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Subject[PATCH 4/6] ARC: build: move symlink creation to arch/arc/Makefile to avoid race
Date
If you run 'make uImage uImage.gz' with the parallel option, uImage.gz
will be created by two threads simultaneously.

This is because arch/arc/Makefile does not specify the dependency
between uImage and uImage.gz. Hence, GNU Make assumes they can be
built in parallel. One thread descends into arch/arc/boot/ to create
uImage, and another to create uImage.gz.

Please notice the same log is displayed twice in the following steps:

$ export CROSS_COMPILE=<your-arc-compiler-prefix>
$ make -s ARCH=arc defconfig
$ make -j$(nproc) ARCH=arc uImage uImage.gz
[ snip ]
LD vmlinux
SORTTAB vmlinux
SYSMAP System.map
OBJCOPY arch/arc/boot/vmlinux.bin
OBJCOPY arch/arc/boot/vmlinux.bin
GZIP arch/arc/boot/vmlinux.bin.gz
GZIP arch/arc/boot/vmlinux.bin.gz
UIMAGE arch/arc/boot/uImage.gz
UIMAGE arch/arc/boot/uImage.gz
Image Name: Linux-5.10.0-rc4-00003-g62f23044
Created: Sun Nov 22 02:52:26 2020
Image Type: ARC Linux Kernel Image (gzip compressed)
Data Size: 2109376 Bytes = 2059.94 KiB = 2.01 MiB
Load Address: 80000000
Entry Point: 80004000
Image arch/arc/boot/uImage is ready
Image Name: Linux-5.10.0-rc4-00003-g62f23044
Created: Sun Nov 22 02:52:26 2020
Image Type: ARC Linux Kernel Image (gzip compressed)
Data Size: 2815455 Bytes = 2749.47 KiB = 2.69 MiB
Load Address: 80000000
Entry Point: 80004000

This is a race between the two threads trying to write to the same file
arch/arc/boot/uImage.gz. This is a potential problem that can generate
a broken file.

I fixed a similar problem for ARM by commit 3939f3345050 ("ARM: 8418/1:
add boot image dependencies to not generate invalid images").

I highly recommend to avoid such build rules that cause a race condition.

Move the uImage rule to arch/arc/Makefile.

Another strangeness is that arch/arc/boot/Makefile compares the
timestamps between $(obj)/uImage and $(obj)/uImage.*:

$(obj)/uImage: $(obj)/uImage.$(suffix-y)
@ln -sf $(notdir $<) $@
@echo ' Image $@ is ready'

This does not work as expected since $(obj)/uImage is a symlink.
The symlink should be created in a phony target rule.

I used $(kecho) instead of echo to suppress the message
'Image arch/arc/boot/uImage is ready' when the -s option is given.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
---

arch/arc/Makefile | 13 ++++++++++++-
arch/arc/boot/Makefile | 11 +----------
2 files changed, 13 insertions(+), 11 deletions(-)

diff --git a/arch/arc/Makefile b/arch/arc/Makefile
index cf9da9aea12a..578bdbbb0fa7 100644
--- a/arch/arc/Makefile
+++ b/arch/arc/Makefile
@@ -102,11 +102,22 @@ libs-y += arch/arc/lib/ $(LIBGCC)

boot := arch/arc/boot

-boot_targets := uImage uImage.bin uImage.gz uImage.lzma
+boot_targets := uImage.bin uImage.gz uImage.lzma

PHONY += $(boot_targets)
$(boot_targets): vmlinux
$(Q)$(MAKE) $(build)=$(boot) $(boot)/$@

+uimage-default-y := uImage.bin
+uimage-default-$(CONFIG_KERNEL_GZIP) := uImage.gz
+uimage-default-$(CONFIG_KERNEL_LZMA) := uImage.lzma
+
+PHONY += uImage
+uImage: $(uimage-default-y)
+ @ln -sf $< $(boot)/uImage
+ @$(kecho) ' Image $(boot)/uImage is ready'
+
+CLEAN_FILES += $(boot)/uImage
+
archclean:
$(Q)$(MAKE) $(clean)=$(boot)
diff --git a/arch/arc/boot/Makefile b/arch/arc/boot/Makefile
index 538b92f4dd25..3b1f8a69a89e 100644
--- a/arch/arc/boot/Makefile
+++ b/arch/arc/boot/Makefile
@@ -1,5 +1,5 @@
# SPDX-License-Identifier: GPL-2.0
-targets := vmlinux.bin vmlinux.bin.gz uImage
+targets := vmlinux.bin vmlinux.bin.gz

# uImage build relies on mkimage being availble on your host for ARC target
# You will need to build u-boot for ARC, rename mkimage to arc-elf32-mkimage
@@ -13,11 +13,6 @@ LINUX_START_TEXT = $$(readelf -h vmlinux | \
UIMAGE_LOADADDR = $(CONFIG_LINUX_LINK_BASE)
UIMAGE_ENTRYADDR = $(LINUX_START_TEXT)

-suffix-y := bin
-suffix-$(CONFIG_KERNEL_GZIP) := gz
-suffix-$(CONFIG_KERNEL_LZMA) := lzma
-
-targets += uImage
targets += uImage.bin
targets += uImage.gz
targets += uImage.lzma
@@ -42,7 +37,3 @@ $(obj)/uImage.gz: $(obj)/vmlinux.bin.gz FORCE

$(obj)/uImage.lzma: $(obj)/vmlinux.bin.lzma FORCE
$(call if_changed,uimage,lzma)
-
-$(obj)/uImage: $(obj)/uImage.$(suffix-y)
- @ln -sf $(notdir $<) $@
- @echo ' Image $@ is ready'
--
2.25.1
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