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kbuild: mark rustc (and others) invocations as recursive
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`rustc` (like Cargo) may take advantage of the jobserver at any time
(e.g. for backend parallelism, or eventually frontend too). In the kernel,
we call `rustc` with `-Ccodegen-units=1` (and `-Zthreads` is 1 so far),
so we do not expect parallelism. However, in the upcoming Rust 1.76.0, a
warning is emitted by `rustc` [1] when it cannot connect to the jobserver
it was passed (in many cases, but not all: compiling and `--print sysroot`
do, but `--version` does not). And given GNU Make always passes
the jobserver in the environment variable (even when a line is deemed
non-recursive), `rustc` will end up complaining about it (in particular
in Make 4.3 where there is only the simple pipe jobserver style).

One solution is to remove the jobserver from `MAKEFLAGS`. However, we
can mark the lines with calls to `rustc` (and Cargo) as recursive, which
looks simpler. This is being documented as a recommendation in `rustc`
[2] and allows us to be ready for the time we may use parallelism inside
`rustc` (potentially now, if a user passes `-Zthreads`). Thus do so.

Similarly, do the same for `rustdoc` and `cargo` calls.

Finally, there is one case that the solution does not cover, which is the
`$(shell ...)` call we have. Thus, for that one, set an empty `MAKEFLAGS`
environment variable.

Link: rust-lang/rust#120515 [1]
Acked-by: Masahiro Yamada <[email protected]>
Link: rust-lang/rust#121564 [2]
Link: https://lore.kernel.org/r/[email protected]
[ Reworded to add link to PR documenting the recommendation. ]
Signed-off-by: Miguel Ojeda <[email protected]>
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ojeda authored and bertschingert committed Mar 9, 2024
1 parent 9d5abc3 commit ef5b8f9
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Showing 4 changed files with 31 additions and 31 deletions.
4 changes: 2 additions & 2 deletions Makefile
Original file line number Diff line number Diff line change
Expand Up @@ -1197,7 +1197,7 @@ prepare0: archprepare
# All the preparing..
prepare: prepare0
ifdef CONFIG_RUST
$(Q)$(CONFIG_SHELL) $(srctree)/scripts/rust_is_available.sh
+$(Q)$(CONFIG_SHELL) $(srctree)/scripts/rust_is_available.sh
$(Q)$(MAKE) $(build)=rust
endif

Expand Down Expand Up @@ -1707,7 +1707,7 @@ $(DOC_TARGETS):
# "Is Rust available?" target
PHONY += rustavailable
rustavailable:
$(Q)$(CONFIG_SHELL) $(srctree)/scripts/rust_is_available.sh && echo "Rust is available!"
+$(Q)$(CONFIG_SHELL) $(srctree)/scripts/rust_is_available.sh && echo "Rust is available!"

# Documentation target
#
Expand Down
48 changes: 24 additions & 24 deletions rust/Makefile
Original file line number Diff line number Diff line change
Expand Up @@ -40,7 +40,7 @@ obj-$(CONFIG_RUST_KERNEL_DOCTESTS) += doctests_kernel_generated_kunit.o
ifdef CONFIG_RUST

# `$(rust_flags)` is passed in case the user added `--sysroot`.
rustc_sysroot := $(shell $(RUSTC) $(rust_flags) --print sysroot)
rustc_sysroot := $(shell MAKEFLAGS= $(RUSTC) $(rust_flags) --print sysroot)
rustc_host_target := $(shell $(RUSTC) --version --verbose | grep -F 'host: ' | cut -d' ' -f2)
RUST_LIB_SRC ?= $(rustc_sysroot)/lib/rustlib/src/rust/library

Expand Down Expand Up @@ -108,14 +108,14 @@ rustdoc-macros: private rustdoc_host = yes
rustdoc-macros: private rustc_target_flags = --crate-type proc-macro \
--extern proc_macro
rustdoc-macros: $(src)/macros/lib.rs FORCE
$(call if_changed,rustdoc)
+$(call if_changed,rustdoc)

rustdoc-core: private rustc_target_flags = $(core-cfgs)
rustdoc-core: $(RUST_LIB_SRC)/core/src/lib.rs FORCE
$(call if_changed,rustdoc)
+$(call if_changed,rustdoc)

rustdoc-compiler_builtins: $(src)/compiler_builtins.rs rustdoc-core FORCE
$(call if_changed,rustdoc)
+$(call if_changed,rustdoc)

# We need to allow `rustdoc::broken_intra_doc_links` because some
# `no_global_oom_handling` functions refer to non-`no_global_oom_handling`
Expand All @@ -124,15 +124,15 @@ rustdoc-compiler_builtins: $(src)/compiler_builtins.rs rustdoc-core FORCE
rustdoc-alloc: private rustc_target_flags = $(alloc-cfgs) \
-Arustdoc::broken_intra_doc_links
rustdoc-alloc: $(src)/alloc/lib.rs rustdoc-core rustdoc-compiler_builtins FORCE
$(call if_changed,rustdoc)
+$(call if_changed,rustdoc)

rustdoc-kernel: private rustc_target_flags = --extern alloc \
--extern build_error --extern macros=$(objtree)/$(obj)/libmacros.so \
--extern bindings --extern uapi
rustdoc-kernel: $(src)/kernel/lib.rs rustdoc-core rustdoc-macros \
rustdoc-compiler_builtins rustdoc-alloc $(obj)/libmacros.so \
$(obj)/bindings.o FORCE
$(call if_changed,rustdoc)
+$(call if_changed,rustdoc)

quiet_cmd_rustc_test_library = RUSTC TL $<
cmd_rustc_test_library = \
Expand All @@ -146,18 +146,18 @@ quiet_cmd_rustc_test_library = RUSTC TL $<
--crate-name $(subst rusttest-,,$(subst rusttestlib-,,$@)) $<

rusttestlib-build_error: $(src)/build_error.rs rusttest-prepare FORCE
$(call if_changed,rustc_test_library)
+$(call if_changed,rustc_test_library)

rusttestlib-macros: private rustc_target_flags = --extern proc_macro
rusttestlib-macros: private rustc_test_library_proc = yes
rusttestlib-macros: $(src)/macros/lib.rs rusttest-prepare FORCE
$(call if_changed,rustc_test_library)
+$(call if_changed,rustc_test_library)

rusttestlib-bindings: $(src)/bindings/lib.rs rusttest-prepare FORCE
$(call if_changed,rustc_test_library)
+$(call if_changed,rustc_test_library)

rusttestlib-uapi: $(src)/uapi/lib.rs rusttest-prepare FORCE
$(call if_changed,rustc_test_library)
+$(call if_changed,rustc_test_library)

quiet_cmd_rustdoc_test = RUSTDOC T $<
cmd_rustdoc_test = \
Expand Down Expand Up @@ -189,7 +189,7 @@ quiet_cmd_rustdoc_test_kernel = RUSTDOC TK $<
$(src)/kernel/lib.rs $(obj)/kernel.o \
$(objtree)/scripts/rustdoc_test_builder \
$(objtree)/scripts/rustdoc_test_gen FORCE
$(call if_changed,rustdoc_test_kernel)
+$(call if_changed,rustdoc_test_kernel)

# We cannot use `-Zpanic-abort-tests` because some tests are dynamic,
# so for the moment we skip `-Cpanic=abort`.
Expand Down Expand Up @@ -254,21 +254,21 @@ quiet_cmd_rustsysroot = RUSTSYSROOT
$(objtree)/$(obj)/test/sysroot/lib/rustlib/$(rustc_host_target)/lib

rusttest-prepare: FORCE
$(call if_changed,rustsysroot)
+$(call if_changed,rustsysroot)

rusttest-macros: private rustc_target_flags = --extern proc_macro
rusttest-macros: private rustdoc_test_target_flags = --crate-type proc-macro
rusttest-macros: $(src)/macros/lib.rs rusttest-prepare FORCE
$(call if_changed,rustc_test)
$(call if_changed,rustdoc_test)
+$(call if_changed,rustc_test)
+$(call if_changed,rustdoc_test)

rusttest-kernel: private rustc_target_flags = --extern alloc \
--extern build_error --extern macros --extern bindings --extern uapi
rusttest-kernel: $(src)/kernel/lib.rs rusttest-prepare \
rusttestlib-build_error rusttestlib-macros rusttestlib-bindings \
rusttestlib-uapi FORCE
$(call if_changed,rustc_test)
$(call if_changed,rustc_test_library)
+$(call if_changed,rustc_test)
+$(call if_changed,rustc_test_library)

ifdef CONFIG_CC_IS_CLANG
bindgen_c_flags = $(c_flags)
Expand Down Expand Up @@ -396,7 +396,7 @@ quiet_cmd_rustc_procmacro = $(RUSTC_OR_CLIPPY_QUIET) P $@
# Therefore, to get `libmacros.so` automatically recompiled when the compiler
# version changes, we add `core.o` as a dependency (even if it is not needed).
$(obj)/libmacros.so: $(src)/macros/lib.rs $(obj)/core.o FORCE
$(call if_changed_dep,rustc_procmacro)
+$(call if_changed_dep,rustc_procmacro)

quiet_cmd_rustc_library = $(if $(skip_clippy),RUSTC,$(RUSTC_OR_CLIPPY_QUIET)) L $@
cmd_rustc_library = \
Expand Down Expand Up @@ -435,36 +435,36 @@ $(obj)/core.o: private skip_flags = -Dunreachable_pub
$(obj)/core.o: private rustc_objcopy = $(foreach sym,$(redirect-intrinsics),--redefine-sym $(sym)=__rust$(sym))
$(obj)/core.o: private rustc_target_flags = $(core-cfgs)
$(obj)/core.o: $(RUST_LIB_SRC)/core/src/lib.rs scripts/target.json FORCE
$(call if_changed_dep,rustc_library)
+$(call if_changed_dep,rustc_library)

$(obj)/compiler_builtins.o: private rustc_objcopy = -w -W '__*'
$(obj)/compiler_builtins.o: $(src)/compiler_builtins.rs $(obj)/core.o FORCE
$(call if_changed_dep,rustc_library)
+$(call if_changed_dep,rustc_library)

$(obj)/alloc.o: private skip_clippy = 1
$(obj)/alloc.o: private skip_flags = -Dunreachable_pub
$(obj)/alloc.o: private rustc_target_flags = $(alloc-cfgs)
$(obj)/alloc.o: $(src)/alloc/lib.rs $(obj)/compiler_builtins.o FORCE
$(call if_changed_dep,rustc_library)
+$(call if_changed_dep,rustc_library)

$(obj)/build_error.o: $(src)/build_error.rs $(obj)/compiler_builtins.o FORCE
$(call if_changed_dep,rustc_library)
+$(call if_changed_dep,rustc_library)

$(obj)/bindings.o: $(src)/bindings/lib.rs \
$(obj)/compiler_builtins.o \
$(obj)/bindings/bindings_generated.rs \
$(obj)/bindings/bindings_helpers_generated.rs FORCE
$(call if_changed_dep,rustc_library)
+$(call if_changed_dep,rustc_library)

$(obj)/uapi.o: $(src)/uapi/lib.rs \
$(obj)/compiler_builtins.o \
$(obj)/uapi/uapi_generated.rs FORCE
$(call if_changed_dep,rustc_library)
+$(call if_changed_dep,rustc_library)

$(obj)/kernel.o: private rustc_target_flags = --extern alloc \
--extern build_error --extern macros --extern bindings --extern uapi
$(obj)/kernel.o: $(src)/kernel/lib.rs $(obj)/alloc.o $(obj)/build_error.o \
$(obj)/libmacros.so $(obj)/bindings.o $(obj)/uapi.o FORCE
$(call if_changed_dep,rustc_library)
+$(call if_changed_dep,rustc_library)

endif # CONFIG_RUST
8 changes: 4 additions & 4 deletions scripts/Makefile.build
Original file line number Diff line number Diff line change
Expand Up @@ -290,27 +290,27 @@ quiet_cmd_rustc_o_rs = $(RUSTC_OR_CLIPPY_QUIET) $(quiet_modtag) $@
cmd_rustc_o_rs = $(rust_common_cmd) --emit=obj=$@ $<

$(obj)/%.o: $(src)/%.rs FORCE
$(call if_changed_dep,rustc_o_rs)
+$(call if_changed_dep,rustc_o_rs)

quiet_cmd_rustc_rsi_rs = $(RUSTC_OR_CLIPPY_QUIET) $(quiet_modtag) $@
cmd_rustc_rsi_rs = \
$(rust_common_cmd) -Zunpretty=expanded $< >$@; \
command -v $(RUSTFMT) >/dev/null && $(RUSTFMT) $@

$(obj)/%.rsi: $(src)/%.rs FORCE
$(call if_changed_dep,rustc_rsi_rs)
+$(call if_changed_dep,rustc_rsi_rs)

quiet_cmd_rustc_s_rs = $(RUSTC_OR_CLIPPY_QUIET) $(quiet_modtag) $@
cmd_rustc_s_rs = $(rust_common_cmd) --emit=asm=$@ $<

$(obj)/%.s: $(src)/%.rs FORCE
$(call if_changed_dep,rustc_s_rs)
+$(call if_changed_dep,rustc_s_rs)

quiet_cmd_rustc_ll_rs = $(RUSTC_OR_CLIPPY_QUIET) $(quiet_modtag) $@
cmd_rustc_ll_rs = $(rust_common_cmd) --emit=llvm-ir=$@ $<

$(obj)/%.ll: $(src)/%.rs FORCE
$(call if_changed_dep,rustc_ll_rs)
+$(call if_changed_dep,rustc_ll_rs)

# Compile assembler sources (.S)
# ---------------------------------------------------------------------------
Expand Down
2 changes: 1 addition & 1 deletion scripts/Makefile.host
Original file line number Diff line number Diff line change
Expand Up @@ -156,7 +156,7 @@ quiet_cmd_host-rust = HOSTRUSTC $@
cmd_host-rust = \
$(HOSTRUSTC) $(hostrust_flags) --emit=link=$@ $<
$(host-rust): $(obj)/%: $(src)/%.rs FORCE
$(call if_changed_dep,host-rust)
+$(call if_changed_dep,host-rust)

targets += $(host-csingle) $(host-cmulti) $(host-cobjs) \
$(host-cxxmulti) $(host-cxxobjs) $(host-rust)

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