cpp_lds_S|dtc|host*|vdso*) : ;; \ + *) echo "Don't know how to preprocess $(1)" >&2; false ;; \ + esac | sed -rn 's/^.*=== __KSYM_(.*) ===.*$$/KSYM_\1/p' + +cmd_and_fixdep = \ + $(echo-cmd) $(cmd_$(1)); \ + $(ksym_dep_filter) | \ + scripts/basic/fixdep -e $(depfile) $@ '$(make-cmd)' \ + > $(dot-target).tmp; \ + rm -f $(depfile); \ + mv -f $(dot-target).tmp $(dot-target).cmd; + +endif + # Usage: $(call if_changed_rule,foo) # Will check if $(cmd_foo) or any of the prerequisites changed, # and if so will execute $(rule_foo). diff --git a/scripts/basic/fixdep.c b/scripts/basic/fixdep.c index 7e90a1f7de0f..746ec1ece614 100644 --- a/scripts/basic/fixdep.c +++ b/scripts/basic/fixdep.c @@ -358,6 +358,7 @@ static void parse_dep_file(void *map, size_t len) /* Ignore certain dependencies */ if (strrcmp(s, "include/generated/autoconf.h") && + strrcmp(s, "include/generated/autoksyms.h") && strrcmp(s, "arch/um/include/uml-config.h") && strrcmp(s, "include/linux/kconfig.h") && strrcmp(s, ".ver")) { -- 2.20.1